Crop a different contrast region

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Hi,

I am so new with opencv and now I am trying to use the 3.0.0 beta with python, my question is just for getting some indications about how I could do the next work:

I have an image and I want to crop a section of the image that have a different (color) I mean 2 levels of grays, so I though the best method is to obtain the contrast of regions and after applying a threshold to crop the region I want, but I have no idea how to do it so if someone could give me some advices I would be so pleased.

Thanks in advance.