I want to write Python code to scan receipt images with clarity. How can I do that?

I have an image of a steak, and my goal is to calculate its marbling by converting the pixels to red for meat and white for fat. However, I have problems with glare in the image, which hinder the calculation. My code helps to improve it somewhat, but I would like to improve it even further. Here is an example, I have this image below, with glare:

I performed some canny edge with large sigma (4.5) in order to identify the main features of my image. So far I’ve gotten to this point where canny edges successfully identified the border of my region of interest (2 semi-parallel lines on top and two semi parallel lines at the bottom – all identified with red boxes).