Self-Supervised Pre-training Strategies for Autonomous Driving Perception

Research Date: 2026-04-11 Focus: Unified survey of SSL pre-training methods for AV perception -- contrastive, masked autoencoder, JEPA, cross-modal distillation, and foundation model adaptation -- with emphasis on LiDAR-primary airside deployment

---

Table of Contents

1. Why Self-Supervised Pre-training for Driving
2. Contrastive Learning Methods
3. Masked Autoencoder (MAE) Methods
4. JEPA and Embedding-Space Prediction
5. DINO/DINOv2 for Driving
6. Multi-Modal Pre-training
7. Pre-training to Fine-tuning Pipeline
8. Pre-training for Specific Tasks
9. Weakly-/Semi-Supervised Learning and Active Learning
10. Training Infrastructure and Efficiency
11. Airside Pre-training Strategy
12. Comprehensive Comparison Table
13. Key Findings Summary
14. References

---

1. Why Self-Supervised Pre-training for Driving

1.1 The Labeled Data Bottleneck

Autonomous driving perception demands enormous quantities of annotated 3D data. A single nuScenes keyframe requires:

• ~40 3D bounding box annotations across 10 classes
• ~100,000 per-point semantic labels (if doing segmentation)
• ~2-4 hours of expert annotator time per frame (3D boxes) or ~8-12 hours (dense segmentation)

At scale, the costs are prohibitive:

Annotation Task	Cost per Frame	10K Frames	100K Frames
3D bounding boxes	$8-15	$80-150K	$800K-1.5M
3D semantic segmentation	$25-50	$250-500K	$2.5-5M
Panoptic 3D	$40-80	$400-800K	$4-8M

For airport airside operations, the problem is compounded:

• No public airside driving datasets exist -- zero labeled 3D LiDAR frames of apron environments
• Novel object taxonomy: GSE (baggage tractors, belt loaders, pushback tugs, catering trucks), aircraft (30-80m wingspan), ramp personnel -- none present in nuScenes/Waymo/KITTI
• Small fleet size: reference airside AV stack operates tens of vehicles, not thousands, limiting organic data collection
• Annotation expertise: Labeling airside objects requires domain knowledge (distinguishing GSE subtypes, identifying FOD, classifying aircraft states)

Self-supervised pre-training directly addresses this bottleneck by learning rich representations from unlabeled data, which is cheap to collect (just drive the vehicle).

1.2 Data Efficiency Gains from Pre-training

Published results consistently show 50-80% label savings:

Method	Pre-training Type	Label Savings	Evidence
GD-MAE (CVPR 2023)	Masked autoencoder	80%	20% Waymo labels matches full-data baseline
GPC (ICLR 2024)	Colorization	95%	5% KITTI labels exceeds scratch training
PSA-SSL (CVPR 2025)	Pose/size-aware SSL	90%	10x fewer labels matches SOTA
Occupancy-MAE (2023)	Occupancy prediction	50%	Halves labeled data for car detection
AD-PT (NeurIPS 2023)	Semi-supervised	60-70%	Cross-dataset transfer with few-shot
ScaLR (CVPR 2024)	Cross-modal distill	40-50%	67.8% mIoU with linear probe only
AD-L-JEPA (AAAI 2026)	Embedding prediction	~80%	1.9-2.7x more efficient than Occupancy-MAE

For airside, this means:

• Without pre-training: Need ~5,000-10,000 labeled airside frames ($150-500K annotation cost)
• With pre-training: Need ~500-1,000 labeled airside frames ($15-30K annotation cost)
• Savings: $100-470K in annotation costs alone

1.3 Representation Quality

Self-supervised pre-training learns fundamentally different representations than supervised training:

Supervised features are optimized for a specific label set. A model trained to detect {car, truck, pedestrian} develops features biased toward those shapes. Transferring to {baggage tractor, aircraft, pushback tug} requires the features to generalize beyond their training objective.

Self-supervised features capture general geometric and semantic structure:

• Surface continuity and curvature (from reconstruction objectives)
• Part-whole relationships (from contrastive objectives)
• Spatial extent and occupancy (from occupancy prediction)
• Cross-modal correspondences (from distillation objectives)

These properties are domain-agnostic. A backbone that understands "this is a large rigid object with a flat surface" transfers from a truck to a baggage loader more readily than one that learned "this is a truck."

Published evidence:

• DINOv2 features transfer to novel tasks with no fine-tuning (frozen linear probe on ADE20k achieves 53.1 mIoU)
• DINO pre-trained driving encoders improve route completion by +9 percentage points on unseen environments vs. supervised ImageNet pre-training (Juneja et al., 2024)
• AD-PT cross-dataset pre-training improves downstream performance beyond single-dataset supervised pre-training

1.4 Domain Adaptation

Pre-training enables efficient domain adaptation through a staged curriculum:

Road driving SSL (unlabeled, abundant)
        |
        v
Road driving supervised (labeled, public datasets)
        |
        v
Airside SSL (unlabeled airside scans, easy to collect)
        |
        v
Airside supervised (small labeled set, expensive but minimal)

Each stage builds on the previous, with the bulk of representation learning happening on cheap unlabeled data. The final supervised stage only needs to calibrate the already-learned features for the target task and domain.

1.5 Taxonomy of SSL Methods for Driving

Self-Supervised Pre-training
├── Contrastive Learning
│   ├── Single-modal (PointContrast, DepthContrast)
│   ├── Cross-modal (SLidR, ScaLR, PPKT, GPC)
│   └── Language-aligned (CLIP, ULIP, OpenScene)
├── Masked Autoencoders
│   ├── Image MAE (MAE, BEiT, SimMIM)
│   ├── Point cloud MAE (Point-MAE, GD-MAE, Voxel-MAE)
│   ├── BEV MAE (BEV-MAE, Occupancy-MAE)
│   └── Multi-modal MAE (M3I, UniPAD)
├── JEPA / Embedding Prediction
│   ├── V-JEPA / V-JEPA 2 (video)
│   ├── AD-L-JEPA (LiDAR)
│   ├── MC-JEPA (motion-content)
│   └── LeJEPA (theoretical)
├── Self-Distillation
│   ├── DINO / DINOv2 (image)
│   ├── Sonata (point cloud)
│   └── Concerto (2D-3D joint)
└── Reconstruction
    ├── Surface reconstruction (ALSO)
    ├── Depth prediction
    └── Neural rendering (UniPAD, PonderV2)

---

2. Contrastive Learning Methods

2.1 Fundamentals of Contrastive Learning for 3D

Contrastive learning learns representations by pulling similar (positive) pairs together and pushing dissimilar (negative) pairs apart in an embedding space. For driving perception, the key question is: what constitutes a meaningful positive pair?

Core loss function (InfoNCE):

import torch
import torch.nn.functional as F

def info_nce_loss(query, positive, negatives, temperature=0.07):
    """
    InfoNCE contrastive loss.
    
    Args:
        query: [B, D] query embeddings
        positive: [B, D] positive embeddings (matched pairs)
        negatives: [B, N, D] negative embeddings
        temperature: scalar temperature for sharpening
    
    Returns:
        loss: scalar contrastive loss
    """
    # Positive similarity: [B]
    pos_sim = torch.sum(query * positive, dim=-1) / temperature
    
    # Negative similarities: [B, N]
    neg_sim = torch.bmm(
        negatives, query.unsqueeze(-1)
    ).squeeze(-1) / temperature
    
    # Concatenate: [B, 1+N]
    logits = torch.cat([pos_sim.unsqueeze(-1), neg_sim], dim=-1)
    
    # Cross-entropy with positive as class 0
    labels = torch.zeros(query.shape[0], dtype=torch.long, device=query.device)
    loss = F.cross_entropy(logits, labels)
    
    return loss

2.2 PointContrast (ECCV 2020)

Paper: "PointContrast: Unsupervised Pre-Training for 3D Point Cloud Understanding" Authors: Xie et al. (Facebook AI Research)

Method:

• Registers two partially overlapping 3D point cloud views of the same scene
• Corresponding points across views form positive pairs
• Non-corresponding points form hard negatives (same scene, different locations)
• Contrastive loss (PointInfoNCE) on point-level features

Architecture:

Point cloud view 1 ──> Sparse convolution backbone ──> Point features F1
Point cloud view 2 ──> Sparse convolution backbone ──> Point features F2
                                                          |
                                                     Correspondence
                                                     matching via
                                                     registration
                                                          |
                                                     PointInfoNCE loss

Results:

Task	Dataset	Pre-trained	From Scratch	Delta
Detection	ScanNet	58.5 AP50	54.2 AP50	+4.3
Detection	SUN RGB-D	37.3 AP50	35.0 AP50	+2.3
Segmentation	ScanNet	74.1 mIoU	72.2 mIoU	+1.9

Limitation for driving: Designed for indoor scenes. Partial-overlap registration is harder for outdoor LiDAR where scenes change rapidly. However, the point-level contrastive formulation is foundational for all subsequent 3D contrastive methods.

2.3 DepthContrast (ICCV 2021)

Paper: "Self-Supervised Pretraining of 3D Features on any Point-Cloud" Authors: Zhang et al. (Facebook AI Research)

Method:

• Applies two different 3D augmentations to the same point cloud
• Point-level and scene-level contrastive objectives simultaneously
• Uses both sparse 3D CNN (U-Net) and point-based (PointNet++) backbones
• Key innovation: works on any 3D point cloud (indoor, outdoor, objects)

Architecture:

Point cloud ──> Augmentation 1 ──> 3D backbone ──> Scene embedding s1 + Point embeddings p1
            └─> Augmentation 2 ──> 3D backbone ──> Scene embedding s2 + Point embeddings p2
                                                          |
                                              Scene-level MoCo loss (s1 vs s2)
                                            + Point-level contrastive loss (p1 vs p2)

Results (outdoor driving):

Task	Dataset	Improvement over scratch
3D Detection	KITTI	+1.2 AP (moderate)
Segmentation	SemanticKITTI	+1.5 mIoU

Airside relevance: DepthContrast is the first method showing that contrastive pre-training on arbitrary 3D data improves outdoor driving perception. The "any point cloud" flexibility means airside LiDAR scans can be used directly for pre-training without any data format requirements.

2.4 SLidR: Image-to-LiDAR Self-Supervised Distillation (CVPR 2022)

Paper: "Image-to-Lidar Self-Supervised Distillation for Autonomous Driving Data" Authors: Sautier et al. (Valeo AI, Ecole des Ponts) GitHub: valeoai/slidr | 330+ stars

Method: SLidR bridges 2D image features to 3D LiDAR points via superpixel correspondences:

1. Superpixel generation: Segment camera images into superpixels (SLIC algorithm)
2. LiDAR projection: Project LiDAR points onto camera images to find which superpixel each point falls in
3. 2D feature extraction: Pass images through a pre-trained image encoder (MoCo v2, DINOv1)
4. Average pooling: Pool 2D features per superpixel to get one feature per superpixel region
5. Contrastive learning: LiDAR point features are pulled toward the pooled 2D feature of their corresponding superpixel, pushed away from features of other superpixels

Key insight: Superpixels provide natural groupings that are more stable than pixel-level correspondences, handling calibration noise and sensor misalignment gracefully.

Results (nuScenes linear probing):

Backbone	Pre-training	mIoU (linear probe)
MinkUNet	From scratch	30.1
MinkUNet	PointContrast	32.4
MinkUNet	DepthContrast	34.2
MinkUNet	SLidR	38.8

SLidR outperforms all prior 3D-only contrastive methods by a large margin, demonstrating that 2D image features provide richer supervisory signal than 3D self-augmentation alone.

Airside relevance: HIGH. If the reference airside AV stack adds cameras (even minimal -- 2-4 cameras), SLidR enables leveraging powerful 2D foundation models to improve LiDAR features without any labels. The superpixel approach is robust to imperfect camera-LiDAR calibration.

2.5 PPKT: Point-to-Pixel Knowledge Transfer (ICCV 2021)

Paper: "Image-to-Point Cloud Knowledge Transfer via Pixel-to-Point Knowledge Transfer" Authors: Liu et al.

Method:

• Direct pixel-to-point correspondence via camera-LiDAR projection
• Contrastive loss between projected image features and LiDAR point features
• Simpler than SLidR (no superpixel step) but more sensitive to calibration noise

Results:

Setting	mIoU Improvement
nuScenes seg (1% labels)	+5.2 over scratch
nuScenes seg (10% labels)	+3.1 over scratch
nuScenes seg (100% labels)	+1.4 over scratch

Key finding: The benefit of cross-modal pre-training is largest in the low-label regime -- exactly the airside scenario.

2.6 ScaLR: Scaled Cross-Modal Distillation (CVPR 2024)

Paper: "Three Pillars Improving Vision Foundation Model Distillation for Lidar" Authors: Sanchez et al. (Valeo AI) GitHub: valeoai/ScaLR | 62 stars

ScaLR extends SLidR with three key improvements (the "three pillars"):

1. Scaling the 2D teacher: Uses DINOv2 instead of MoCo v2 / DINO v1. The jump from DINO v1 to DINOv2 alone provides +8.2 mIoU improvement on linear probing.

2. Scaling the 3D student: Replaces MinkUNet with WaffleIron (a more efficient 3D backbone), enabling larger model capacity without proportionally more compute.

3. Scaling the training data: Pre-trains on multiple driving datasets simultaneously (nuScenes + other) rather than single-dataset pre-training.

Results (nuScenes linear probing -- frozen backbone evaluation):

Method	2D Teacher	3D Student	mIoU
SLidR	MoCo v2	MinkUNet	38.8
SLidR	DINOv1	MinkUNet	44.2
ScaLR	DINOv2	MinkUNet	52.4
ScaLR	DINOv2	WaffleIron	67.8

ScaLR achieves 67.8% mIoU with linear probing alone -- meaning the frozen LiDAR backbone already produces features good enough for semantic segmentation without any task-specific fine-tuning. This is the best LiDAR-only self-supervised result published as of early 2026.

Airside relevance: VERY HIGH. ScaLR is directly applicable to the reference airside AV stack:

• Requires camera-LiDAR pairs (available if cameras are added)
• Pre-trains LiDAR backbone using only the camera teacher -- no 3D labels needed
• Frozen features already achieve strong segmentation -- fine-tuning on airside data should yield even better results
• See lidar-foundation-models.md for integration recommendations

2.7 ProposalContrast (CVPR 2023)

Paper: "ProposalContrast: Unsupervised Pre-training for LiDAR-Based 3D Object Detection" Authors: Yin et al. (Huawei Noah's Ark Lab)

Method: ProposalContrast addresses a limitation of point-level contrastive methods -- they learn features for individual points but not for object-level proposals needed by detection heads.

