I have obtained the image below after applying thresholding. The white areas inside the boundaries represent the defective part. I only want these defective parts on the image, like the ones inside the blue boxes. The approach in my mind is to use any edge detection algorithm and then subtract that image from the original one so that only defective parts remain on the image. I have used many edge detection algorithms, but I failed to obtain the desired result. Kindly tell me how to extract boundaries or edges from the given image. Or is there any better way to solve this problem?