For test purpose, I selected an image and a subset of the same image. Then under image analysis - utilities - Image difference selected. here for the before theme ( i selected the actual image) and before layer as layer_1 and After theme I selected ( subset image) and before layer was again layer_1. Everything else of default. I was thinking results would come as "unchanged" rather it came all increased 10 % . I am confused here. why didnt it came unchanged?when its same image just an subset. Please help.