support creating bitmasks from empty polygons

Reviewed By: HannaMao

Differential Revision: D30795434

fbshipit-source-id: 7a841e0f980f6c8ff40411acff9c732a53ac2c3b

Author: ppwwyyxx

detectron2/structures/masks.py

@@ -170,7 +170,10 @@ def from_polygon_masks(
         if isinstance(polygon_masks, PolygonMasks):
             polygon_masks = polygon_masks.polygons
         masks = [polygons_to_bitmask(p, height, width) for p in polygon_masks]
-        return BitMasks(torch.stack([torch.from_numpy(x) for x in masks]))
+        if len(masks):
+            return BitMasks(torch.stack([torch.from_numpy(x) for x in masks]))
+        else:
+            return BitMasks(torch.empty(0, height, width, dtype=torch.bool))
 
     @staticmethod
     def from_roi_masks(roi_masks: "ROIMasks", height: int, width: int) -> "BitMasks":

tests/structures/test_masks.py

@@ -38,6 +38,10 @@ def test_get_bounding_box(self):
             reconstruct_box = PolygonMasks([[poly]]).get_bounding_boxes()[0].tensor
             self.assertTrue(torch.all(box == reconstruct_box).item())
 
+    def test_from_empty_polygons(self):
+        masks = BitMasks.from_polygon_masks([], 100, 100)
+        self.assertEqual(masks.tensor.shape, (0, 100, 100))
+
 
 if __name__ == "__main__":
     unittest.main()