The framework calls this member function when Windows or an application makes a request to repaint a portion of an application's window.
A window may receive internal paint messages as a result of calling the RedrawWindow member function with the RDW_INTERNALPAINT flag set. In this case, the window may not have an update region. An application should call the GetUpdateRect member function to determine whether the window has an update region. If GetUpdateRect returns 0, the application should not call the BeginPaint and EndPaint member functions.
It is an application's responsibility to check for any necessary internal repainting or updating by looking at its internal data structures for each WM_PAINT message because a WM_PAINT message may have been caused by both an invalid area and a call to the RedrawWindow member function with the RDW_INTERNALPAINT flag set.
An internal WM_PAINT message is sent only once by Windows. After an internal WM_PAINT message is sent to a window by the UpdateWindow member function, no further WM_PAINT messages will be sent or posted until the window is invalidated or until the RedrawWindow member function is called again with the RDW_INTERNALPAINT flag set.
For information on rendering an image in document/view applications, see CView::OnDraw.
For more information about using WM_Paint, see the following topics in the Windows SDK: