Canny on a concave grayscale, the result contours can not end in closure

[victor]

@crackwitz

you don’t need a highpass or an edge map. you just needed to threshold the picture to clearly differentiate black from gray. in your case, the background is already black (zero) and the foreground is gray (nonzero), so that can already be interpreted as a binary mask.

thank you for your explanation about Canny.
Let me simplify my case. My goal is to extract polygon (rectangle is preferred if possible) for gray pattern in the image.I have already generated a binarized image and smoothed it, so just findContours on it will work.
is there anything else i should take care, or any more advice?