1. Region proposal generation: Use unsupervised clustering (DBSCAN) to generate 3D region proposals from raw LiDAR
2. Proposal-level contrastive learning: Augmented views of the same proposal form positive pairs
3. Hard negative mining: Nearby but distinct proposals form hard negatives
4. Multi-scale features: Proposals at different spatial scales capture both local detail and global context

Results (Waymo 3D detection):

Backbone	Pre-training	Vehicle APH (L2)	Pedestrian APH (L2)
CenterPoint	Scratch	66.2	62.1
CenterPoint	PointContrast	67.1	63.0
CenterPoint	ProposalContrast	68.5	64.8

Airside relevance: ProposalContrast is particularly relevant because it directly optimizes for the detection task (proposal-level features) rather than per-point features. For airside, the DBSCAN clustering would naturally group GSE, aircraft parts, and personnel clusters, providing a strong unsupervised initialization for the detection pipeline.

2.8 Cross-Modal Contrastive: LiDAR-Camera, LiDAR-Radar

Beyond the image-to-LiDAR methods above, cross-modal contrastive learning extends to additional sensor pairs:

LiDAR-Camera (general paradigm):

Camera images ──> 2D encoder (frozen) ──> 2D features
                                              |
                                         Projection &
                                         Correspondence
                                              |
LiDAR points  ──> 3D encoder (trainable) ──> 3D features
                                              |
                                    Cross-modal contrastive loss

LiDAR-Radar contrastive (2024-2025):

• Radar provides velocity and reflectivity that LiDAR lacks
• LiDAR provides geometric precision that radar lacks
• Contrastive alignment of LiDAR-radar features at matched spatial locations
• Particularly relevant for airside where 4D radar is recommended for adverse weather robustness (see 20-av-platform/sensors/4d-radar.md)

Cross-modal self-supervised learning study (2024): A systematic study demonstrated that cross-modality contrastive learning outperforms single-modality alternatives for self-driving point clouds. The key insight: using a modality with richer semantics (images) to supervise a modality with better geometry (LiDAR) yields better features than either modality can learn alone.

2.9 CLIP and OpenCLIP for Driving Scene Understanding

CLIP (Contrastive Language-Image Pre-training): CLIP learns a shared embedding space between images and natural language via contrastive learning on 400M image-text pairs. For driving, CLIP enables:

• Open-vocabulary detection: Detect objects described in natural language, including novel classes not in training data ("pushback tug," "baggage dolly chain," "marshaller with wands")
• Scene classification: "Is this a normal apron operation or an emergency scenario?"
• Zero-shot transfer: Apply to airside without any airside-specific training

OpenCLIP (open-source reproduction) extends CLIP with:

• Training on LAION-5B (5 billion image-text pairs)
• Larger model variants (ViT-G/14, ViT-bigG/14)
• Better performance on downstream driving tasks

CLIP for 3D via lifting:

Method	Venue	Approach	Result
ULIP-2	CVPR 2024	Align 3D point embeddings to CLIP space	Open-vocabulary 3D recognition
OpenScene	CVPR 2023	Dense CLIP features lifted to 3D via projection	Language-queried 3D segmentation
Concerto	NeurIPS 2025	CLIP translator in PTv3 backbone	Open-world 3D perception

Airside application: CLIP-aligned 3D features enable querying the LiDAR scene with natural language -- "show me all ground support equipment within 10m of aircraft Alpha" -- without needing explicit class definitions. This is valuable for the airside domain where the object taxonomy is complex and evolving.

---

3. Masked Autoencoder (MAE) Methods

3.1 MAE for Vision (MAE, BEiT, SimMIM)

Masked autoencoders learn by masking portions of the input and predicting the missing content. The approach was first proven for NLP (BERT) and adapted to vision:

MAE (He et al., CVPR 2022):

• Masks 75% of image patches randomly
• Asymmetric encoder-decoder: heavy ViT encoder processes only visible patches, lightweight decoder reconstructs masked patches
• Reconstruction target: normalized pixel values of masked patches
• Result: 87.8% ImageNet top-1 with ViT-H/14 (surpasses supervised pre-training)

BEiT (Bao et al., ICLR 2022):

• Uses a discrete visual tokenizer (dVAE from DALL-E) to convert patches to discrete tokens
• Predicts token IDs of masked patches rather than raw pixels
• This discrete target provides a more semantic reconstruction objective

SimMIM (Xie et al., CVPR 2022):

• Simplifies MAE: random masking, direct pixel regression, simple L1 loss
• Shows that the masking strategy matters more than the reconstruction target
• Works with both ViT and Swin Transformer architectures

3.2 MAE Training Loop (Reference Implementation)

import torch
import torch.nn as nn
import torch.nn.functional as F
from einops import rearrange

class MAEPretrainer(nn.Module):
    """
    Masked Autoencoder pre-trainer for ViT backbone.
    Reference implementation for driving perception pre-training.
    """
    def __init__(
        self,
        encoder,           # ViT encoder (to be pre-trained)
        decoder_dim=512,
        decoder_depth=4,
        decoder_heads=8,
        mask_ratio=0.75,
        patch_size=14,
    ):
        super().__init__()
        self.encoder = encoder
        self.mask_ratio = mask_ratio
        self.patch_size = patch_size
        
        encoder_dim = encoder.embed_dim  # e.g., 768 for ViT-B
        
        # Encoder-to-decoder projection
        self.enc_to_dec = nn.Linear(encoder_dim, decoder_dim)
        
        # Learnable mask token
        self.mask_token = nn.Parameter(torch.randn(1, 1, decoder_dim))
        
        # Decoder (lightweight)
        decoder_layer = nn.TransformerEncoderLayer(
            d_model=decoder_dim,
            nhead=decoder_heads,
            dim_feedforward=decoder_dim * 4,
            activation='gelu',
            batch_first=True,
        )
        self.decoder = nn.TransformerEncoder(
            decoder_layer, num_layers=decoder_depth
        )
        
        # Reconstruction head: predict normalized pixel values
        self.pred_head = nn.Linear(
            decoder_dim, patch_size * patch_size * 3
        )
    
    def random_masking(self, x, mask_ratio):
        """
        Random masking: keep (1-mask_ratio) of patches.
        
        Args:
            x: [B, N, D] patch embeddings
            mask_ratio: fraction of patches to mask
        Returns:
            x_visible: [B, N_vis, D] visible patch embeddings
            mask: [B, N] binary mask (1 = masked, 0 = visible)
            ids_restore: [B, N] indices for unshuffling
        """
        B, N, D = x.shape
        n_keep = int(N * (1 - mask_ratio))
        
        # Random permutation per sample
        noise = torch.rand(B, N, device=x.device)
        ids_shuffle = torch.argsort(noise, dim=1)
        ids_restore = torch.argsort(ids_shuffle, dim=1)
        
        # Keep first n_keep patches
        ids_keep = ids_shuffle[:, :n_keep]
        x_visible = torch.gather(
            x, dim=1, index=ids_keep.unsqueeze(-1).expand(-1, -1, D)
        )
        
        # Binary mask
        mask = torch.ones(B, N, device=x.device)
        mask[:, :n_keep] = 0
        mask = torch.gather(mask, dim=1, index=ids_restore)
        
        return x_visible, mask, ids_restore
    
    def forward(self, images):
        """
        Forward pass: mask, encode visible, decode all, compute loss.
        
        Args:
            images: [B, 3, H, W] input images
        Returns:
            loss: reconstruction loss on masked patches
        """
        # Patchify and embed
        patches = self.encoder.patch_embed(images)  # [B, N, D]
        B, N, D = patches.shape
        
        # Add positional embeddings before masking
        patches = patches + self.encoder.pos_embed[:, 1:N+1, :]
        
        # Random masking
        visible, mask, ids_restore = self.random_masking(
            patches, self.mask_ratio
        )
        
        # Encode visible patches only (efficiency gain)
        for block in self.encoder.blocks:
            visible = block(visible)
        visible = self.encoder.norm(visible)
        
        # Project to decoder dimension
        visible = self.enc_to_dec(visible)
        
        # Prepare full sequence with mask tokens
        mask_tokens = self.mask_token.expand(
            B, N - visible.shape[1], -1
        )
        full_seq = torch.cat([visible, mask_tokens], dim=1)
        
        # Unshuffle to original order
        full_seq = torch.gather(
            full_seq, dim=1,
            index=ids_restore.unsqueeze(-1).expand(-1, -1, full_seq.shape[-1])
        )
        
        # Decode
        decoded = self.decoder(full_seq)
        
        # Predict pixel values
        pred = self.pred_head(decoded)  # [B, N, patch_size^2 * 3]
        
        # Compute target: normalized pixel patches
        target = self._patchify(images)  # [B, N, patch_size^2 * 3]
        
        # Per-patch normalization (as in original MAE)
        mean = target.mean(dim=-1, keepdim=True)
        var = target.var(dim=-1, keepdim=True)
        target = (target - mean) / (var + 1e-6).sqrt()
        
        # Loss only on masked patches
        loss = (pred - target) ** 2
        loss = loss.mean(dim=-1)  # [B, N] per-patch loss
        loss = (loss * mask).sum() / mask.sum()  # avg over masked
        
        return loss
    
    def _patchify(self, images):
        """Convert images to patch sequences."""
        p = self.patch_size
        B, C, H, W = images.shape
        h, w = H // p, W // p
        patches = rearrange(
            images, 'b c (h p1) (w p2) -> b (h w) (p1 p2 c)',
            p1=p, p2=p
        )
        return patches


# Training loop example
def train_mae_epoch(model, dataloader, optimizer, device):
    """One epoch of MAE pre-training."""
    model.train()
    total_loss = 0
    
    for batch_idx, images in enumerate(dataloader):
        images = images.to(device)
        
        loss = model(images)
        
        optimizer.zero_grad()
        loss.backward()
        
        # Gradient clipping (important for stable MAE training)
        torch.nn.utils.clip_grad_norm_(model.parameters(), max_norm=1.0)
        
        optimizer.step()
        total_loss += loss.item()
    
    return total_loss / len(dataloader)

3.3 Voxel-MAE for LiDAR (2022)

Paper: "Voxel-MAE: Masked Autoencoders for Pre-Training Large-Scale Point Clouds" Authors: Hess et al.

Method:

• Voxelizes LiDAR point cloud into regular 3D grid
• Randomly masks voxels at 60-80% ratio
• Encoder: 3D sparse convolution (only on visible voxels)
• Decoder: predicts occupancy and point positions of masked voxels
• Dual reconstruction target: (1) binary occupancy, (2) point coordinates within voxel

Results (Waymo 3D detection):

Backbone	Pre-training	Vehicle APH (L1)
VoxelNet	Scratch	71.8
VoxelNet	Voxel-MAE	73.2 (+1.4)

Airside relevance: Works with voxel-based backbones compatible with the reference airside AV stack OpenPCDet stack. The dual reconstruction (occupancy + position) learns both coarse spatial structure and fine geometric detail.

3.4 Occupancy-MAE (IEEE TIV 2023)

Paper: "Self-supervised Pre-training Large-scale LiDAR Point Clouds with Masked Occupancy Autoencoders" Authors: Min et al. GitHub: 280 stars

Architecture:

• Designed specifically for voxel-based outdoor LiDAR point clouds
• Range-aware random masking: Accounts for LiDAR density variation with distance -- masks more densely sampled near-range voxels at higher rates to balance learning difficulty
• Pretext task: Binary occupancy prediction (does a masked voxel contain points?)
• Even with 90% masking ratio, learns representative features
• Compatible with SECOND, CenterPoint, PV-RCNN detectors

Key results:

Task	Dataset	Improvement
3D Detection (Car)	KITTI	Reduces labeled data by 50% for car detection
3D Detection (Small objects)	Waymo	~2% AP improvement
3D Segmentation	Multiple	~2% mIoU improvement

Airside relevance: HIGH. The range-aware masking is particularly important for airside because LiDAR density patterns on aprons differ from roads (large open areas with sparse structure punctuated by dense aircraft returns). Works with existing OpenPCDet backbones.

3.5 GD-MAE: Geometry-Enhanced MAE (CVPR 2023)

Paper: "Generative Decoder for MAE Pre-training on LiDAR Point Clouds" Authors: Yang et al. GitHub: 124 stars

Architecture:

• First MAE method designed specifically for outdoor LiDAR point clouds
• Generative decoder that hierarchically merges surrounding context to restore masked geometric knowledge
• Works with voxel-based 3D backbones (VoxelBackBone, SECOND)
• Compatible with CenterPoint and PV-RCNN detection heads

Critical data efficiency result:

• Achieves comparable accuracy with only 20% of labeled Waymo data -- GD-MAE_0.2 demonstrates that pre-training on unlabeled data followed by fine-tuning on 20% of labels matches full-label performance

Results:

Dataset	Backbone	Metric	Score
Waymo (Vehicle L1)	Two-stage	mAPH	80.2/79.8
KITTI (Car)	SECOND	AP	82.01 (moderate)
ONCE (Vehicle)	CenterPoint	AP	76.79 (vs 74.10 scratch)

Airside relevance: VERY HIGH. GD-MAE works with the exact voxel-based backbones used in the reference airside AV stack CenterPoint pipeline. The 80% label reduction directly addresses the zero-airside-dataset problem. See lidar-foundation-models.md Section 2.4 for full details.

3.6 BEV-MAE (AAAI 2024)

Paper: "Bird's Eye View Masked Autoencoders for Point Cloud Pre-training in Autonomous Driving" Authors: Ren et al. (Peking University)

Architecture:

• Projects LiDAR point cloud onto 2D BEV grid
• BEV-guided masking: randomly masks non-empty BEV grid cells
• Pretext task: Predicts point density per masked BEV cell (not just occupancy) -- a harder objective that learns richer features
• Leverages LiDAR density-distance correlation
• Avoids complex 3D decoder design by operating in BEV space

Results:

Setting	Metric	Improvement
100% pretrain, 20% finetune (Waymo)	mAP	+1.42 over baseline
100% pretrain, 20% finetune (Waymo)	APH	+1.34 over baseline

Airside relevance: The BEV formulation aligns with the reference airside AV stack's BEV-based planning representation. Pre-training in BEV space means learned features are directly compatible with downstream planning.

3.7 NOMAE: Multi-Scale Occupancy MAE (CVPR 2025)

Paper: "Non-Overlapping Masked Autoencoders for 3D Point Cloud Pre-training" Authors: 2025

Key innovation: Multi-scale occupancy prediction with non-overlapping mask strategy that prevents information leakage between encoder and decoder. NOMAE achieves the remarkable result of beating fully supervised baselines with self-supervised pre-training alone, demonstrating that SSL representations can surpass task-specific supervision.

Airside relevance: VERY HIGH. If NOMAE's SSL pre-training beats supervised baselines on road driving data, the gap should be even larger for airside where labeled data is scarce and expensive.

3.8 MAE vs. JEPA: Efficiency Comparison

AD-L-JEPA (AAAI 2026) provides a direct comparison between MAE-style and JEPA-style pre-training for LiDAR:

Metric	Occupancy-MAE	AD-L-JEPA	Advantage
GPU hours (pre-training)	1x	0.37-0.53x	1.9-2.7x faster
GPU memory	1x	0.25-0.36x	2.8-4x less memory
3D detection (KITTI)	Baseline	Comparable/better	Equivalent quality
3D detection (Waymo)	Baseline	Comparable/better	Equivalent quality
3D detection (ONCE)	Baseline	Comparable/better	Equivalent quality

The efficiency gap arises because MAE methods must reconstruct raw voxel/point content (pixel-space prediction), while JEPA predicts in a learned embedding space that discards irrelevant detail. See Section 4 for full JEPA analysis.

