Object Segmentation method

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Hello,

I need your help, I have an image, and I want to extract the red part of this image (contours), my idea is giving the user the possibility to select a point inside the red part and use this information to extract the contours.

My problem is : I think a method that can do that exist, but I can't find it, I find Convex Hull but I don't think that the best solution for my problem.

And thank you.