Here BPR is defined as the ratio of the number of ground-truth boxes a detector can recall at the most divided by all ground-truth boxes.
For FCOS, at the first glance one may think that the BPR can be much lower than anchor-based detectors because it is impossible to recall an object which no location on the final feature maps encodes due to a large stride. Here, we empirically show that even with a large stride, FCN-based FCOS is still able to produce a good BPR, and it can even better than the BPR of the anchor-based detector RetinaNet [15] in the official implementation Detectron [7] (refer to Table 1).
class FCOSModule