---

4. JEPA and Embedding-Space Prediction

4.1 Core Principle: Predict Embeddings, Not Pixels

The Joint Embedding Predictive Architecture (JEPA), proposed by Yann LeCun (2022), represents a fundamentally different approach to self-supervised learning:

• MAE/Generative: Mask input, predict raw pixels/voxels of masked regions
• Contrastive: Pull positive pairs together, push negatives apart in embedding space
• JEPA: Mask input, predict the learned embeddings of masked regions (not raw content)

Why this matters for driving: Driving scenes contain enormous amounts of irrelevant detail -- exact cloud patterns, precise shadow shapes, specific reflectance values. MAE methods waste capacity modeling this noise. JEPA learns to predict only the semantically meaningful aspects by operating in a learned embedding space that naturally filters out low-level variation.

            MAE approach:
            Input ──> Encoder ──> [mask] ──> Decoder ──> Reconstruct raw input
                                                          (pixel-level detail)

            JEPA approach:
            Input x ──> Encoder ──> Features f(x)
            Input y ──> Encoder ──> Features f(y)  [target, stop-gradient]
                                        |
            Predictor: f(x) ──> predicted f(y)
                                        |
                              L1 loss in embedding space
                              (semantic-level prediction)

4.2 V-JEPA: Video JEPA (Meta, 2024)

Paper: "V-JEPA: Latent Video Prediction for Visual Representation Learning" Authors: Bardes, Garrido, et al. (Meta FAIR, Yann LeCun) GitHub: facebookresearch/jepa

Architecture:

• ViT encoder processes video frames as spatiotemporal patches
• Context encoder sees visible (unmasked) patches
• Target encoder (EMA of context encoder) produces target embeddings for masked patches
• Predictor maps context encoder outputs to target encoder space
• Trained on 2 million video clips from various sources

Masking strategy:

• Multi-block masking: masks 8-10 large spatiotemporal blocks per video
• Each block spans multiple frames, forcing temporal prediction
• ~90% of spatiotemporal tokens are masked (high masking ratio)

Results:

Benchmark	V-JEPA	VideoMAE v2	OmniMAE
Kinetics-400 (frozen)	81.9	77.0	74.2
Something-Somethingv2	71.4	69.6	65.1
ImageNet-1k (frozen)	82.0	76.8	74.9

V-JEPA outperforms all video MAE methods while being more sample-efficient, validating the embedding-space prediction hypothesis.

4.3 V-JEPA 2 and V-JEPA 2-AC (Meta, 2025)

Paper: "V-JEPA 2: Self-Supervised Video Models Enable Understanding, Prediction, and Planning" Authors: Assran, Bardes, et al. (Meta FAIR) GitHub: facebookresearch/vjepa2

V-JEPA 2 scales the approach with:

• Larger models: Up to ViT-H and beyond
• More data: Trained on diverse video datasets
• Multi-task transfer: Single frozen backbone supports classification, action recognition, and depth estimation simultaneously

V-JEPA 2-AC (Action-Conditioned): The action-conditioned variant enables model-predictive control:

• Predicts future state embeddings conditioned on candidate actions
• Uses Cross-Entropy Method (CEM) to optimize action sequences in embedding space
• Planning in embedding space is ~15x faster than pixel-space prediction (no need for expensive pixel generation)

Relevance to driving: V-JEPA 2-AC demonstrates that JEPA embeddings are sufficient for planning and control -- the predicted future state embeddings carry enough information to evaluate action quality without ever generating pixels.

4.4 AD-L-JEPA: JEPA for LiDAR Driving (AAAI 2026)

Paper: "AD-L-JEPA: Self-Supervised Spatial World Model for Autonomous Driving with Joint-Embedding Predictive Architecture" Authors: Zhu et al. GitHub: haoranzhuexplorer/ad-l-jepa-release

AD-L-JEPA is the first JEPA-based pre-training method for autonomous driving LiDAR. It adapts the JEPA framework to Bird's-Eye-View representations of driving scenes:

Architecture:

1. Voxelize LiDAR point cloud and project to BEV features
2. Context encoder processes visible BEV regions
3. Target encoder (EMA) produces target embeddings for masked regions
4. Predictor maps context embeddings to target embedding space
5. Variance regularization prevents representation collapse (replacing the contrastive-based collapse prevention used in image JEPA)

Key properties:

• Neither generative nor contrastive -- uses explicit variance regularization
• Predicts BEV embeddings, not raw occupancy/points
• Compatible with standard voxel-based detection heads (CenterPoint, SECOND)

Results:

Dataset	Metric	AD-L-JEPA	Occupancy-MAE	Improvement
KITTI3D (Car)	AP (mod)	Improved	Baseline	Comparable/better
Waymo (Vehicle)	APH (L2)	Improved	Baseline	Comparable/better
ONCE (Vehicle)	AP	Improved	Baseline	Comparable/better

Efficiency:

Resource	AD-L-JEPA vs Occupancy-MAE
GPU hours	1.9-2.7x fewer
GPU memory	2.8-4x less
Downstream quality	Comparable or better

Airside relevance: VERY HIGH. AD-L-JEPA provides the most efficient pre-training for the LiDAR-primary reference airside AV stack:

• Works with existing BEV-based pipeline
• Dramatically reduces pre-training compute costs
• Compatible with CenterPoint detection heads
• The BEV embedding prediction is well-suited for the flat, open apron environment
• See tokenized-and-jepa.md Section 2.4 for context on JEPA paradigm

4.5 MC-JEPA: Motion-Content Separation (2023)

Paper: "MC-JEPA: A Joint-Embedding Predictive Architecture for Self-Supervised Learning of Motion and Content Features" Authors: Bardes et al.

MC-JEPA jointly learns optical flow (motion) and content features within a shared encoder using a JEPA objective. The key innovation is simultaneously interpreting dynamic and static elements of video.

Relevance to airside: Airport aprons have a mix of static structure (terminal, jet bridges) and dynamic objects (GSE, personnel, taxiing aircraft) with very different motion patterns. MC-JEPA's explicit motion-content separation could yield better features for tracking moving GSE while maintaining robust understanding of static infrastructure.

4.6 LeJEPA: Theoretical Foundation

Paper: "Provable JEPA Objective" Authors: Balestriero et al. (Meta FAIR) GitHub: rbalestr-lab/lejepa

LeJEPA provides theoretical grounding for why JEPA works:

• Proves that the JEPA objective is equivalent to learning a conditional expectation in feature space
• Shows that the target encoder converges to capture the mutual information between context and target
• Provides guarantees on representation quality under specific assumptions about data distribution

Practical implication: LeJEPA validates that JEPA is not just an empirical trick but has principled statistical foundations. The theoretical guarantees suggest JEPA representations should transfer well to new domains (like airside) because they capture the underlying data structure rather than surface statistics.

4.7 Embedding-Space vs. Pixel-Space Prediction: Tradeoffs

Dimension	Pixel-Space (MAE)	Embedding-Space (JEPA)
Reconstruction	Can visualize what the model learned	Latent -- harder to interpret
Compute	Heavier decoder needed for pixel reconstruction	Lightweight predictor sufficient
Memory	Must store full pixel targets	Only store compressed embeddings
Noise modeling	Models all pixel variation (including irrelevant)	Filters out unpredictable detail
Feature quality	Good spatial features, may overfit to texture	Better semantic features, more robust
Collapse risk	None (reconstruction provides natural gradient signal)	Requires regularization (EMA, variance)
Data efficiency	Moderate	Higher (focuses on structure not detail)
Driving suitability	Good for reconstruction tasks (depth, flow)	Better for semantic tasks (detection, segmentation)

Recommendation for airside: Use JEPA (AD-L-JEPA) for LiDAR pre-training when the goal is detection/segmentation. Use MAE (GD-MAE, Occupancy-MAE) when you need to learn fine geometric detail or when using the pre-trained decoder for auxiliary tasks like occupancy prediction.

---

5. DINO/DINOv2 for Driving

5.1 DINOv2 Feature Quality for Driving

DINOv2 (Meta, 2023) is a family of self-supervised Vision Transformers trained on LVD-142M (142M curated images, no labels). It produces universal visual features via a combination of:

• DINO self-distillation loss (image-level, multi-crop)
• iBOT masked image modeling loss (patch-level)
• KoLeo regularizer (uniform feature distribution)

Why DINOv2 features are valuable for driving:

1. Domain-agnostic: Features learned from 142M diverse images transfer to driving without driving-specific training
2. Dense spatial quality: Patch-level features capture object boundaries and parts without segmentation supervision
3. Robustness: DINOv2+LoRA-adapted BEV achieves 2.5x better performance under motion blur and maintains >60% under darkness vs. SimpleBEV dropping below 30% (nuScenes-C benchmark)
4. Multi-task: Same frozen features work for detection, segmentation, depth estimation simultaneously

Frozen feature benchmarks:

Task	ViT-S	ViT-B	ViT-L	ViT-g
ImageNet k-NN (%)	79.0	82.1	83.5	83.5
ADE20k Seg (mIoU)	47.2	51.3	53.1	53.0
NYU Depth (RMSE)	0.417	0.362	0.333	0.298

See dinov2-foundation-models-driving.md for complete architecture details.

5.2 Direct Backbone Replacement Fails

Critical finding: When DINOv2-Small was used as a drop-in replacement for ResNet-18 in a 3D detection pipeline (without adapter modules), the model achieved 0.0% mAP and only 6% NDS -- effectively complete failure.

Why this happens:

• DINOv2 outputs patch tokens at 14x14 resolution -- different spatial structure than CNN feature pyramids
• ViT patch tokens are 1D sequences that need architectural adaptation for 2D spatial tasks
• Detection heads expect multi-scale feature pyramids (FPN), not flat patch sequences
• The embedding dimension (384-1536) doesn't match what downstream heads expect

Solution: Adapter-mediated integration:

DINOv2 ViT backbone (frozen or LoRA-adapted)
       |
Feature adapter (conv layers for resolution/dimension matching)
       |
Feature Pyramid Network (multi-scale features)
       |
View transform (LSS / BEVFormer / SimpleBEV)
       |
BEV feature grid → Detection / segmentation heads

5.3 LoRA Rank 32 Optimal for Fine-tuning

Paper: "Robust Bird's Eye View Segmentation by Adapting DINOv2" (arXiv:2409.10228)

LoRA (Low-Rank Adaptation) updates only query and value projections in ViT attention layers. For DINOv2 applied to BEV segmentation:

Configuration	Learnable Params	mIoU
SimpleBEV (ResNet-101, full)	37M	42.3
DINOv2 ViT-B + LoRA (r=32)	1M	42.3
DINOv2 ViT-L + LoRA (r=32)	3M	43.4
DINOv2 ViT-L + LoRA (r=32, hi-res)	3M	47.6

DINOv2 ViT-B + LoRA matches ResNet-101 with 37x fewer learnable parameters.

LoRA adapter setup code:

import torch
import torch.nn as nn
import math

class LoRALinear(nn.Module):
    """
    LoRA adapter for DINOv2 / ViT attention layers.
    Wraps an existing nn.Linear with low-rank update.
    """
    def __init__(
        self,
        original_linear: nn.Linear,
        rank: int = 32,
        alpha: float = 32.0,
        dropout: float = 0.0,
    ):
        super().__init__()
        self.original_linear = original_linear
        self.rank = rank
        self.alpha = alpha
        self.scaling = alpha / rank
        
        in_features = original_linear.in_features
        out_features = original_linear.out_features
        
        # Low-rank decomposition: delta_W = B @ A
        self.lora_A = nn.Linear(in_features, rank, bias=False)
        self.lora_B = nn.Linear(rank, out_features, bias=False)
        self.lora_dropout = nn.Dropout(dropout) if dropout > 0 else nn.Identity()
        
        # Initialize A with Kaiming, B with zeros (start as identity)
        nn.init.kaiming_uniform_(self.lora_A.weight, a=math.sqrt(5))
        nn.init.zeros_(self.lora_B.weight)
        
        # Freeze original weights
        self.original_linear.weight.requires_grad = False
        if self.original_linear.bias is not None:
            self.original_linear.bias.requires_grad = False
    
    def forward(self, x):
        # Original output + low-rank update
        original_output = self.original_linear(x)
        lora_output = self.lora_B(self.lora_A(self.lora_dropout(x)))
        return original_output + lora_output * self.scaling


def add_lora_to_dinov2(model, rank=32, alpha=32.0, target_modules=('qkv',)):
    """
    Add LoRA adapters to DINOv2 ViT model.
    
    Args:
        model: DINOv2 ViT model (e.g., from torch.hub)
        rank: LoRA rank (32 is optimal per published results)
        alpha: LoRA scaling factor
        target_modules: which linear layers to adapt
            'qkv' -- query/key/value projection (standard)
            'proj' -- output projection
            'mlp' -- MLP layers in transformer blocks
    
    Returns:
        model with LoRA adapters added
    """
    # Freeze all parameters first
    for param in model.parameters():
        param.requires_grad = False
    
    lora_params = 0
    total_params = sum(p.numel() for p in model.parameters())
    
    for name, module in model.named_modules():
        # Target attention QKV projections
        if 'qkv' in target_modules and name.endswith('.attn.qkv'):
            if isinstance(module, nn.Linear):
                lora_layer = LoRALinear(module, rank=rank, alpha=alpha)
                # Replace in parent module
                parent_name = name.rsplit('.', 1)[0]
                attr_name = name.rsplit('.', 1)[1]
                parent = dict(model.named_modules())[parent_name]
                setattr(parent, attr_name, lora_layer)
                lora_params += rank * (module.in_features + module.out_features)
        
        # Target output projection
        if 'proj' in target_modules and name.endswith('.attn.proj'):
            if isinstance(module, nn.Linear):
                lora_layer = LoRALinear(module, rank=rank, alpha=alpha)
                parent_name = name.rsplit('.', 1)[0]
                attr_name = name.rsplit('.', 1)[1]
                parent = dict(model.named_modules())[parent_name]
                setattr(parent, attr_name, lora_layer)
                lora_params += rank * (module.in_features + module.out_features)
    
    trainable = sum(p.numel() for p in model.parameters() if p.requires_grad)
    print(f"LoRA params: {lora_params:,}")
    print(f"Total trainable: {trainable:,} / {total_params:,} "
          f"({100*trainable/total_params:.2f}%)")
    
    return model


# Usage example:
# dinov2 = torch.hub.load('facebookresearch/dinov2', 'dinov2_vitl14')
# dinov2 = add_lora_to_dinov2(dinov2, rank=32, target_modules=('qkv', 'proj'))
# Only ~3M trainable params out of 304M total (< 1%)

5.4 Feature Lifting from 2D to 3D

OccFeat approach: Lift DINOv2 2D features to 3D space for volumetric perception:

1. Extract multi-scale DINOv2 features from each camera view
2. Project features onto LiDAR point clouds using camera-LiDAR calibration
3. Accumulate across multiple frames for temporal density
4. Voxelize projected features to produce 3D feature volumes

DualViewDistill (2025) implements this pipeline:

• Creates BEV pseudo-labels from DINOv2 features projected onto LiDAR
• Achieves 0.669 AMOTA on nuScenes test (SOTA tracking), 0.621 mAP detection
• ID switches reduced from 699 to 407 vs. previous SOTA

5.5 DINOv2 + BEVFormer for Camera-Based Perception

The combination of DINOv2 as the image backbone with BEVFormer as the view transform provides a strong camera-based perception pipeline:

Multi-camera images (6 views)
       |
DINOv2 ViT-L (frozen + LoRA r=32)  →  ~3M trainable params
       |
Feature adapter (conv 1x1, 1024 → 256)
       |
BEVFormer (spatial cross-attention + temporal self-attention)
       |
BEV features (200x200 grid, 0.5m resolution)
       |
├── CenterPoint head (3D detection)
├── Segmentation head (BEV semantic map)
└── Occupancy head (3D occupancy prediction)

Performance with DINOv2 + LSS:

Configuration	Vehicle Seg IoU
LSS (EfficientNet)	33.0
EfficientNet + Metric3Dv2	40.5
DINOv2 + Metric3Dv2	41.9 (+8.9)
DINOv2 + Metric3Dv2 (half data)	40.4 (+7.4)

DINOv2-based pipeline converges in <150K iterations vs. 300K+ for baseline, while achieving 22.4% relative improvement.

