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When the IDirectDrawSurface5::Flipmethod 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 DDERR_WASSTILLDRAWING. In the samples included with the SDK, the IDirectDrawSurface5::Flipcall continues to loop until it returns DD_OK. Also, a IDirectDrawSurface5::Flipcall 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 DDERR_WASSTILLDRAWING value is not returned is very inefficient. Instead, you could create a function in your application that calls the IDirectDrawSurface5::GetFlipStatusmethod on the back buffer to determine if the previous flip has finished.
If the previous flip has not finished and the call returns DDERR_WASSTILLDRAWING, 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 IDirectDrawSurface5::GetBltStatusmethod in much the same way to determine whether a blit has finished. Because IDirectDrawSurface5::GetFlipStatusand IDirectDrawSurface5::GetBltStatusreturn immediately, you can use them periodically in your application with little loss in speed.
Last updated on Tuesday, May 18, 2004