Learning-AI

Sparse R-CNN: End-to-End Object Detection with Learnable Proposals

November 2020

tl;dr: Sparse proposal and iterative refinement for a two-stage end-to-end object detector.

Overall impression

This paper rethinks the necessity of dense priors (either anchor boxes or reference points) in object detection, very similar to TSP. Sparse RCNN uses a number of sparse proposals (N « HWk dense priors) for object detection.

There are several papers on improving the training speed of DETR.

• Deformable DETR: sparse attention
• TSP: sparse attention
• Sparse RCNN: sparse proposal and iterative refinement

The iterative head design is quite inefficient in capturing the context and relationship with other parts of the image, and thus needs quite a few iterations (~6 cascaded stage). In comparison, the sparse cross attention in Deformable DETR and TSP may be a better way to go.

The authors also wrote OneNet which is a single-stage easy-to-deploy end to end object detection model.

Key ideas

• Sparse-in and sparse out
  • DETR uses sparse set of object queries to interact with global (dense) image feature. It is also dense-to-sparse.
  • Sparse RCNN proposes both sparse proposals and sparse features
    • Sparse RCNN uses only 100 proposals, same as DETR. 300 proposal adds very little inference cost due to light design of dynamic head.
  • Sparse Proposal feature: High dim (256-d) latent features encoding pose and shape of instances. Proposal feature generates a series of customized parameters for its exclusive object recognition head via dynamic conv.
• Three Input:
  • Image
  • A set of learned proposal boxes (fixed, and dataset independent, and may not be symmetric or shift-equivariant) –> This looks to be an inefficiency that can be optimized. Maybe something like the FoI in TSP-FCOS.
  • A set of learnable proposal features. The initial values of the proposal feature is actually not that important, given the auto-regressive iterative refinement scheme (similar to IterDet).
• Dynamic instance interactive head –> More like a trick and an afterthought rather than intuition inspired design.
  • The interaction between Proposal feature and RoI features is modeled by a dynamic convolution. The parameters are generate by the proposal features.
  • Newly generated object boxes and object features of the next stage in iterative process. Features need to be RoI aligned again, similar to Cascade RCNN CVPR 2018. This only introduces a marginal computational overhead as the dynamic conv is very light.

Technical details

• Set prediction loss exactly the same as DETR.
• Ablation studies
• The paper has a very clear evolution path of model, in Table 2, 3 and 4. According to the author’s reply on Zhihu知乎.

• 18.5到20.5：加上cascade r-cnn那样的结构
• 20.5到32.2: cascade结构 + 上一个stage的obj feature concat到下一个stage的obj feature
• 32.2到37.2: cascade结构 + 上一个stage的obj feature先self-att，再concat到下一个stage的obj feature
• 37.2到42.3: cascade结构 + 上一个stage的obj feature先self-att，在作为proposal feature与下一个stage的RoI做instace interaction

• Initialization of proposal box does not matter much
• Number of proposals from 100 to 500 improves performance, but also requires longer training time
• Stage increase saturates around stage 6. The iterative heads gradually refines box and removes duplicate ones.

Notes

• Code on github
• The Sparse RCNN paper is not very well written. Luckily the first author clarified most of the important details in Zhihu知乎.
• The proposal can be seen as an averaged GT distribution. This should be improved to be data dependent, and the authors are working on a v2 of Sparse RCNN. The authors also argue that a reasonable statistic can already be qualified candidates. Maybe this is similar to the FoI classifier in TSP-FCOS.