fixup cv2.findContours to only return 2 values

[aaronlelevier]

This PR is to fix an error that I ran in to when running demo/Mask_R-CNN_demo.ipynb

The PR removes the first _ assigned variable. Based on the docs for opencv cv2.findContours only returns 2 values. Here's the docs link:

https://docs.opencv.org/master/d3/dc0/group__imgproc__shape.html#gadf1ad6a0b82947fa1fe3c3d497f260e0

Here is the stack trace that I got from the current code on master:

ValueError                                Traceback (most recent call last)
<ipython-input-10-ba06eae97bcc> in <module>
      1 # compute predictions
----> 2 predictions = coco_demo.run_on_opencv_image(image)
      3 imshow(predictions)

~/github/maskrcnn-benchmark/demo/predictor.py in run_on_opencv_image(self, image)
    177         result = self.overlay_boxes(result, top_predictions)
    178         if self.cfg.MODEL.MASK_ON:
--> 179             result = self.overlay_mask(result, top_predictions)
    180         result = self.overlay_class_names(result, top_predictions)
    181 

~/github/maskrcnn-benchmark/demo/predictor.py in overlay_mask(self, image, predictions)
    289             thresh = mask[0, :, :, None]
    290             _, contours, hierarchy = cv2.findContours(
--> 291                 thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE
    292             )
    293             image = cv2.drawContours(image, contours, -1, color, 3)

ValueError: not enough values to unpack (expected 3, got 2)

I was using these package versions when I ran into the error:

Python 3.6.7
opencv-python==4.0.0.21

[fmassa]

Hi,

This is a problem with OpenCV versions.

In my case, I have OpenCV 3.4.3, and the function signature is

findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> image, contours, hierarchy

Could you maybe add a helper function findContours, which internally handles different versions of OpenCV instead?

Thanks!

[ardiya]

@aaronlelevier Would you mind changing it to this code? this won't break other opencv version

if cv2.__version__.startswith('2') or cv2.__version__.startswith('4'):
    contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
else:
    _, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

[fmassa]

Yes, I'd be willing to merge such a PR, as propose by @ardiya

[csguanhu]

fix this bug following these steps: 1 pip uninstall opencv-python; 2 conda install opencv;