Fix inconsistent NMS implementation between CPU and CUDA (#1556)

* Fix inconsistent NMS implementation

* Improve tests for NMS

* Remove unnecessary using statement

Author: fmassa

test/test_ops.py

@@ -1196,26 +1196,33 @@ def reference_nms(self, boxes, scores, iou_threshold):
 
         return torch.as_tensor(picked)
 
-    def _create_tensors(self, N):
+    def _create_tensors_with_iou(self, N, iou_thresh):
+        # force last box to have a pre-defined iou with the first box
+        # let b0 be [x0, y0, x1, y1], and b1 be [x0, y0, x1 + d, y1],
+        # then, in order to satisfy ops.iou(b0, b1) == iou_thresh,
+        # we need to have d = (x1 - x0) * (1 - iou_thresh) / iou_thresh
         boxes = torch.rand(N, 4) * 100
-        boxes[:, 2:] += torch.rand(N, 2) * 100
+        boxes[:, 2:] += boxes[:, :2]
+        boxes[-1, :] = boxes[0, :]
+        x0, y0, x1, y1 = boxes[-1].tolist()
+        boxes[-1, 2] += (x1 - x0) * (1 - iou_thresh) / iou_thresh
         scores = torch.rand(N)
         return boxes, scores
 
     def test_nms(self):
-        boxes, scores = self._create_tensors(1000)
         err_msg = 'NMS incompatible between CPU and reference implementation for IoU={}'
         for iou in [0.2, 0.5, 0.8]:
+            boxes, scores = self._create_tensors_with_iou(1000, iou)
             keep_ref = self.reference_nms(boxes, scores, iou)
             keep = ops.nms(boxes, scores, iou)
             self.assertTrue(torch.allclose(keep, keep_ref), err_msg.format(iou))
 
     @unittest.skipIf(not torch.cuda.is_available(), "CUDA unavailable")
     def test_nms_cuda(self):
-        boxes, scores = self._create_tensors(1000)
         err_msg = 'NMS incompatible between CPU and CUDA for IoU={}'
 
         for iou in [0.2, 0.5, 0.8]:
+            boxes, scores = self._create_tensors_with_iou(1000, iou)
             r_cpu = ops.nms(boxes, scores, iou)
             r_cuda = ops.nms(boxes.cuda(), scores.cuda(), iou)
 

torchvision/csrc/cpu/nms_cpu.cpp

@@ -61,7 +61,7 @@ at::Tensor nms_cpu_kernel(
       auto h = std::max(static_cast<scalar_t>(0), yy2 - yy1);
       auto inter = w * h;
       auto ovr = inter / (iarea + areas[j] - inter);
-      if (ovr >= iou_threshold)
+      if (ovr > iou_threshold)
         suppressed[j] = 1;
     }
   }

torchvision/csrc/cuda/nms_cuda.cu

@@ -72,7 +72,6 @@ __global__ void nms_kernel(
 at::Tensor nms_cuda(const at::Tensor& dets,
     const at::Tensor& scores,
     float iou_threshold) {
-  using scalar_t = float;
   AT_ASSERTM(dets.type().is_cuda(), "dets must be a CUDA tensor");
   AT_ASSERTM(scores.type().is_cuda(), "scores must be a CUDA tensor");
   at::cuda::CUDAGuard device_guard(dets.device());