What does the python interface to opencv2.fillPoly want as input?

The AssertionError is telling you that OpenCV wants a signed, 32-bit integer. The array of polygon points should have that particular data type (e.g. points = numpy.array(A,dtype='int32') ). You could also just cast it for the function call (i.e. my_array.astype('int32') ) or as a friend put it once...

" Changing

   cv2.fillConvexPoly(binary_image, np.array(rect['boundary']), 255) to

   cv2.fillConvexPoly(binary_image, np.array(rect['boundary'], 'int32'), 255) "

import numpy as np
import cv2
import matplotlib.pyplot as plt

a3 = np.array( [[[10,10],[100,10],[100,100],[10,100]]], dtype=np.int32 )
im = np.zeros([240,320],dtype=np.uint8)
cv2.fillPoly( im, a3, 255 )

plt.imshow(im)
plt.show()

Check on colab.research.google.com