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Below are four images. The first is the red channel extracted from the original image. The 2nd is this extracted image thresholded at about 200. The 3rd is the result of using findContours with external option used and then only keep the largest contour. The 4th is the contour mask after some morphology to clean up the corners: 1 iteration of erosion with a very wide kernel about 50 x 3 and 1 iteration with a very tall kernel about 3 x 50