How to capture dark object in a bright environment

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Hi,

I am looking for a method which can recognize the dark object in a bright environment. I need to get the position of the center of the dark object. I tried several packages in opencv_apps. However, it only finds a bright object and ignores dark objects. Even the edge is clear. It will work if I can reverse white and black of my image but can not find a method to do so.

Does anyone know how to find the dark object? or how to reverse W/B of image topic?

Thank you in advance