---

6. Multi-Modal Pre-training

6.1 UniPAD: Universal Pre-training for Accelerated Deployment (CVPR 2024)

Paper: "UniPAD: A Universal Pre-training Paradigm for Autonomous Driving" Authors: Yang et al. GitHub: 204 stars

UniPAD introduces a unified encoder that pre-trains on both 2D images and 3D point clouds simultaneously via differentiable neural rendering:

Architecture:

1. Unified 3D encoder: Encodes both camera features (lifted to 3D via LSS) and LiDAR features into a shared 3D volume
2. Pre-training objective: Differentiable volume rendering -- predicts depth maps, RGB images, and 3D occupancy from the 3D volume
3. Neural rendering as supervision: The rendering objective forces the encoder to learn accurate 3D geometry without explicit 3D annotations

Results (nuScenes):

Metric	UniPAD	Previous Best
NDS	73.2	~72
mAP	~68	~66
3D Segmentation (mIoU)	79.4	~77

Airside relevance: UniPAD's unified pre-training means a single pre-training run benefits both LiDAR and camera perception pathways. This is particularly valuable for the Phase 2 transition from LiDAR-only to LiDAR+camera fusion in the reference airside AV stack.

6.2 BEVDistill: Cross-Modal BEV Distillation (NeurIPS 2023)

Paper: "BEVDistill: Cross-Modal BEV Knowledge Distillation for Multi-View 3D Object Detection"

BEVDistill transfers knowledge from a LiDAR-based BEV teacher to a camera-only BEV student:

Pipeline:

LiDAR point cloud ──> LiDAR BEV encoder (teacher, frozen) ──> BEV features (target)
                                                                    |
Camera images ──> Camera BEV encoder (student, trainable) ──> BEV features (predicted)
                                                                    |
                                                     BEV-level distillation loss
                                                   + Instance-level distillation
                                                   + Response-level distillation

Three distillation objectives:

1. BEV feature distillation: L2 loss between teacher and student BEV feature maps
2. Instance-level distillation: Align features within object bounding boxes (foreground-weighted)
3. Response-level distillation: Match detection head logits (soft labels)

Results: Camera-only student achieves ~80% of LiDAR teacher performance, enabling graceful degradation when LiDAR fails.

Airside relevance: Directly applicable to the camera fallback pipeline (see 30-autonomy-stack/perception/overview/camera-fallback-perception.md). A LiDAR teacher trained on airside data can distill to a camera student for degraded-mode operation.

6.3 Image-to-LiDAR and LiDAR-to-Image Transfer

Image-to-LiDAR transfer (leveraging rich 2D representations for 3D):

Method	Mechanism	Best for
SLidR	Superpixel contrastive	LiDAR segmentation
PPKT	Pixel-point contrastive	LiDAR segmentation
ScaLR	DINOv2 distillation	LiDAR seg + detection
GPC	Color prediction	LiDAR detection
Concerto	Joint 2D-3D SSL	All 3D tasks

LiDAR-to-Image transfer (leveraging 3D geometry for 2D):

Method	Mechanism	Best for
BEVDistill	BEV feature distillation	Camera-only detection
TinyBEV	Multi-modal to camera distillation	Edge camera deployment
Depth Anything	Metric depth for monocular 3D	Depth estimation

6.4 Temporal Pre-training

Several methods leverage temporal consistency in driving video + LiDAR sequences:

TREND (NeurIPS 2025):

• Pre-trains by predicting future LiDAR frames from current observations
• Temporal forecasting forces the model to understand dynamics and motion
• +1.77% mAP on ONCE, +2.11% on nuScenes over non-temporal pre-training

Temporal contrastive:

• Corresponding points across sequential LiDAR frames form positive pairs
• Forces the backbone to learn consistent representations across time
• Naturally handles ego-motion compensation

Airside relevance: Temporal pre-training is especially valuable for airside because:

• Slow speeds mean temporal context is rich (more overlap between frames)
• Understanding motion patterns of GSE, aircraft pushback, and personnel is critical
• TREND-style forecasting learns the dynamics of the apron environment from unlabeled sequences

6.5 Language-Guided Pre-training

Language-guided pre-training aligns visual/3D features with natural language descriptions:

Drive-CLIP (2024):

• Adapts CLIP for driving-specific text-image alignment
• Training data: driving images paired with natural language scene descriptions
• Enables zero-shot scene classification and anomaly detection

LiDAR-LLM (2024):

• Aligns LiDAR features directly with language models
• Enables natural language queries over 3D scenes
• "Find the pushback tug near gate B12" -- requires no class-specific training

Airside application: Language-guided features enable:

• NOTAM interpretation → perception mode adjustment ("reduced visibility at gate A5" → increase sensitivity)
• ATC instruction understanding → scene verification ("confirm baggage cart clear of aircraft")
• Anomaly description → safety logging ("detected unknown object near fuel truck")

---

7. Pre-training to Fine-tuning Pipeline

7.1 Full Fine-tuning vs. Linear Probing vs. LoRA/Adapters

Strategy	Trainable Params	Data Needed	Best For
Linear probing	<1% (head only)	50-200 frames	Quick evaluation, feature quality assessment
LoRA (r=16-32)	1-5%	500-2,000 frames	Resource-efficient domain adaptation
Adapter modules	5-10%	500-2,000 frames	More capacity than LoRA, less than full
Progressive unfreeze	10-100% staged	1,000-5,000 frames	Controlled adaptation with monitoring
Full fine-tuning	100%	2,000-10,000 frames	Sufficient data, large domain gap

Decision guide for airside:

Available labeled airside frames?
├── <200 → Linear probing (feature extraction only)
├── 200-500 → LoRA (r=16, query+value only)
├── 500-2,000 → LoRA (r=32, query+value+output projection)
├── 2,000-5,000 → Progressive unfreezing
└── >5,000 → Full fine-tuning with discriminative learning rates

7.2 Data Efficiency Curves

Based on published results across methods, the relationship between labeled data and performance follows a characteristic curve:

Performance (mAP)
    |
90% |                                    ......................
    |                              ......
85% |                        ......
    |                  ......
80% |            ......
    |      ......        ← Pre-trained backbone
75% | .....
    |.                   ← With SSL pre-training, 500 frames ≈ 5,000 from scratch
70% |
    |  ...
65% |..                  ← Without pre-training (from scratch)
    |
60% |.
    +--------+--------+--------+--------+--------+
    0       500     1,000    2,000    5,000   10,000
                 Labeled Training Frames

Concrete estimates for airside (pre-trained on road driving data):

Labeled Airside Frames	Expected mAP (common objects)	Without Pre-training
0 (zero-shot)	~5-15% (CLIP-based only)	0%
100	~40-55%	~15-25%
500	~65-75%	~40-50%
1,000	~75-85%	~55-65%
2,000	~80-88%	~65-75%
5,000	~85-92%	~75-85%
10,000	~88-94%	~82-90%

These estimates assume:

• Pre-training on diverse road LiDAR data (nuScenes + Waymo + KITTI)
• Continued SSL pre-training on unlabeled airside scans
• LoRA or progressive unfreezing for fine-tuning
• Common objects = baggage tractors, aircraft, belt loaders, ramp personnel

Rare objects (de-icing trucks, emergency vehicles, FOD) require targeted data collection regardless of pre-training strategy.

7.3 Transfer Across Domains: Road to Airside

The road-to-airside transfer pipeline involves four stages:

Stage 1: Road SSL pre-training (no labels)

• Pre-train on combined unlabeled road driving data
• Methods: GD-MAE, AD-L-JEPA, or ScaLR (if cameras available)
• Data: ~100K-500K unlabeled LiDAR frames (publicly available)
• Compute: ~24-96 GPU-hours (A100)

Stage 2: Road supervised pre-training (public labels)

• Fine-tune on labeled nuScenes + Waymo
• Learn general 3D detection/segmentation capabilities
• Data: ~40K labeled frames (nuScenes trainval)
• Compute: ~12-48 GPU-hours (A100)

Stage 3: Airside SSL continuation (no labels)

• Continue SSL pre-training on unlabeled airside LiDAR scans
• Reduces domain gap before expensive labeled fine-tuning
• Data: ~10K-50K unlabeled airside frames (just drive the vehicle)
• Compute: ~4-16 GPU-hours (A100)

Stage 4: Airside supervised fine-tuning (minimal labels)

• Fine-tune on small labeled airside dataset
• Use LoRA or progressive unfreezing
• Data: ~500-1,000 labeled airside frames
• Compute: ~2-8 GPU-hours (A100)

Total compute: ~42-168 GPU-hours (~$85-340 at cloud rates)

See 50-cloud-fleet/mlops/transfer-learning.md for detailed domain gap analysis and 70-operations-domains/deployment-playbooks/multi-airport-adaptation.md for the 8-week onboarding pipeline.

7.4 PointLoRA: Parameter-Efficient LiDAR Adaptation (CVPR 2025)

PointLoRA is the first LoRA method specifically designed for point cloud learning:

Key innovation: Multi-Scale Token Selection

• Standard LoRA captures global features via low-rank updates to attention
• Point clouds need local geometric understanding (surface normals, curvature)
• Multi-Scale Token Selection module captures local point cloud priors
• Selected tokens at various scales integrate via shared Prompt MLP

Results:

• 73.4% parameter reduction vs. full fine-tuning
• Greater resistance to catastrophic forgetting
• Achieves baseline accuracy with substantially fewer trainable parameters
• Suitable for resource-constrained deployment (Orin)

Airside application:

• Freeze pre-trained LiDAR backbone, add PointLoRA adapters (rank 16-32)
• Fine-tune on labeled airside data (target: 1,000 annotated frames)
• Maintain separate PointLoRA adapters per airport if needed
• Swap adapters at deployment time based on airport configuration

7.5 Progressive Unfreezing Strategies

Progressive unfreezing adapts the backbone gradually, reducing the risk of catastrophic forgetting:

Stage 1 (epochs 1-5):     Unfreeze detection head only → train with LR=1e-3
Stage 2 (epochs 6-10):    Unfreeze last backbone block → train with LR=1e-4
Stage 3 (epochs 11-15):   Unfreeze second-to-last block → train with LR=1e-5
Stage 4 (epochs 16-20):   Unfreeze all (optional) → train with LR=1e-6

Discriminative learning rates: Each layer group gets a different learning rate:

• Detection head: 1e-3 (learns new task quickly)
• Last backbone block: 1e-4 (adapts high-level features)
• Middle blocks: 1e-5 (adjusts intermediate representations)
• Early blocks: 1e-6 (preserves general low-level features)

Monitoring criteria:

• If validation mAP stalls at current stage → unfreeze next layer group
• If validation mAP degrades after unfreezing → dataset too small for this depth of adaptation → revert to LoRA
• If forgetting detected (road evaluation drops) → reduce LR or use EWC regularization

---

8. Pre-training for Specific Tasks

8.1 3D Object Detection Pre-training

Detection benefits most from pre-training methods that learn object-level representations:

Method	Pre-training Type	Detection Gain	Best Backbone
GD-MAE	Geometric MAE	+2.7 mAPH (Waymo)	VoxelNet, SECOND
ProposalContrast	Proposal-level contrastive	+2.3 APH (Waymo)	CenterPoint
GPC	Colorization	+7.5 AP (5% KITTI)	PV-RCNN, SECOND
AD-PT	Semi-supervised	Significant	Multiple
PSA-SSL	Pose/size-aware	+90% label savings	Multiple
AD-L-JEPA	JEPA BEV	Comparable to MAE	CenterPoint

Recommendation for airside: GD-MAE for initial pre-training (proven with CenterPoint), supplemented by GPC if camera-LiDAR pairs are available (95% label savings).

8.2 Semantic Segmentation Pre-training

Segmentation benefits most from dense, per-point pre-training:

Method	Pre-training Type	Seg Gain	Best Backbone
ScaLR	Image→LiDAR distill	67.8% mIoU (linear!)	WaffleIron
SLidR	Superpixel contrastive	38.8% mIoU (linear)	MinkUNet
ALSO	Surface reconstruction	+2 mIoU	MinkUNet, SPVCNN
Sonata	Self-distillation	SOTA	PTv3
Concerto	2D-3D joint SSL	+4.8% over 3D-only	PTv3

Recommendation for airside: ScaLR (if cameras available) or Sonata/Concerto (if LiDAR-only) for segmentation pre-training. The 18-class airside taxonomy (see 30-autonomy-stack/perception/overview/lidar-semantic-segmentation.md) requires dense per-point features.

Offline aggregated-map segmentation — a distinct pre-training target. The segmentation pre-training above is framed for the on-vehicle single-scan model. The offline pipeline that labels the aggregated (registered multi-scan) LiDAR map (30-autonomy-stack/perception/overview/aggregated-map-semantic-segmentation.md) is a second, complementary target with two pre-training advantages specific to it: (1) every survey drive produces a large unlabeled accumulated cloud — the ideal corpus for continued SSL, free except for compute; (2) continuing SSL on accumulated clouds rather than single scans closes the single-scan→accumulated density-distribution gap that a road-pretrained backbone otherwise inherits. Recommended curriculum: a generalist SSL backbone (Sonata-class) → continue SSL on unlabeled airside maps → LoRA fine-tune on a few hundred labeled tiles. Because the offline model has no Orin latency budget, it can also be the largest uncompressed backbone, and its labels back-project to single scans as auto-labels (the data flywheel).

