This function retrieves the update region of a window by copying it into the specified region. The coordinates of the update region are relative to the upper left corner of the window; that is, they are client coordinates.
int GetUpdateRgn( HWND hWnd, HRGN hRgn, BOOL bErase );
Handle to the window with an update region that is to be retrieved.
Handle to the region to receive the update region.
Boolean that specifies whether the window background should be erased and whether nonclient areas of child windows should be drawn.
If this parameter is FALSE, no drawing is done.
The bErase parameter is ignored, the window background is not erased and no drawing is done.
The BeginPaint function automatically validates the update region, so any call to GetUpdateRgn made immediately after the call to BeginPaint retrieves an empty update region.