Segmentation based on pixel density

I have an image with different regions marked with different pixel densities. I want to segment those different regions. Please guide for the same.

[Image Analyst]

Define what you mean by densities. All images have pixels that are side-by-side. Do you mean resolution like one scene was captured with a 8 megapixel camera and another image of the same scene was captured by a 18 megapixel camera? Or do you mean optical density (like x-ray terminology) which is basically gray level?