How do Windows know the clip region?

Well, for each window, they can easily get the clipping region of their own, and then they can redraw the region.

However, i'm wondering how Windows know the clipping region of each one (process)? There must be some centralized mechanism in the system that do the calculation. However, it seems it's behinded the documentation?

Is there any guru here that know how Windows do the clipping region calcuation (in the background)?

Thanks.

Not sure I undertsnad your query, but you could use any of the following :
GetDCEx
SetMetaRgn
OffsetClipRgn
ExcludeClipRect
ExtSelectClipRgn
IntersectClipRect
SelectClipRgn
GetClipBox
GetRandomRgn

Thanks, but the above API are the "result" after each Window already get it's clipping region.

My question should be something like undocumented, and maybe we need some gurus who happen to have chances (like working in the gov?) to trace the Windows system source code or read some undocumented payper to know how the Windows internal system works.

Maybe "Painter's algorithm" can help explain some?

PS:
Tracing Windows mobile/CE code, and you will get "?SelectClipRgn@CDC@@QAAHPAVCRgn@@@Z @ 2269 NONAME". Microsoft still didn't release this kernel part code yet.

It's said that in Windows CE 6.0, there is no hidden code of Windows kernel?

If this is true, then the way Windows handle clipping region can be clarify. Any guru happens to know how?