Retrieving the Update Region
The GetUpdateRect and GetUpdateRgn functions retrieve the current update region for the window. GetUpdateRect retrieves the smallest rectangle (in logical coordinates) that encloses the entire update region. GetUpdateRgn retrieves the update region itself. These functions can be used to calculate the current size of the update region to determine where to carry out a drawing operation.
BeginPaint also retrieves the dimensions of the smallest rectangle enclosing the current update region, copying the dimensions to the rcPaint member in the PAINTSTRUCT structure. Because BeginPaint validates the update region, any call to GetUpdateRect and GetUpdateRgn immediately after a call to BeginPaint returns an empty update region.