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BEVFormer: Learning Bird’s-Eye-View Representation from Multi-Camera Images via Spatiotemporal Transformers

March 2022

tl;dr: Spatiotemporal transformer for BEV perception of both dynamic and static elements.

Overall impression

BEVFormer exploits both spatial and temporal information by interacting with spatial and temporal space through predefined grid-shaped BEV queries. This includes a spatial cross-attention and a temporal self-attention. --> actually also a type of cross-attention.

The paper believes the previous methods based on depth prediction (Lift Splat Shoot, CaDDN, PseudoLidar) are subject to compounding errors, and thus favors direct method.

It significantly outperforms previous SOTA DETR3D by 9 points in NDS score on nuScenes dataset.

The introduction of temporal module helps in 3 directions: 1) more accurate velocity estimation 2) more stable location and orientation 3) higher recall on heavily occluded objects.

Key ideas

  • The BEV perception of BEVFormer includes 3D object detection and map segmentation.
  • Spatiotemporal transformer encoder that projects multicam video input to BEV representation.
  • Deformable DETR is adppted as efficient backbone and its formulation is a bit diff from global attention. DeformAttn(Q, K=p, V=input), where feature V at p is bilaterally sampled.
  • Three tailored designs of BEVFormer: BEV queries, spatial cross-attention and temporal self-attention.
  • BEV queries
    • HxWxC = 200x200x256, s grid size = 0.512 m. This corresponds to a grid = 50 m to all sides in nuScenes. The query responsible for located at p=(x, y) is $Q_p \in R^{1\times C}$.
  • Spatial cross-attention (SCA)
    • For each pillar in the BEV grid, 4 points are sampled along diff height (every 2 m from -5 m to 3 m), and projected to diff images to form reference points.
    • $SCA(Q_p, F_t)$ can be written as global attention $\text{Attn}(Q=Q, K=F_t, V=F_t)$.
    • BEVFormer used Deformable attention, and therefore $\sum_i\sum_j^{N_{ref}}\text{DeformAttn}(Q_p, P(p, i, j), F_t^i)$. i iterates through all camera view, and j iterates through N-ref points in deformable attention. P(p, i, j) is in F_t space.
  • Temporal self-attention
    • Align $B_{t-1}$ to Q at current timestamp with ego-motion, then denote as $B'_{t-1}$
    • $TSA(Q_p, {Q, B'{t-1}})$ can be written as global attention $\text{Attn}(Q=Q, K=Q, V=Q)+ \text{Attn}(Q=Q, K=B'{t-1}, V=B'_{t-1})$. Essentially it is a cross-attention of BEV query of B_t-1 feature, but also enforced with a self-attention of Q on itself.
    • BEVFormer used Deformable attention, and therefore $\text{DeformAttn}(Q_p, p, Q) + \text{DeformAttn}(Q_p, p, B'_{t-1})$. p is in BEV space, the same space as Q and B'_t-1.

Technical details

  • This is the first work that leverages the temporal information to predict velocity, which got the SOTA results among all vision-based method. It reduced the velocity estimation error by almost 50%. It is unbelievable that previous work used single images to predict velocity.
  • Map segmentation uses Panoptic SegFormer head.
  • Temporal training: 3+1 for the past 2 seconds. All but the last step require no gradient. --> This can help speed up training. Maybe we can do the same for convLSTM.
  • There are 6 encoder layers, the output of each layer is used as the updated queries as the next layer. B_t-1 remain the same for each encoder layer and require no gradient.
  • For nuscenes, HxWxC = 200x200x256, s grid size = 0.512 m, spanning 50 meters to all 4 directions. For waymo open dataset, HxWxC = 300x220x256, s grid size = 0.5 m, spanning almost 75 meters in all direction except the rear.
  • Negative transfer in joint training: when two tasks regress each other in joint training.

Notes

  • github (code open sourced in 2022/06)