BOOL bSuccess = FALSE;
HRGN hrgn = CreateRectRgn(0,0,1,1);     // create a generic rgn
if ( GetClipRgn(hdc, hrgn) != -1 )      // get the clip rgn from the given hdc
{
      if ( OffsetRgn(hrgn, xDiff, yDiff) != ERROR )  // offset the clip rgn
      {
           if ( SelectClipRgn(hdc, hrgn) != ERROR )  // select the modified rgn into
                   bSuccess = TRUE;      // the device context
      }
}
DeleteObject(hrgn);          // delete the rgn we created above