The paper reports that "having an RoI pooling layer that is differentiable w.r.t the box coordinates is a nontrivial problem" and refers to "ROI Warping" (crops and resizes the features to a fixed shape) that makes it fully differentiable w.r.t the box coordinates.
I can't figure out why RoI pooling layer is not differentiable and ROI Warping is?
