python opencv TypeError: Layout of the output array incompatible with cv::Mat

Question

I\'m using the selective search here: http://koen.me/research/selectivesearch/ This gives possible regions of interest where an object might be. I want to do some processing

Answer 1

Opencv appears to have issues drawing to numpy arrays that have the data type np.int64, which is the default data type returned by methods such as np.array and np.full:

>>> canvas = np.full((256, 256, 3), 255)
>>> canvas
array([[255, 255, 255],
       [255, 255, 255],
       [255, 255, 255]])
>>> canvas.dtype
dtype('int64')
>>> cv2.rectangle(canvas, (0, 0), (2, 2), (0, 0, 0))
Traceback (most recent call last):
  File "", line 1, in 
TypeError: Layout of the output array img is incompatible with cv::Mat (step[ndims-1] != elemsize or step[1] != elemsize*nchannels)

The solution is to convert the array to np.int32 first:

>>> cv2.rectangle(canvas.astype(np.int32), (0, 0), (2, 2), (0, 0, 0))
array([[  0,   0,   0],
       [  0, 255,   0],
       [  0,   0,   0]], dtype=int32)