Call findContours in a compatible way (#119, #9) (#847)

Summary:
OpenCV 2, 3, 4 have different signatures for this function. Using [-2] is compatible with them.
Pull Request resolved: https://github.com/facebookresearch/Detectron/pull/847

Reviewed By: rbgirshick

Differential Revision: D14703716

Pulled By: ir413

fbshipit-source-id: 7329c5d45a0b0d2a3125198e6ac64bc79b42ff46

detectron/utils/vis.py

@@ -104,8 +104,8 @@ def vis_mask(img, mask, col, alpha=0.4, show_border=True, border_thick=1):
     img[idx[0], idx[1], :] += alpha * col
 
     if show_border:
-        _, contours, _ = cv2.findContours(
-            mask.copy(), cv2.RETR_CCOMP, cv2.CHAIN_APPROX_NONE)
+        contours = cv2.findContours(
+            mask.copy(), cv2.RETR_CCOMP, cv2.CHAIN_APPROX_NONE)[-2]
         cv2.drawContours(img, contours, -1, _WHITE, border_thick, cv2.LINE_AA)
 
     return img.astype(np.uint8)
@@ -328,8 +328,8 @@ def vis_one_image(
                 img[:, :, c] = color_mask[c]
             e = masks[:, :, i]
 
-            _, contour, hier = cv2.findContours(
-                e.copy(), cv2.RETR_CCOMP, cv2.CHAIN_APPROX_NONE)
+            contour = cv2.findContours(
+                e.copy(), cv2.RETR_CCOMP, cv2.CHAIN_APPROX_NONE)[-2]
 
             for c in contour:
                 polygon = Polygon(