How to obtain histogram of image blocks(16*16) . please help me

i have compound image and i divided image into blocks. How can i find gradients for each blocks so that i can differentiat image into 3 layers Mask, FG and BG please help me...............

[Image Analyst]

What is a compound image? What does a histogram have to do with gradients? And we have no idea what would be in the "3 layers" that you call Mask, FG, and BG. Please read this http://www.mathworks.com/matlabcentral/answers/6200-tutorial-how-to-ask-a-question-on-answers-and-get-a-fast-answer