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Requirements for Decoders

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[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

Decoders that deliver samples to the VMR must observe the following rules:

  • There should be one subpicture frame delivered to the VMR for each video frame. The two frames should have the same time stamps.
  • If the subpicture has not changed, use the AM_GBF_NOTASYNCPOINT flag in the IMemAllocator::GetBuffer method to force the allocator to return a buffer containing the last frame delivered to the VMR. Just put a new time stamp on the sample and deliver it to the VMR again. If the subpicture fame is blank, you should still deliver it. The VMR will detect the empty frame and not blend it with the video. This test is performed by the VGA chip and does not affect the playback performance.
  • All samples, except for live streams, must have valid start and stop time stamps attached. (DVD is not a live stream.)
  • The media sample time stamps must be contiguous
  • The decoder must identify itself as VMR-capable for use by graph builders.
  • The subpicture stream should now contain embedded per-pixel alpha values. The ARGB4444 surface type is ideal for subpictures.
  • Do not assume that the stride of the subpicture is the same as the surface width. This is not always the case with the VMR.

About DirectX Video Acceleration