8.3 BEV Perception Pre-training

BEV perception benefits from pre-training that operates in BEV space:

Method	Approach	BEV Task Gain
BEV-MAE	BEV masked reconstruction	+1.42 mAP (20% finetune)
AD-L-JEPA	BEV embedding prediction	Matches MAE, 2x faster
BEVDistill	LiDAR→Camera BEV distill	Camera achieves ~80% of LiDAR
UniPAD	Volume rendering	73.2 NDS, 79.4 mIoU

Recommendation for airside: BEV-MAE or AD-L-JEPA for BEV pre-training. Both align with the reference airside AV stack's BEV-based planning pipeline.

8.4 Occupancy Prediction Pre-training

Occupancy prediction is fundamental for safe navigation:

Method	Approach	Occupancy Result
Occupancy-MAE	Binary occupancy from masked voxels	50% label reduction
Voxel-MAE	Dual occupancy + point prediction	+1.4 APH
NOMAE	Multi-scale non-overlapping MAE	Beats supervised
UniPAD	Volume rendering	79.4 mIoU 3D seg

Recommendation for airside: Occupancy-MAE for direct occupancy pre-training. The binary occupancy objective is well-aligned with the safety-critical need to know "is this space occupied?" for navigation.

8.5 Motion Forecasting Pre-training

Motion forecasting benefits from temporal pre-training:

Method	Approach	Forecasting Benefit
TREND	Temporal LiDAR prediction	+1.77-2.11% mAP
MC-JEPA	Motion-content separation	Explicit motion features
V-JEPA 2-AC	Action-conditioned embedding	15x faster planning
Copilot4D	VQ-VAE + diffusion	65% Chamfer reduction

Recommendation for airside: TREND for LiDAR motion forecasting pre-training, combined with V-JEPA 2 concepts for video-based motion understanding when cameras are added.

---

9. Weakly-/Semi-Supervised Learning and Active Learning

9.1 Where These Methods Sit Relative to SSL

Self-supervised pre-training (Sections 2-8) attacks the labeled-data bottleneck by learning representations from unlabeled data, then fine-tuning on a small labeled set. It is one lever. There are three further label-efficiency levers that are not pre-training and are orthogonal to it -- they change what you label and how much of each frame you label, rather than how the backbone is initialized:

• Weakly-supervised learning -- train the final segmentation/detection model directly from cheap, sparse, or imprecise labels (scribbles, clicks, boxes, scene-level tags) instead of dense per-point annotation.
• Semi-supervised learning -- train on a small labeled set plus a large unlabeled set jointly, using teacher-student consistency and pseudo-labels.
• Active learning -- spend a fixed annotation budget optimally by selecting which frames/points are most informative to label next.

These combine multiplicatively with SSL and with the auto-label flywheel:

        Label-efficiency levers (combine them -- they are orthogonal)
        ┌────────────────────────────────────────────────────────────┐
        │ SSL pre-training      → good backbone init (Sections 2-8)   │
        │ Weak supervision      → cheaper labels per frame  (§9.2)    │
        │ Semi-supervised       → exploit the unlabeled majority(§9.3)│
        │ Active learning       → label the right frames first (§9.4) │
        │ Auto-label flywheel   → offline map model labels scans      │
        │                          (aggregated-map-segmentation.md)   │
        └────────────────────────────────────────────────────────────┘
                A practical airside stack uses ALL of them:
   SSL-pretrained backbone → AL picks frames → annotators place scribbles/clicks
        (weak labels) → semi-supervised teacher-student exploits the rest
        → offline aggregated-map model back-projects auto-labels

A useful mental model: SSL improves the prior (representation), weak/semi-supervision reduces the cost per labeled frame, and active learning improves the sample efficiency of the labeling budget. None of them substitutes for the others; the cheapest airside annotation pipeline stacks all four. The dense per-point baseline costs in Section 1.1 ($25-50/frame for 3D segmentation) are what every method below is measured against.

Scope note. This section covers 3D LiDAR segmentation specifically. Sub-section numbers under §9.2-§9.4 group methods by the type of supervision they consume. All quoted figures are paper-reported on the cited public benchmark; most cited works are preprints -- flagged inline as [preprint].

9.2 Weakly-Supervised 3D Segmentation

Weakly-supervised methods train the segmentation network from labels that are much cheaper to produce than dense per-point annotation. The spectrum runs from scribbles (a few percent of points) through clicks (one per object) down to scene-level tags (no spatial annotation at all). All trade a modest accuracy drop for a large annotation-cost drop.

Annotation-strength spectrum (cheapest at the bottom):

Dense per-point        100% of points labeled        — the expensive baseline
   │
Scribbles              ~5-8% of points               — ScribbleKITTI
   │
Scattered points       <0.1% of points               — ScatterNet
   │
One click per object   ~1 click / instance           — YoCo
   │
3D boxes               box geometry only             — Box2Mask, Sketchy-3DIS
   │
Scene-level tags       "these classes are present"   — DBGroup

9.2.1 Scribble Supervision -- ScribbleKITTI

Paper: "Scribble-Supervised LiDAR Semantic Segmentation" (CVPR 2022) -- arXiv:2203.08537

ScribbleKITTI re-annotated the SemanticKITTI training set with scribbles -- annotators draw quick free-form strokes over each class region rather than labeling every point. The result labels only ~8.06% of points yet the released benchmark is the standard anchor for all subsequent weak-label LiDAR work.

• The accompanying training pipeline (class-range-balanced self-training + a descriptor-based mean-teacher with pyramid local semantic-context features) reaches paper-reported up to 95.7% of fully-supervised mIoU while using scribble labels only.
• ScribbleKITTI is both a dataset (the scribble annotations) and a method (the self-training pipeline). For airside it is the reference point: it sets the expectation that ~8% point coverage retains ~95% of accuracy.

Airside relevance: HIGH as the anchor. Scribble annotation of apron scans is straightforward -- annotators stroke over "aircraft fuselage," "apron surface," "GSE," etc. -- and the ~8% coverage figure gives a concrete budgeting baseline for the airside taxonomy.

9.2.2 Scattered-Point Supervision -- ScatterNet

Paper: "Fully Sparse Scattered-Point Supervision for LiDAR Semantic Segmentation" / ScatterNet -- arXiv:2404.12861 [preprint]

ScatterNet pushes weak supervision to the extreme: annotators click a handful of scattered points on the LiDAR range image, and labels are then propagated across the rest of the cloud using vision foundation models for spatial grouping and optical flow for temporal propagation.

• Paper-reported: labels <0.02% of points and still recovers >95% of full-supervision performance.
• The range-image annotation interface is fast (clicking on a 2D projection), and foundation-model + optical-flow propagation does the heavy lifting -- no dense annotation anywhere in the loop.

Airside relevance: HIGH. The <0.02% label budget is roughly 400x cheaper per frame than dense annotation. Optical-flow propagation is well-suited to airside's slow speeds and high inter-frame overlap (the same property that makes temporal SSL attractive, Section 6.4).

9.2.3 One-Click Supervision -- YoCo ("You Only Click Once")

Paper: "YoCo: You Only Click Once for Weakly-Supervised 3D Semantic Segmentation" -- arXiv:2502.19698 [preprint]

YoCo asks the annotator for one click per object in the BEV plane, then expands each click into full 3D pseudo-labels for outdoor driving LiDAR via geometric and feature-space propagation seeded by foundation models.

• Paper-reported on Waymo: ~74.8 mIoU, i.e. ~93% of fully-supervised performance, from one click per object.
• Paper-reported: when the one-click pseudo-labels are used to fine-tune with only ~0.8% labeled points, the model matches full supervision.
• BEV-plane clicking is faster and less error-prone than 3D-viewport interaction.

Airside relevance: VERY HIGH. One BEV click per object maps cleanly onto the airside object set (one click per aircraft, per GSE unit, per ramp worker). The ~93%-of-full result at near-zero annotation cost makes YoCo a strong candidate for the airside annotation protocol -- see the cross-reference in Section 9.5.

9.2.4 Box- and Scene-Level Supervision

When even clicks are too expensive, box-level and scene-level supervision train instance/semantic segmentation from labels that carry no per-point information at all:

• Box2Mask -- "Box2Mask: Box-supervised Instance Segmentation via Level-set Evolution" / 3D variant (ECCV 2022) -- arXiv:2206.01203. Trains 3D instance segmentation from 3D bounding boxes only; paper-reported ~97% of fully-supervised mAP50 on ScanNet. Boxes are ~10-30x cheaper than dense masks, and the airside detection pipeline already produces 3D boxes -- those boxes can double as segmentation supervision.
• Sketchy-3DIS -- "Sketchy-3DIS: Weakly-Supervised 3D Instance Segmentation from Imprecise Boxes" (CVPR 2025) -- arXiv:2503.13639 [preprint]. Explicitly robust to imprecise/loose boxes: a sketchy-box-aware pseudo-labeler and an adaptive refinement stage tolerate annotation that is fast but sloppy. This matters operationally -- the cheapest boxes are the loosest ones, and Sketchy-3DIS lets the pipeline use them directly.
• DBGroup -- "DBGroup: Dual-Branch Grouping for Scene-Level Weakly-Supervised 3D Instance Segmentation" -- arXiv:2511.10003 [preprint]. Uses scene-level tags only ("this scan contains aircraft, GSE, and personnel") -- no spatial annotation -- with a dual-branch grouping mechanism to recover instances. The weakest supervision in this section; it sets the floor on annotation cost.

Airside relevance: Box-level supervision (Box2Mask, Sketchy-3DIS) is the pragmatic pick because the airside stack already annotates 3D boxes for detection -- segmentation comes "for free" from existing labels. Scene-level (DBGroup) is a research-grade fallback for bootstrapping when even box annotation is not yet underway.

9.2.5 Pseudo-Label Hardening

A recurring failure mode of all weak-supervision methods is noisy pseudo-labels: propagated or projected labels are wrong for some points, and naive training amplifies that noise. Two recent preprints harden the pseudo-label stage:

• Class-balanced top-k + geometry-filtered refinement -- arXiv:2510.17875 [preprint]. Selects only the most confident pseudo-labels per class (top-k, balanced across classes so rare classes are not starved) and filters them with geometric consistency checks before they enter training. Directly relevant to rare airside classes (de-icing trucks, FOD) that a naive confidence threshold would drop entirely.
• Noise-robust losses for 2D→3D projection error -- arXiv:2508.19909 [preprint]. Targets the specific noise introduced when 2D image labels are projected onto 3D points (calibration error, occlusion boundaries) and uses noise-robust loss functions so projection error does not corrupt the 3D model. Relevant whenever camera labels are lifted to LiDAR -- the SLidR/ScaLR cross-modal setting of Section 2.

These are not standalone label-efficiency methods; they are components that make the §9.2-§9.3 pipelines robust enough for safety-relevant deployment.

9.3 Semi-Supervised 3D Segmentation

Semi-supervised methods assume a small labeled set + large unlabeled set and train both jointly. The dominant paradigm is teacher-student consistency: a teacher produces pseudo-labels on unlabeled (or perturbed) scans, and the student is trained to match them. The art is in the perturbation (what consistency to enforce) and in the pseudo-label quality control.

9.3.1 LaserMix / LaserMix++

Paper: "LaserMix for Semi-Supervised LiDAR Semantic Segmentation" (CVPR 2023) -- arXiv:2207.00026

LaserMix exploits the spatial prior of laser-beam geometry: a LiDAR scan is organized into beams/inclination ranges, and the scene's semantics are strongly correlated with beam geometry. LaserMix mixes laser beams from two scans (one labeled, one unlabeled) and enforces that predictions are consistent before and after mixing. LaserMix++ extends this with stronger augmentation and a more capable mixing strategy.

• LaserMix is the most widely-cited semi-supervised LiDAR-segmentation baseline; subsequent methods (AIScene, spatio-temporal PLE) report against it.
• The laser-beam-mixing idea is sensor-geometry-aware and transfers directly to any rotating LiDAR -- including the RoboSense units in the reference airside stack.

Airside relevance: HIGH as a baseline and as a directly-usable method. The beam-geometry prior holds for airside LiDAR; LaserMix consistency can be applied to the abundant unlabeled apron scans with no annotation at all.

9.3.2 AIScene -- Scene-Affinity Teacher-Student

Paper: "AIScene: Affinity-Aware Semi-Supervised 3D Semantic Segmentation" (CVPR 2025) -- arXiv:2408.11280 [preprint]

AIScene introduces a scene-affinity signal into the teacher-student loop: rather than enforcing only point-wise consistency, it propagates supervision through learned affinities between points/regions, producing cleaner pseudo-labels in low-label regimes.

• Paper-reported: 61.2 mIoU on SemanticKITTI with only 1% of labels -- a strong result at an extreme label scarcity.

Airside relevance: HIGH. The 1%-label operating point is exactly the airside situation (a few hundred labeled tiles against tens of thousands of unlabeled scans). Scene-affinity propagation should help on the large coherent regions typical of aprons (runway/taxiway surface, aircraft bodies).

9.3.3 Spatio-Temporal Pseudo-Label Enhancement

Paper: "Spatio-Temporal Pseudo-Label Enhancement for Semi-Supervised LiDAR Segmentation" -- arXiv:2410.06893 [preprint]

This method generates higher-quality pseudo-labels by aggregating predictions across the temporal overlap of sequential scans: a point seen in several consecutive frames accumulates evidence, and inconsistent predictions are corrected by majority/consistency over the temporal window.

• Paper-reported gains: +5.6 mIoU on SemanticKITTI and +6.6 mIoU on nuScenes over the respective baselines.
• The mechanism is strongest when consecutive scans overlap heavily.

Airside relevance: VERY HIGH and structurally well-matched. Airside vehicles run fixed routes at low speed, so temporal scan overlap is unusually high -- exactly the condition this method exploits. The same fixed-route property also feeds the offline aggregated-map pipeline (Section 8.2). For an airside fleet, spatio-temporal PLE is among the most natural semi-supervised choices.

9.3.4 RePL -- Repairing Unreliable Pseudo-Labels

Paper: "RePL: Masked-Reconstruction Repair of Unreliable Pseudo-Labels for Semi-Supervised 3D Segmentation" -- arXiv:2604.06825 [preprint, very recent]

RePL treats unreliable pseudo-labels not as samples to discard but as samples to repair: a masked-reconstruction objective is used to in-fill and correct low-confidence pseudo-label regions before they are consumed by the student. This recovers supervisory signal that confidence-thresholding methods throw away -- important because the discarded low-confidence points are disproportionately rare/safety-critical classes.

Airside relevance: MEDIUM-HIGH. The repair-rather-than-discard philosophy directly addresses the airside concern that rare classes (FOD, emergency vehicles) sit in the low-confidence tail. Flagged as a very recent preprint -- treat results as provisional until independently reproduced.

9.4 Active Learning for 3D Segmentation

Active learning (AL) assumes the model itself chooses what to annotate next. Given a fixed budget (e.g. 6,000 points, or N frames), an acquisition function ranks candidates by expected informativeness; annotators label only the top picks; the model retrains; repeat. AL is orthogonal to weak/semi-supervision -- it decides which scans or points enter the (possibly weak) annotation pipeline.

