  We present Sparse R-CNN, a purely sparse method for object detection in
images. Existing works on object detection heavily rely on dense object
candidates, such as $k$ anchor boxes pre-defined on all grids of image feature
map of size $H\times W$. In our method, however, a fixed sparse set of learned
object proposals, total length of $N$, are provided to object recognition head
to perform classification and location. By eliminating $HWk$ (up to hundreds of
thousands) hand-designed object candidates to $N$ (e.g. 100) learnable
proposals, Sparse R-CNN completely avoids all efforts related to object
candidates design and many-to-one label assignment. More importantly, final
predictions are directly output without non-maximum suppression post-procedure.
Sparse R-CNN demonstrates accuracy, run-time and training convergence
performance on par with the well-established detector baselines on the
challenging COCO dataset, e.g., achieving 45.0 AP in standard $3\times$
training schedule and running at 22 fps using ResNet-50 FPN model. We hope our
work could inspire re-thinking the convention of dense prior in object
detectors. The code is available at: https://github.com/PeizeSun/SparseR-CNN.
