help on flux kontext low res mapped onto higher resolution

I want to create a comfyui nodes/workflow that takes a large image i.e. 3072x3072, downscales it to 1024x1024 so that it actually runs, this rescaling has already been done by S&R FluxKontextImageRescale then rescales the output to 3072x3072 detects the differences and overlays the differences over the original 3072x3072 image. How would I go about doing this? Do I figure out someway to iterate over every pixel and see if theres a large enough difference to warrant replacing the pixel or how does this work