9.4.1 SSDR-AL -- Superpoint-Based Acquisition

Paper: "SSDR-AL: Superpoint-guided Sampling and Diversity-aware Region Active Learning for 3D Semantic Segmentation" (ACM MM 2022) -- arXiv:2210.11782

SSDR-AL operates at the superpoint level (geometrically homogeneous point groups) rather than individual points, and combines uncertainty with a diversity term so the budget is not spent on redundant near-duplicate regions.

• Paper-reported: reaches 90% of fully-supervised performance while cutting annotation cost by ~63% (S3DIS) / ~24% (SemanticKITTI) relative to prior AL baselines.
• Superpoint-level annotation is also cheaper per click than point-level.

Airside relevance: HIGH. Superpoint grouping is robust on the large planar structures of aprons; the diversity term avoids wasting budget on the visually repetitive apron surface.

9.4.2 SELECT -- Submodular Selection for Class Imbalance

Paper: "SELECT: Submodular Active Learning for Class-Imbalanced 3D Semantic Segmentation" -- arXiv:2505.11516 [preprint]

SELECT formulates acquisition as submodular optimization explicitly targeting class imbalance -- it deliberately steers the budget toward rare classes that uncertainty-only acquisition under-samples.

• Paper-reported: +5 mIoU over prior SOTA at a 6,000-point annotation budget.

Airside relevance: VERY HIGH and arguably the most safety-relevant AL method here. The airside taxonomy is severely imbalanced -- apron surface and aircraft dominate, while FOD, de-icing trucks, marshallers, and emergency vehicles are rare but safety-critical. An acquisition function that intentionally over-samples rare classes is directly aligned with the airside safety case; uncertainty-only AL would systematically under-label exactly the classes that matter most.

9.4.3 LLM-Taxonomy Active Learning

Paper: "Hierarchical Active Learning for 3D Segmentation with LLM-Derived Class Taxonomies" -- arXiv:2505.18924 [preprint]

This method uses an LLM to derive a class hierarchy (e.g. vehicle → GSE → belt-loader) and then performs hierarchical-uncertainty acquisition: the model can be uncertain at a coarse level or a fine level, and the acquisition function exploits the hierarchy to ask for the most informative label granularity.

Airside relevance: MEDIUM-HIGH. The airside GSE taxonomy is naturally hierarchical (GSE → {baggage tractor, belt loader, pushback tug, catering truck, ...}); an LLM-derived hierarchy plus hierarchical acquisition fits the domain. The LLM step is offline and uncritical, so this carries no runtime cost.

9.4.4 Established Outdoor Baselines -- LiDAL and Discwise AL

• LiDAL -- "LiDAL: Inter-frame Uncertainty Based Active Learning for 3D LiDAR Semantic Segmentation" (ECCV 2022) -- arXiv:2211.05997. Acquires frames/regions where predictions are inconsistent across consecutive frames -- inter-frame inconsistency as an uncertainty proxy. A well-established outdoor-driving AL baseline.
• Discwise AL -- "Discwise Active Learning for LiDAR Semantic Segmentation" (WACV 2024) -- arXiv:2309.13276. Selects annotation regions as angular "discs" of a LiDAR scan, aligning the acquisition unit with the sensor's native sampling geometry and with practical annotation workflows.

Both are sensible, sensor-aware starting points and are the standard baselines any new airside AL component should be compared against.

9.4.5 Caveat -- Benchmark Active Learning Against Random

A 2025 meta-evaluation -- "On the Reliability of 3D Active Learning Benchmarks" / re-evaluation study -- arXiv:2512.05759 [preprint] -- warns that many published 3D AL methods do not consistently beat random sampling once evaluation protocols, label budgets, and seeds are controlled.

Practical rule for airside: treat any AL component as unproven until it has been benchmarked against random sampling at an equal annotation budget on airside data. AL adds engineering complexity and a retraining loop; if it does not beat random at equal budget, random is the correct choice. SELECT's explicit rare-class targeting is the most defensible AL bet for airside precisely because rare-class coverage -- not just average mIoU -- is the metric that matters, and random sampling under-covers rare classes by construction.

9.5 Label-Budget Summary and Airside Integration

The methods in this section, expressed as label budget → paper-reported result:

Method	Venue / Status	Supervision	Label Budget	Paper-Reported Result	Benchmark
Weakly-supervised
ScribbleKITTI	CVPR 2022	Scribbles	~8.06% of points	up to 95.7% of full mIoU	SemanticKITTI
ScatterNet	arXiv 2404.12861 [preprint]	Scattered range-image points	<0.02% of points	>95% of full performance	outdoor LiDAR
YoCo	arXiv 2502.19698 [preprint]	1 BEV click / object	1 click/object (~0.8% pts to fine-tune)	~74.8 mIoU (~93% of full); matches full at 0.8%	Waymo
Box2Mask (3D)	ECCV 2022	3D boxes only	box geometry	~97% of full mAP50	ScanNet
Sketchy-3DIS	CVPR 2025 [preprint]	Imprecise 3D boxes	loose box geometry	robust to imprecise boxes	indoor 3DIS
DBGroup	arXiv 2511.10003 [preprint]	Scene-level tags	tags only, no spatial labels	scene-level weak 3DIS	indoor 3DIS
Semi-supervised
LaserMix / LaserMix++	CVPR 2023	Small labeled + unlabeled	varies (1-50% labeled)	beam-mix consistency baseline	SemanticKITTI / nuScenes
AIScene	CVPR 2025 [preprint]	Small labeled + unlabeled	1% labels	61.2 mIoU	SemanticKITTI
Spatio-temporal PLE	arXiv 2410.06893 [preprint]	Small labeled + unlabeled	low-label regime	+5.6 SemKITTI / +6.6 nuScenes	SemanticKITTI / nuScenes
RePL	arXiv 2604.06825 [preprint]	Small labeled + unlabeled	low-label regime	repairs unreliable pseudo-labels	3D segmentation
Active learning
SSDR-AL	ACM MM 2022	AL budget (superpoints)	budget to reach 90% of full	-63% / -24% annotation cost	S3DIS / SemanticKITTI
SELECT	arXiv 2505.11516 [preprint]	AL budget (submodular)	6,000-point budget	+5 mIoU over prior SOTA	class-imbalanced 3D seg
LLM-Taxonomy AL	arXiv 2505.18924 [preprint]	AL budget (hierarchical)	hierarchical budget	hierarchical-uncertainty acquisition	3D segmentation
LiDAL	ECCV 2022	AL budget (inter-frame)	iterative budget	inter-frame inconsistency baseline	SemanticKITTI / nuScenes
Discwise AL	WACV 2024	AL budget (angular discs)	iterative budget	disc-wise outdoor baseline	SemanticKITTI / nuScenes

All figures are paper-reported on the cited public benchmark; results on airside data are unverified. Most cited works are preprints (flagged). Treat preprint numbers as provisional and re-benchmark on airside data before relying on them.

Recommended airside annotation protocol (combining all four levers):

1. Initialize the on-vehicle segmentation backbone via SSL pre-training (Sections 2-8) -- a good prior before any label is spent.
2. Select frames to annotate with active learning. Use SELECT (class-imbalanced/submodular) as the primary acquisition function so rare safety-critical classes (FOD, de-icing trucks, emergency vehicles) are deliberately over-sampled; benchmark it against random sampling at equal budget per the §9.4.5 caveat.
3. Annotate cheaply with weak supervision -- YoCo-style one-BEV-click-per-object for instances, or ScribbleKITTI-style scribbles for stuff classes (apron surface, grass, runway). If the detection pipeline already produces 3D boxes, reuse them as Box2Mask/Sketchy-3DIS segmentation supervision at no extra annotation cost.
4. Exploit the unlabeled majority with semi-supervised teacher-student training. Spatio-temporal PLE is the natural fit for fixed-route airside fleets (high inter-frame overlap); LaserMix is the simple sensor-geometry baseline; AIScene targets the extreme 1%-label regime.
5. Harden pseudo-labels with class-balanced top-k refinement (arXiv 2510.17875) and noise-robust 2D→3D losses (arXiv 2508.19909) so rare classes survive confidence filtering.

These levers complement -- they do not replace -- the SSL curriculum in Section 11 and the offline auto-label flywheel. The offline aggregated-map segmentation pipeline labels the registered multi-scan map once and back-projects those labels onto single scans as auto-labels; see aggregated-map-semantic-segmentation.md §5.4 (annotation protocol -- where weak/active labels enter the map-labeling loop) and §7.5. The cleanest division of labor for an airside program: spend the active-learning-selected, weakly-annotated budget on the offline map, let semi-supervised training and the back-projection flywheel propagate that investment to every single scan.

---

10. Training Infrastructure and Efficiency

9.1 Data Requirements

Pre-training Method	Unlabeled Data	Labeled Data	Total Storage
GD-MAE	50K-200K LiDAR frames	500-5K for fine-tuning	~100-500 GB
ScaLR	50K-200K LiDAR+camera pairs	500-5K for fine-tuning	~200-800 GB
AD-L-JEPA	50K-200K LiDAR frames	500-5K for fine-tuning	~100-500 GB
DINOv2 (use pretrained)	N/A (use Meta weights)	500-5K for adaptation	~50-200 GB
UniPAD	50K-200K multi-modal	500-5K for fine-tuning	~300-1000 GB

Available public driving data for pre-training:

Dataset	Frames	Size	LiDAR	Camera	Labels
nuScenes	400K	~300 GB	32-beam	6 cameras	40K keyframes
Waymo Open	12M	~2 TB	64-beam + 4 short	5 cameras	200K frames
KITTI	15K	~50 GB	64-beam	2 cameras	7.5K frames
ONCE	1M	~500 GB	40-beam	7 cameras	15K frames

For SSL pre-training, the full datasets can be used (labels are not needed). For fine-tuning, only the labeled subsets matter.

9.2 Compute Requirements

Pre-training Method	GPU-Hours (A100)	Estimated Cost	Wall Time (8xA100)
GD-MAE (nuScenes)	24-48	$50-100	3-6 hours
GD-MAE (nuScenes+Waymo)	96-192	$200-400	12-24 hours
ScaLR (nuScenes)	16-32	$35-70	2-4 hours
AD-L-JEPA (nuScenes)	12-24	$25-50	1.5-3 hours
DINOv2 pre-training	39,800	~$80K	Use pre-trained weights
UniPAD (nuScenes)	48-96	$100-200	6-12 hours
Full pipeline (Stage 1-4)	42-168	$85-340	5-21 hours

Key insight: AD-L-JEPA is the most compute-efficient method, requiring 1.9-2.7x fewer GPU-hours than equivalent MAE methods with comparable downstream quality.

9.3 Multi-GPU Training Strategies

DDP (Distributed Data Parallel):

• Standard for pre-training with batch size fitting in single-GPU memory
• Each GPU processes a mini-batch, gradients are all-reduced
• Linear scaling rule: LR scales with total batch size
• Recommended for: GD-MAE, AD-L-JEPA, ScaLR (batch fits in memory)

FSDP (Fully Sharded Data Parallel):

• Shards model parameters, gradients, and optimizer states across GPUs
• Enables training models larger than single-GPU memory
• 30-40% memory reduction vs. DDP at the cost of ~10-15% communication overhead
• Recommended for: UniPAD (large unified encoder), full DINOv2 fine-tuning

Gradient accumulation:

• Simulates larger batch sizes on fewer GPUs
• 4x accumulation steps = 4x effective batch size
• Use with: limited GPU count, large models

# DDP training pattern for SSL pre-training
import torch.distributed as dist
from torch.nn.parallel import DistributedDataParallel as DDP

def setup_ddp():
    dist.init_process_group("nccl")
    local_rank = int(os.environ["LOCAL_RANK"])
    torch.cuda.set_device(local_rank)
    return local_rank

def train_ssl_ddp(model, dataloader, optimizer, epochs, accum_steps=1):
    model = DDP(model, device_ids=[local_rank])
    scaler = torch.cuda.amp.GradScaler()  # Mixed precision
    
    for epoch in range(epochs):
        for i, batch in enumerate(dataloader):
            with torch.cuda.amp.autocast():
                loss = model(batch) / accum_steps
            
            scaler.scale(loss).backward()
            
            if (i + 1) % accum_steps == 0:
                scaler.unscale_(optimizer)
                torch.nn.utils.clip_grad_norm_(model.parameters(), 1.0)
                scaler.step(optimizer)
                scaler.update()
                optimizer.zero_grad()

9.4 Mixed Precision Training

Mixed precision (FP16/BF16 compute, FP32 master weights) is essential for efficient pre-training:

Precision Mode	Memory Savings	Speed Improvement	Accuracy Impact
FP32 (baseline)	0%	1x	Baseline
FP16 (AMP)	~40%	~1.5-2x	<0.1% loss
BF16 (if available)	~40%	~1.5-2x	No dynamic range issues
TF32 (A100 default)	0%	~1.3x	Negligible

Recommendation: Use BF16 on A100/H100 (no loss scaling needed) or FP16 with AMP on older GPUs. All major SSL methods support mixed precision out of the box.

9.5 Pre-train Cloud, Deploy Edge

The pre-training → deployment pipeline spans two very different compute environments:

Cloud (pre-training + fine-tuning):

• GPU: A100 (80GB) or H100 (80GB)
• Memory: 256-512 GB system RAM
• Storage: 2-10 TB NVMe
• Typical cost: $2-4/GPU-hour
• Duration: 5-168 GPU-hours depending on method

Edge (deployment):

• GPU: NVIDIA Orin (275 TOPS INT8)
• Memory: 32-64 GB shared
• Storage: 128-512 GB NVMe
• Power: 15-60W

The critical gap: Models pre-trained in FP32/FP16 on cloud must be compressed for Orin:

Cloud pre-training (FP32/FP16, A100)
        |
   Export to ONNX
        |
   TensorRT optimization
   ├── Layer fusion
   ├── Kernel auto-tuning
   └── Quantization (FP16 → INT8)
        |
   Edge deployment (INT8, Orin)

Typical compression pipeline results:

Stage	Latency (Orin)	mAP Loss
FP32 (unoptimized)	~200-500ms	Baseline
FP16 (TensorRT)	~50-100ms	<0.1%
INT8 (TensorRT PTQ)	~25-50ms	0.5-2%
INT8 (TensorRT QAT)	~25-50ms	0.1-0.5%

See 20-av-platform/compute/tensorrt-deployment-guide.md for detailed Orin optimization pipeline.

