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how to find dent in a circle?
Not exactly sure what you mean by "find" - for example is it just a binary image of the dent alone, or is it an array of all the (row, column) pairs of pixels that comprise the dent. But anyway, you can just get a binary image of the bojects and then use bwconvhull
dentsImage = bwconvhull(binaryImage) - binaryImage; % Get image. [rows columns] = find(dentsImage); % Get (y,x) coordinates.