Recognize holes with inserted plate

Question

I have a lot of photos with tanks. Each tank has holes in which to insert a metal plate.

I need to find all the holes without a plate.

I tried to find circles on HoughCircles, used the haar training classifier, but did not get an acceptable result.

The holes are small (about 30x30 pixels), lighting is not constant.

I would be grateful for any ideas.

Thank!

Answer 1

If the problem is with lightning, you can try to do the following: (the result depends on how problematic these shadows are)

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Answer 2

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