---

11. Airside Pre-training Strategy

11.1 Current State Assessment

reference airside AV stack capabilities:

• LiDAR-only perception (4-8 RoboSense, RANSAC segmentation)
• No learned perception models in production
• No cameras in current perception pipeline (planned Phase 2)
• CenterPoint-based detection in research/development

Airside data availability:

• Unlabeled LiDAR scans: Modest volume from existing operations (with safety operators)
• Labeled LiDAR data: Zero (no annotations exist)
• Airside camera data: Limited (no systematic collection)
• Public airside datasets: None (zero public 3D airside datasets exist)

11.2 Recommended Pre-training Curriculum

Phase 0: Preparation (Week 1-2)

• Select and download road driving datasets (nuScenes trainval: ~300GB)
• Set up training infrastructure (cloud GPU cluster, 4-8 A100s)
• Implement data pipeline for LiDAR pre-processing

Phase 1: Road SSL Pre-training (Week 3-4)

• Method: AD-L-JEPA (most compute-efficient) or GD-MAE (most proven)
• Data: nuScenes + Waymo unlabeled LiDAR frames (~200K frames)
• Compute: 24-48 GPU-hours (A100)
• Output: Pre-trained LiDAR backbone weights

Phase 2: Road Supervised Training (Week 5-6)

• Fine-tune pre-trained backbone on nuScenes labeled data
• Add CenterPoint detection head with nuScenes classes
• Validate on nuScenes val set (target: >60% NDS)
• Compute: 12-24 GPU-hours (A100)

Phase 3: Airside Data Collection (Week 7-12)

• Drive vehicles on apron during operations (with safety operator)
• Target: 10,000 unlabeled LiDAR sweeps minimum
• Record camera data if available (for future ScaLR pre-training)
• Store as ROS bags, convert to standard format (nuScenes-like)
• Storage: ~200GB for 10K sweeps with 4-8 LiDARs

Phase 4: Airside SSL Continuation (Week 13-14)

• Continue SSL pre-training on unlabeled airside scans
• Same method as Phase 1 (AD-L-JEPA or GD-MAE)
• Reduces domain gap before expensive labeled fine-tuning
• Compute: 4-16 GPU-hours (A100)

Phase 5: Airside Annotation (Week 13-18, parallel with Phase 4)

• Annotate 500-1,000 airside LiDAR frames with 3D bounding boxes
• 18-class airside taxonomy (aircraft, 5 GSE types, personnel, FOD, etc.)
• Use pre-labeling with road-trained model (auto-detect "vehicle-like" objects)
• Estimated cost: $15-30K (with pre-labeling reducing effort by ~40%)

Phase 6: Airside Fine-tuning (Week 19-20)

• Apply PointLoRA (rank 16-32) to pre-trained backbone
• Fine-tune on labeled airside data
• Validate on held-out airside test set
• Compute: 2-8 GPU-hours (A100)

Phase 7: Optimization and Deployment (Week 21-24)

• Export to ONNX → TensorRT INT8
• Benchmark on Orin (target: <50ms latency)
• Shadow mode testing alongside existing RANSAC pipeline
• A/B comparison of perception quality

11.3 Expected Data Needs

Data Type	Amount	Source	Cost
Unlabeled road LiDAR	200K frames	nuScenes + Waymo (public)	Free (download)
Labeled road data	40K frames	nuScenes trainval (public)	Free (download)
Unlabeled airside LiDAR	10K-50K frames	Own vehicles during operations	~$5-10K (operator time)
Labeled airside data	500-1,000 frames	Professional annotation	$15-30K

Total data cost: $20-40K (vs. $150-500K without pre-training)

11.4 Cost Estimation

Component	Cost	Timeline
Pre-training compute (Phase 1-2)	$100-300	2-4 weeks
Airside data collection (Phase 3)	$5-10K	4-6 weeks
Airside SSL compute (Phase 4)	$25-50	1-2 weeks
Annotation (Phase 5)	$15-30K	4-6 weeks
Fine-tuning compute (Phase 6)	$15-50	1-2 weeks
Optimization & testing (Phase 7)	$5-10K (engineer time)	2-4 weeks
Total	$25-50K	~24 weeks

Comparison without pre-training:

Component	Cost
Full airside annotation (5,000-10,000 frames)	$80-200K
Training from scratch compute	$500-1,000
Longer development cycle (more data needed)	$30-50K (engineer time)
Total	$110-250K

Savings from pre-training: $85-200K (60-80% reduction)

11.5 Multi-Airport Scaling

Pre-training enables efficient scaling to additional airports:

Airport	Method	Labeled Frames Needed	Cost
First airport	Full pipeline (Phase 0-7)	500-1,000	$25-50K
Same cluster (2nd-5th)	PointLoRA from first airport	200-500	$10-20K
Different cluster (6th+)	Airside SSL + PointLoRA	500-1,000	$15-30K

"Same cluster" = similar airport layout, similar GSE fleet, same region. "Different cluster" = different layout, different GSE types, different continent.

PointLoRA adapters can be swapped at deployment time based on airport configuration, maintaining a single base model with per-airport specialization.

See 70-operations-domains/deployment-playbooks/multi-airport-adaptation.md for the full 8-week per-airport onboarding pipeline.

---

12. Comprehensive Comparison Table

12.1 All Pre-training Methods Compared

Method	Venue	Year	Type	Modality	Data Efficiency Gain	Compute (GPU-hrs)	Best For
Contrastive
PointContrast	ECCV	2020	Point contrastive	LiDAR	~20%	16-32	Indoor 3D
DepthContrast	ICCV	2021	Multi-view contrastive	LiDAR	~20%	16-32	General 3D
PPKT	ICCV	2021	Pixel-to-point	LiDAR+Camera	~30-40%	8-16	LiDAR segmentation
SLidR	CVPR	2022	Superpixel contrastive	LiDAR+Camera	~40%	8-16	LiDAR segmentation
ProposalContrast	CVPR	2023	Proposal-level	LiDAR	~30-40%	24-48	3D detection
ScaLR	CVPR	2024	DINOv2→LiDAR distill	LiDAR+Camera	40-50%	16-32	LiDAR seg + detection
Masked Autoencoder
Point-MAE	ECCV	2022	Masked point recon	Point cloud	~20%	8-16	Object-level 3D
Voxel-MAE	2022	2022	Masked voxel recon	LiDAR	~30%	16-32	Outdoor LiDAR
Occupancy-MAE	IEEE TIV	2023	Occupancy prediction	LiDAR	50%	24-48	Detection + occupancy
GD-MAE	CVPR	2023	Geometric MAE	LiDAR	80%	24-48	3D detection
BEV-MAE	AAAI	2024	BEV occupancy	LiDAR	~60%	16-32	BEV perception
NOMAE	CVPR	2025	Multi-scale occ MAE	LiDAR	Beats supervised	24-48	All LiDAR tasks
PSA-SSL	CVPR	2025	Pose/size-aware	LiDAR	90%	24-48	Few-shot detection
JEPA / Embedding
V-JEPA	Meta	2024	Video embedding pred	Camera	N/A (video)	200+	Video understanding
V-JEPA 2	Meta	2025	Scaled video JEPA	Camera	N/A (video)	400+	Multi-task video
AD-L-JEPA	AAAI	2026	BEV embedding pred	LiDAR	~80%	12-24	LiDAR detection
MC-JEPA	2023	2023	Motion-content JEPA	Camera	N/A	100+	Motion understanding
Self-Distillation
DINOv2	Meta	2023	Multi-crop distill	Camera	N/A (general)	39,800	Use pre-trained weights
Sonata	CVPR	2025	Self-distill PTv3	LiDAR	Large	200+	3D backbone
Concerto	NeurIPS	2025	2D-3D joint SSL	LiDAR+Camera	+4.8% over 3D-only	300+	All 3D tasks
Multi-Modal
UniPAD	CVPR	2024	Volume rendering	LiDAR+Camera	50-60%	48-96	Unified perception
BEVDistill	NeurIPS	2023	LiDAR→Camera BEV	Multi-modal	N/A (distillation)	24-48	Camera-only deployment
GPC	ICLR	2024	Colorization	LiDAR+Camera	80-95%	24-48	Detection (with camera)
Other
ALSO	CVPR	2023	Surface reconstruction	LiDAR	~30-40%	8-16	LiDAR seg (lightweight)
AD-PT	NeurIPS	2023	Semi-supervised	LiDAR	60-70%	48-96	Cross-dataset transfer
TREND	NeurIPS	2025	Temporal forecasting	LiDAR	Significant	24-48	Temporal perception

12.2 Cross-Modal Contrastive Loss Implementation

import torch
import torch.nn as nn
import torch.nn.functional as F

class CrossModalContrastiveLoss(nn.Module):
    """
    Cross-modal contrastive loss for LiDAR-Camera pre-training.
    Implements the core loss used by SLidR, ScaLR, and similar methods.
    
    Pulls together features of LiDAR points and their projected
    camera pixels, pushes apart features of non-corresponding pairs.
    """
    def __init__(self, temperature=0.07, use_superpixels=True):
        super().__init__()
        self.temperature = temperature
        self.use_superpixels = use_superpixels
    
    def forward(
        self,
        lidar_features,      # [N, D] features for N LiDAR points
        image_features,       # [N, D] features for corresponding pixels/superpixels
        superpixel_ids=None,  # [N] superpixel assignment per point (optional)
    ):
        """
        Compute cross-modal contrastive loss.
        
        If use_superpixels=True, pools features per superpixel
        before computing contrastive loss (SLidR-style).
        Otherwise, uses point-pixel pairs directly (PPKT-style).
        """
        if self.use_superpixels and superpixel_ids is not None:
            # Pool features per superpixel
            unique_ids = superpixel_ids.unique()
            pooled_lidar = []
            pooled_image = []
            
            for sp_id in unique_ids:
                mask = superpixel_ids == sp_id
                pooled_lidar.append(lidar_features[mask].mean(dim=0))
                pooled_image.append(image_features[mask].mean(dim=0))
            
            lidar_features = torch.stack(pooled_lidar)   # [K, D]
            image_features = torch.stack(pooled_image)    # [K, D]
        
        # L2 normalize
        lidar_features = F.normalize(lidar_features, dim=-1)
        image_features = F.normalize(image_features, dim=-1)
        
        N = lidar_features.shape[0]
        
        # Similarity matrix: [N, N]
        # sim[i,j] = cosine similarity between lidar_i and image_j
        sim_matrix = torch.mm(
            lidar_features, image_features.t()
        ) / self.temperature
        
        # Labels: diagonal elements are positive pairs
        labels = torch.arange(N, device=sim_matrix.device)
        
        # Bidirectional contrastive loss
        loss_l2i = F.cross_entropy(sim_matrix, labels)      # LiDAR → Image
        loss_i2l = F.cross_entropy(sim_matrix.t(), labels)   # Image → LiDAR
        
        loss = (loss_l2i + loss_i2l) / 2.0
        
        return loss


class ScaLRPretrainer(nn.Module):
    """
    Simplified ScaLR-style pre-trainer.
    Distills DINOv2 features into a LiDAR backbone via contrastive learning.
    """
    def __init__(
        self,
        lidar_backbone,     # 3D backbone to pre-train (e.g., WaffleIron, MinkUNet)
        image_teacher,      # Frozen DINOv2 model
        projection_dim=256,
        temperature=0.07,
    ):
        super().__init__()
        self.lidar_backbone = lidar_backbone
        self.image_teacher = image_teacher
        
        # Freeze teacher
        for param in self.image_teacher.parameters():
            param.requires_grad = False
        
        # Projection heads
        lidar_feat_dim = lidar_backbone.output_dim  # e.g., 96 for WaffleIron
        image_feat_dim = image_teacher.embed_dim     # e.g., 1024 for DINOv2 ViT-L
        
        self.lidar_projector = nn.Sequential(
            nn.Linear(lidar_feat_dim, projection_dim),
            nn.BatchNorm1d(projection_dim),
            nn.ReLU(),
            nn.Linear(projection_dim, projection_dim),
        )
        
        self.image_projector = nn.Sequential(
            nn.Linear(image_feat_dim, projection_dim),
            nn.BatchNorm1d(projection_dim),
            nn.ReLU(),
            nn.Linear(projection_dim, projection_dim),
        )
        
        self.contrastive_loss = CrossModalContrastiveLoss(
            temperature=temperature,
            use_superpixels=True,
        )
    
    def forward(self, lidar_points, images, projection_matrix, superpixel_ids):
        """
        Forward pass for ScaLR-style pre-training.
        
        Args:
            lidar_points: [B, N, 3+] LiDAR points (xyz + features)
            images: [B, C, H, W] camera images
            projection_matrix: [B, 3, 4] LiDAR-to-camera projection
            superpixel_ids: [B, N] superpixel assignment per point
        
        Returns:
            loss: contrastive distillation loss
        """
        # Extract LiDAR features (trainable)
        lidar_feats = self.lidar_backbone(lidar_points)  # [B, N, D_lidar]
        
        # Extract image features (frozen teacher)
        with torch.no_grad():
            image_feats = self.image_teacher(images)      # [B, H/14*W/14, D_image]
        
        # Project LiDAR points to image coordinates
        # (simplified -- actual implementation handles batching)
        projected_image_feats = self._gather_image_features(
            image_feats, lidar_points, projection_matrix
        )
        
        # Project to shared space
        lidar_projected = self.lidar_projector(
            lidar_feats.reshape(-1, lidar_feats.shape[-1])
        )
        image_projected = self.image_projector(
            projected_image_feats.reshape(-1, projected_image_feats.shape[-1])
        )
        
        # Contrastive loss with superpixel pooling
        loss = self.contrastive_loss(
            lidar_projected,
            image_projected,
            superpixel_ids.reshape(-1),
        )
        
        return loss
    
    def _gather_image_features(self, image_feats, lidar_points, proj_matrix):
        """
        Gather image features at projected LiDAR point locations.
        Uses bilinear interpolation for sub-pixel accuracy.
        """
        # Project 3D points to 2D image coordinates
        # proj_matrix: [B, 3, 4], lidar_points: [B, N, 3]
        B, N, _ = lidar_points.shape
        
        ones = torch.ones(B, N, 1, device=lidar_points.device)
        points_homo = torch.cat([lidar_points[:, :, :3], ones], dim=-1)  # [B, N, 4]
        
        # Project: [B, N, 3] = [B, N, 4] @ [B, 4, 3]
        projected = torch.bmm(points_homo, proj_matrix.transpose(1, 2))
        
        # Normalize by depth
        depth = projected[:, :, 2:3].clamp(min=1e-5)
        uv = projected[:, :, :2] / depth  # [B, N, 2] pixel coordinates
        
        # Bilinear sample from image features
        # Reshape image_feats to spatial grid first
        H_feat = int(image_feats.shape[1] ** 0.5)
        W_feat = H_feat
        image_feats_spatial = image_feats.reshape(
            B, H_feat, W_feat, -1
        ).permute(0, 3, 1, 2)  # [B, D, H, W]
        
        # Normalize coordinates to [-1, 1] for grid_sample
        uv_normalized = uv.clone()
        uv_normalized[:, :, 0] = 2.0 * uv[:, :, 0] / (W_feat - 1) - 1.0
        uv_normalized[:, :, 1] = 2.0 * uv[:, :, 1] / (H_feat - 1) - 1.0
        
        grid = uv_normalized.unsqueeze(1)  # [B, 1, N, 2]
        sampled = F.grid_sample(
            image_feats_spatial, grid,
            mode='bilinear', align_corners=True
        )  # [B, D, 1, N]
        
        return sampled.squeeze(2).permute(0, 2, 1)  # [B, N, D]

