|Microsoft DirectX 9.0|
The UseWhenFullScreen method determines whether DirectShow should change display mode when going to full-screen mode.
HRESULT UseWhenFullScreen( long UseWhenFullScreen );
Value specifying whether to change the display mode. Set to OATRUE to cause the renderer to use DirectShow in full-screen mode; otherwise, set to OAFALSE.
Returns an HRESULT value.
When asked to go to full-screen mode, DirectShow has a number of choices. The first choice is to determine if any filters in the graph can support full-screen playback directly; if one can, it will be asked to do so.
The second choice is to automatically add a special full-screen renderer to the filter graph that uses DirectDraw mode-changing services to play back the video. By changing display modes, the video effectively fills more (but not necessarily all) of the display. So, for example, if the current mode is 1024 x 768 pixels, a video might look relatively small, but when displayed in a 320 x 240 display mode it might look very presentable.
The third and final choice is to simply take any renderer supporting the IVideoWindow interface and have its window stretched full-screen. This will typically offer lower performance than the second option (swapping in a full-screen DirectDraw-enabled renderer). If the UseWhenFullScreen parameter is set to On (OATRUE), the window will always be stretched full-screen for full-screen playback; if set to Off (the default), the filter graph manager is free to swap in the DirectDraw-enabled full-screen renderer.