Measure closeness of changed image pixels

I am doing image quality assessment and need to measure "closeness"/"unity" of degraded pixel regions. For MWE (min working example) I use 2 images, one with randomly distributed pixel degradation and one with aggregated degradation area. Mapping looks like this:

and this:

I tried std2 but is it not working, it is almost the same for above quality maps. What is simple method to measure and determine pixel/value (above some threshold or in binary-mode) based on some mathematical model, eg. the value of "closeness" is exponentially related to number of grouped pixels?

Answer 1

[Image Analyst]

If you think that pixels in your scene have just moved to different pixels in the image you can use optical flow. It's a vector field that say where every pixel moved to. If you want to find differences in color at every pixel location, you can use simple subtraction, or much better, use the delta E color difference.

There are a number of image quality metrics http://en.wikipedia.org/wiki/Image_quality, perhaps SSIM being the most famous and widespread.