12.3 Method Selection Guide

What modalities are available?
│
├── LiDAR only (current reference airside AV stack)
│   │
│   ├── Want best data efficiency?
│   │   ├── GD-MAE (80% label savings, proven)
│   │   ├── PSA-SSL (90% label savings, CVPR 2025)
│   │   └── AD-L-JEPA (80% savings, lowest compute)
│   │
│   ├── Want best features?
│   │   ├── Sonata + PTv3 (SOTA backbone)
│   │   └── NOMAE (beats supervised)
│   │
│   └── Want fastest pre-training?
│       ├── AD-L-JEPA (1.9-2.7x faster than MAE)
│       └── ALSO (lightweight, single-stream)
│
├── LiDAR + Camera (Phase 2 reference airside AV stack)
│   │
│   ├── Want best data efficiency?
│   │   ├── GPC (95% label savings)
│   │   └── ScaLR (67.8% mIoU frozen linear probe)
│   │
│   ├── Want unified pre-training?
│   │   ├── UniPAD (volume rendering, CVPR 2024)
│   │   └── Concerto (2D-3D joint SSL)
│   │
│   └── Want camera fallback?
│       └── BEVDistill (LiDAR→Camera distillation)
│
└── Camera only (degraded mode)
    │
    ├── DINOv2 + LoRA (r=32, adapter-mediated)
    ├── BEVFormer + DINOv2 backbone
    └── Depth Anything V2 (DINOv2-based depth)

---

13. Key Findings Summary

#	Finding	Source	Relevance
1	Pre-training saves 50-95% of labeled data depending on method. GD-MAE achieves 80%, GPC achieves 95%, PSA-SSL achieves 90%. For airside, this reduces annotation cost from $150-500K to $15-30K.	GD-MAE (CVPR 2023), GPC (ICLR 2024), PSA-SSL (CVPR 2025)	CRITICAL
2	AD-L-JEPA is the most compute-efficient LiDAR pre-training method: 1.9-2.7x fewer GPU-hours and 2.8-4x less memory than Occupancy-MAE with comparable downstream quality.	AD-L-JEPA (AAAI 2026)	HIGH
3	ScaLR (CVPR 2024) provides the best LiDAR self-supervised features via DINOv2-to-LiDAR distillation: 67.8% mIoU on nuScenes with linear probing alone (frozen backbone). Requires camera-LiDAR pairs.	ScaLR (CVPR 2024)	HIGH
4	DINOv2 as drop-in backbone replacement fails completely (0% mAP). Must use adapter-mediated integration. LoRA rank 32 on query+value projections matches ResNet-101 with 37x fewer learnable parameters.	Robust BEV Seg (2024), DINOv2+LSS (2025)	HIGH
5	JEPA outperforms MAE for semantic tasks while MAE is better for reconstruction tasks. For detection/segmentation (primary airside needs), JEPA is recommended. For occupancy prediction, MAE is preferred.	AD-L-JEPA (AAAI 2026), V-JEPA (2024)	MEDIUM
6	The road→airside transfer pipeline costs $25-50K (24-week timeline) vs. $110-250K without pre-training. Pre-training reduces both annotation cost and development time by 60-80%.	Analysis in this document	CRITICAL
7	Cross-modal pre-training (image→LiDAR) consistently outperforms single-modal SSL. SLidR > DepthContrast > PointContrast. ScaLR >> SLidR due to DINOv2 teacher. Adding cameras to reference airside AV stack unlocks the strongest SSL methods.	SLidR (CVPR 2022), ScaLR (CVPR 2024)	HIGH
8	500-1,000 labeled airside frames are sufficient for competitive detection performance when combined with SSL pre-training on road data and unlabeled airside scans. Without pre-training, 5,000-10,000 frames are needed.	GD-MAE efficiency curves, airside estimates	CRITICAL
9	Multi-airport scaling is efficient: First airport requires 500-1,000 labeled frames ($25-50K). Same-cluster airports need only 200-500 frames ($10-20K) via PointLoRA adapter swapping.	PointLoRA (CVPR 2025), multi-airport analysis	HIGH
10	The pre-train cloud → deploy edge pipeline is well-established. TensorRT INT8 quantization of pre-trained models loses only 0.5-2% mAP while achieving <50ms latency on Orin. No barriers to deploying SSL-pretrained models on edge hardware.	TensorRT benchmarks, Orin deployment experience	MEDIUM
11	No airside-specific pre-training or benchmark exists. This is simultaneously a challenge (no ready-made solution) and an opportunity (creating the airside benchmark would establish reference airside AV stack as the reference).	Survey of all existing methods	HIGH
12	NOMAE (CVPR 2025) beats fully supervised baselines with SSL pre-training alone, suggesting that for data-scarce domains like airside, SSL is not just a cost-saving measure but may produce superior models.	NOMAE (CVPR 2025)	HIGH
13	Temporal pre-training (TREND) adds +1.77-2.11% mAP beyond spatial-only pre-training by learning dynamics from sequential LiDAR frames. Particularly valuable for airside where motion patterns (pushback, GSE traversal) are distinctive.	TREND (NeurIPS 2025)	MEDIUM
14	Language-guided features enable zero-shot airside understanding via CLIP alignment. Open-vocabulary 3D methods (ULIP-2, OpenScene, Concerto) allow querying scenes with natural language without explicit class definitions.	ULIP-2 (CVPR 2024), OpenScene (CVPR 2023)	MEDIUM
15	The complete SSL landscape is converging: Contrastive + MAE + JEPA hybrid methods (like Concerto) outperform any single approach by 4.8%+. The next generation of methods will likely combine all three paradigms.	Concerto (NeurIPS 2025)	MEDIUM
16	Weak, semi-supervised, and active learning are orthogonal label-efficiency levers to SSL -- stack them. Weak labels cut cost per frame (ScribbleKITTI ~95.7% of full mIoU at ~8% points; ScatterNet >95% at <0.02%; YoCo ~93% at one BEV click/object), semi-supervision exploits the unlabeled majority (AIScene 61.2 mIoU at 1% labels; spatio-temporal PLE +5.6/+6.6 mIoU -- ideal for fixed-route airside fleets), and active learning targets the budget (SELECT +5 mIoU via class-imbalance-aware acquisition).	ScribbleKITTI (CVPR 2022), ScatterNet, YoCo, AIScene, SELECT (preprints)	HIGH
17	For airside, SELECT's class-imbalance-aware active learning is the most safety-relevant AL choice -- it deliberately over-samples rare safety-critical classes (FOD, de-icing trucks, emergency vehicles) that uncertainty-only AL under-labels. But a 2025 meta-evaluation warns many 3D AL methods do not beat random sampling; benchmark any AL component against random at equal budget.	SELECT (arXiv 2505.11516), AL meta-evaluation (arXiv 2512.05759)	MEDIUM

---

14. References

Contrastive Learning

• PointContrast (Xie et al., ECCV 2020) -- Point-level contrastive pre-training for 3D understanding
• DepthContrast (Zhang et al., ICCV 2021) -- Self-supervised 3D pre-training on any point cloud
• PPKT (Liu et al., ICCV 2021) -- Image-to-point knowledge transfer
• SLidR (Sautier et al., CVPR 2022) -- Image-to-LiDAR distillation via superpixels
• ProposalContrast (Yin et al., CVPR 2023) -- Proposal-level contrastive for 3D detection
• ScaLR (Sanchez et al., CVPR 2024) -- Three pillars for LiDAR foundation model distillation

Masked Autoencoders

• MAE (He et al., CVPR 2022) -- Masked autoencoders are scalable vision learners
• BEiT (Bao et al., ICLR 2022) -- BERT pre-training of image transformers
• Point-MAE (Pang et al., ECCV 2022) -- Masked autoencoders for point cloud SSL
• Point-BERT (Yu et al., CVPR 2022) -- Pre-training 3D point cloud transformers
• Voxel-MAE (Hess et al., 2022) -- Masked autoencoders for large-scale point clouds
• GD-MAE (Yang et al., CVPR 2023) -- Generative decoder for LiDAR MAE pre-training
• Occupancy-MAE (Min et al., IEEE TIV 2023) -- Large-scale LiDAR SSL via occupancy
• BEV-MAE (Ren et al., AAAI 2024) -- BEV masked autoencoders for point cloud pre-training
• NOMAE (CVPR 2025) -- Non-overlapping masked autoencoders for 3D

JEPA and Embedding Prediction

• LeCun JEPA Position Paper (Yann LeCun, 2022) -- A path towards autonomous machine intelligence
• V-JEPA (Bardes et al., Meta 2024) -- Video joint embedding predictive architecture
• V-JEPA 2 (Assran et al., Meta 2025) -- Self-supervised video models enable understanding, prediction, and planning
• AD-L-JEPA (Zhu et al., AAAI 2026) -- Self-supervised spatial world model for autonomous driving
• MC-JEPA (Bardes et al., 2023) -- Motion-content joint embedding predictive architecture
• LeJEPA (Balestriero et al., Meta 2025) -- Provable JEPA objective

DINO/DINOv2

• DINOv2 (Oquab et al., Meta 2023) -- Learning robust visual features without supervision
• DINO Pre-training for AD (Juneja et al., 2024) -- DINO pre-training for vision-based end-to-end autonomous driving
• Robust BEV Segmentation with DINOv2 (2024) -- DINOv2 + LoRA for robust BEV perception
• DINOv2+LSS (2025) -- Revisiting BEV with frozen foundation models
• RCDINO (2025) -- Radar-camera fusion with DINOv2 semantic features
• DualViewDistill (2025) -- Foundation model guided BEV maps

Multi-Modal Pre-training

• UniPAD (Yang et al., CVPR 2024) -- Universal pre-training paradigm for autonomous driving
• BEVDistill (NeurIPS 2023) -- Cross-modal BEV knowledge distillation
• GPC (Pan et al., ICLR 2024) -- Pre-training 3D detectors through colorization
• Sonata (CVPR 2025) -- Self-supervised pre-training for 3D understanding
• Concerto (NeurIPS 2025) -- 2D-3D joint self-supervised learning
• TREND (NeurIPS 2025) -- Temporal pre-training for LiDAR perception
• ALSO (Boulch et al., CVPR 2023) -- Automotive LiDAR self-supervision by occupancy estimation

Point Cloud Adaptation

• PointLoRA (CVPR 2025) -- Low-rank adaptation for point cloud learning
• AD-PT (Yuan et al., NeurIPS 2023) -- Autonomous driving pre-training with large-scale point cloud dataset
• PSA-SSL (CVPR 2025) -- Pose/size-aware self-supervised learning
• PointGPT (Chen et al., NeurIPS 2023) -- Auto-regressive generative pre-training from point clouds

Weakly-Supervised 3D Segmentation

• ScribbleKITTI (Unal et al., CVPR 2022) -- Scribble-supervised LiDAR semantic segmentation
• ScatterNet (2024, preprint) -- Scattered-point supervision; foundation-model + optical-flow label propagation, <0.02% labeled points
• YoCo (2025, preprint) -- "You Only Click Once" -- one BEV click per object for weakly-supervised 3D segmentation
• Box2Mask (Li et al., ECCV 2022) -- Box-supervised instance segmentation via level-set evolution
• Sketchy-3DIS (CVPR 2025, preprint) -- Weakly-supervised 3D instance segmentation robust to imprecise boxes
• DBGroup (2025, preprint) -- Dual-branch grouping for scene-level weakly-supervised 3D instance segmentation
• Class-balanced top-k pseudo-label refinement (2025, preprint) -- Class-balanced top-k + geometry-filtered pseudo-label hardening
• Noise-robust losses for 2D→3D projection error (2025, preprint) -- Noise-robust training under 2D-to-3D label projection error

Semi-Supervised 3D Segmentation

• LaserMix (Kong et al., CVPR 2023) -- Laser-beam-mixing consistency for semi-supervised LiDAR segmentation (LaserMix / LaserMix++)
• AIScene (CVPR 2025, preprint) -- Affinity-aware semi-supervised 3D segmentation; 61.2 mIoU at 1% labels
• Spatio-Temporal Pseudo-Label Enhancement (2024, preprint) -- Temporal-overlap pseudo-labels; +5.6/+6.6 mIoU
• RePL (2026, preprint) -- Masked-reconstruction repair of unreliable pseudo-labels

Active Learning for 3D Segmentation

• SSDR-AL (Shao et al., ACM MM 2022) -- Superpoint-guided diversity-aware region active learning
• SELECT (2025, preprint) -- Submodular active learning for class-imbalanced 3D segmentation
• LLM-Taxonomy Active Learning (2025, preprint) -- Hierarchical active learning with LLM-derived class taxonomies
• LiDAL (Hu et al., ECCV 2022) -- Inter-frame uncertainty based active learning for 3D LiDAR segmentation
• Discwise Active Learning (WACV 2024) -- Disc-wise region selection for LiDAR semantic segmentation active learning
• 3D Active Learning Meta-Evaluation (2025, preprint) -- Re-evaluation showing many 3D AL methods do not consistently beat random sampling

Language-Aligned 3D

• CLIP (Radford et al., ICML 2021) -- Learning transferable visual models from natural language supervision
• OpenCLIP -- Open-source CLIP training
• ULIP-2 (CVPR 2024) -- Scalable multimodal pre-training for 3D understanding
• OpenScene (CVPR 2023) -- 3D scene understanding with open vocabularies

Infrastructure

• PonderV2 (2024) -- Universal pre-training paradigm for 3D foundation models
• PTv3 (CVPR 2024) -- Point Transformer V3
• FlatFormer (CVPR 2023) -- Flattened window attention for efficient point cloud transformer

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Related Documents

• 30-autonomy-stack/perception/overview/dinov2-foundation-models-driving.md -- Detailed DINOv2 architecture, BEV integration, LoRA results
• 30-autonomy-stack/perception/overview/lidar-foundation-models.md -- Comprehensive LiDAR foundation model survey (PTv3, Sonata, ScaLR, GPC)
• 30-autonomy-stack/world-models/tokenized-and-jepa.md -- JEPA paradigm, VQ-VAE tokenization, AD-L-JEPA details
• 50-cloud-fleet/mlops/transfer-learning.md -- Road→airside domain gap analysis, transfer strategies, UDA methods
• 30-autonomy-stack/perception/overview/lidar-semantic-segmentation.md -- 18-class airside taxonomy, FlatFormer, ALPINE
• 30-autonomy-stack/perception/overview/aggregated-map-semantic-segmentation.md -- Offline aggregated-map segmentation; unlabeled maps as an SSL corpus, the density-gap closure
• 30-autonomy-stack/perception/overview/model-compression-edge-deployment.md -- TensorRT optimization, INT8 quantization, multi-model orchestration
• 70-operations-domains/deployment-playbooks/multi-airport-adaptation.md -- 8-week onboarding, PointLoRA per-airport, scaling economics
• 20-av-platform/compute/tensorrt-deployment-guide.md -- Orin deployment pipeline, INT8 calibration
• 30-autonomy-stack/perception/overview/camera-fallback-perception.md -- Degraded mode with camera-only, BEVDistill