Getting the Flip and Blit Status
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4/8/2010
When the IDirectDrawSurface::Flip method is called, the primary surface and back buffer are exchanged. However, the exchange may not occur immediately.
For example, if a previous flip has not finished, or if it did not succeed, this method returns DirectDraw Return Values.
In the samples included with the SDK, the IDirectDrawSurface::Flip call continues to loop until it returns DD_OK. Also, a IDirectDrawSurface::Flip call does not complete immediately. It schedules a flip for the next time a vertical blank occurs on the system.
An application that waits until the DirectDraw Return Values value is not returned is very inefficient. Instead, you could create a function in your application that calls the IDirectDrawSurface::GetFlipStatus method on the back buffer to determine if the previous flip has finished.
If the previous flip has not finished and the call returns DirectDraw Return Values, your application can use the time to perform another task before it checks the status again. Otherwise, you can perform the next flip.
The following example demonstrates this concept.
while(lpDDSBack->GetFlipStatus(DDGFS_ISFLIPDONE) ==
DDERR_WASSTILLDRAWING);
// Waiting for the previous flip to finish. The application can
// perform another task here.
ddrval = lpDDSPrimary->Flip(NULL, 0);
You can use the IDirectDrawSurface::GetBltStatus method in much the same way to determine whether a blit has finished. Because IDirectDrawSurface::GetFlipStatus and IDirectDrawSurface::GetBltStatus return immediately, you can use them periodically in your application with little loss in speed.