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I believe findContours treads white as foreground and black as background:

The pointer first_contour is filled by the function. It will contain a pointer to the first outermost contour or NULL if no contours are detected (if the image is completely black)

So for findContours your image is a big white rectangle with a black hole in it.

You could negate the image first, otherwise I think you will have to discard the biggest contour.

I believe findContours treads white as foreground and black as background:

The pointer first_contour is filled by the function. It will contain a pointer to the first outermost contour or NULL if no contours are detected (if the image is completely black)

So for findContours your image is a big white rectangle with a black hole in it.

You could negate the image first, first; otherwise I think you will have to discard the biggest contour.