Find dirtyrect or cliprect using objective C

Question

Is it possible to capture the dirty rect alone in Mac OS using Objective C?

Is there any api which will give, just the dirty rectangles or clip rect of the areas that are modified, when there is a screen change in the display?

I'm in need of capturing the screen and sending across the screen changes to server repeatedly.

What would be the best way to do it?

Please help.

Thanks, Priya