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TSP: Rethinking Transformer-based Set Prediction for Object Detection

November 2020

tl;dr: Train DETR faster and better with sparse attention and guided Hungarian matching.

Overall impression

The paper digs into the reasons why DETR is so hard to train, primarily the issues with Hungarian loss and the Transformer cross attention mechanism. The paper proposed two improved version of DETR based on existing solutions, TSP-FCOS (improved FCOS) and TSP-RCNN (improved Faster RCNN).

This work basically borrows the best practice in modern object detector (FPN, FCOS and Faster-RCNN) and replaces the dense prior heads with DETR encoder, and uses set prediction loss.

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

A standard 36 epochs (3x schedule) can yield a SOTA object detector.

Object detection is essentially a set prediction problem, as the ordering of the predicted objects is not required. Most modern object detectors uses a detect-and-merge strategy, and makes predictions on a set of dense priors. The dense priors makes NMS necessary. The detection model is trained agnostically wrt the merging step, so the optimization is not end-to-end and arguably sub-optimal.

DETR removes the handcrafted parts such as dense prior design, many-to-one label assignment problem and NMS postprocessing.

DETR removes the necessity of NMS as self-attention component can learn to remove duplicated detection. The Hungarian loss encourages one target per object in the bipartite matching.

Key ideas

  • Cross attention in decoder impedes convergence
    • Transformer attention maps are nearly uniform in the initialization stage but becomes increasingly more sparse during the training process toward convergence.
    • Sparsity of cross attention consistently increases even beyond 100 training epochs. It is super hard to train.
    • Encoder self-attention is essentially identical to the self-attention in a non-autoregressive decoder, so a set prediction scheme is still feasible for encoder-only DETR. --> Just that no query is needed anymore, and all predictions are wrt the input feature point or region of interest (FOI or ROI).
  • New bipartite matching scheme
    • Instability in bipartite matching in original DETR affects convergence, but a matching distillation experiment reveals that this is not the bottleneck after 15 epochs
    • Faster set prediction loss: borrows GT assignment rules from FCOS and Faster RCNN. For TSP-FCOS, A feature point can be assigned to a GT only when the point is in the bounding box of the object and in the proper feature pyramid level.
  • TSP-FCOS
    • FoI classifier: the computation complexity of transformer's self-attention module scales quadratically wrt the sequence length, quartically with image size. A binary feature point classifier selects a limited portion of features (FoI).
    • FoI and their positional embedding feed into the encoder. Self-attention module is used to aggregate information of different FoIs.

Technical details

  • Sparsity of attention map is measured in terms of negative entropy
  • The input of encoder head: 700 top-scoring FoI from FoI classifier are selected in TSP-FCOS, and 700 top-scoring RoI from RPN are selected in TSP-RCNN.
  • During training, 70% of inputs to encoder is randomly dropped to improve robustness of set prediction.

Notes

  • Questions and notes on how to improve/revise the current work