How to find only visible portion of the window?

Hi,

I had put lot of controls in form window and some dialog's also embedded. Suppose if I want to know whether my mouse pointer coordinates in side the dialog or not.

I am using PtInRect function by passing window rect But my requirement is, I wanted to know whether my coordinates are falls in visible part of the window.

example : Actual window rect is(0,0,1000,1000) but only (0,0,500,500) portion of Rect is visible to user. I wanted to know (700,700) coordinates are in side the visible part of window ?

Please help to Improve my knowledge

Thanks,

Yogisha