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The CVideoTransformFilterclass is designed primarily as a base class for AVI decompressor filters. It is based on a "copying" transform class and assumes that the output buffer will likely be a video buffer or Microsoft DirectDraw buffer, although this could be used as a base class for other types of transform filters. The main feature of this class is that it enables quality-control management in a transform filter. This means that it decides to drop frames based on receiving a quality notification from the renderer, and taking into account other factors about the media stream it is processing and the filter's own behavior.
Every time the CVideoTransformFilter::Receivemember function is called, it calls CVideoTransformFilter::ShouldSkipFrameto determine whether to start, continue, or stop skipping frames. This member function starts skipping samples only if all the following conditions are true.
- The average time to decode is more than one fourth of the frame time.
- The filter is running at least one frame late.
- The next anticipated key frame is estimated to be no more than one frame early.
- The occurrence of key frames is sufficiently frequent.
Once the class starts to skip frames, it will skip all frames until a key frame appears, at which time it resets the m_bSkippingflag and processes the sample.
Key frames are defined as AVI key frames or MPEG I frames. These require no history to decode and, if they are skipped, no other frames can be decoded until the next key frame. Non-key frames include AVI non-key frames, MPEG P frames, and MPEG B frames. MPEG B frames are treated the same as other non-key frames by this class. (MPEG B frames can be dropped without the need to skip further frames; however, because this class is aimed primarily at AVI decompressors, it does not allow for this. Once any frame is skipped, all frames are skipped up to the next key frame.)
Protected Data Members
m_bNoSkip Set to TRUE to prevent skipping to the next key frame (for debugging the filter). m_bQualityChanged Status flag that indicates if the stream has degraded. This is set to TRUE in CVideoTransformFilter::Receiveif the call to the derived class Transformmember function fails. ( Receivereturns NOERROR in this case because returning S_FALSE indicates that end-of-stream has arrived.) m_bSkipping Set to TRUE if the filter is skipping to the next key frame. m_idFrameType Performance-measuring frame type identifier (available if PERFis defined). Logs 1 for key frames; logs 2 for nonkey frames. m_idLate Performance identifier for measuring lateness (available if PERFis defined). m_idSkip Performance identifier for measuring frame skipping (available if PERFis defined). m_idTimeTillKey Performance identifier that represents an estimate of the time in milliseconds until the next key frame arrives (available if PERFis defined). m_itrAvgDecode Average time required to decode (transform) the sample. If this is less than one-fourth of the frame time, it is assumed the quality problems are not being generated by this filter and no frames are dropped. m_itrLate Amount of time that the current frame is late. This is originally set to the value of the Qualitystructure's Latemember passed in the quality control message from the renderer filter. It is decremented by the frame time of each frame that is skipped. m_nFramesSinceKeyFrame Used to count frames since the last key frame. m_nKeyFramePeriod The largest observed interval between key frames. m_nWaitForKey Used to ensure output after a format change before getting the first key frame. When nonzero, frames are not passed to the renderer. Set to 30 when format is changed and decremented on each non-key frame. m_tDecodeStart Time since the start of the decoding. Member Functions
AlterQuality Receives a quality-control notification from the output pin and provides an opportunity to alter the quality of the media stream. CVideoTransformFilter Constructs a CVideoTransformFilterobject. ShouldSkipFrame Determines if the filter should start, continue, or stop skipping frames. Overridable Member Functions
EndFlush Receives notification of leaving the flushing state and passes it downstream. Receive Receives the media sample and either skips the sample or transforms and delivers the media sample. RegisterPerfId Registers a performance measurement identifier. StartStreaming Overrides CTransformFilter::StartStreamingto reset the quality control information when streaming starts or flushing starts.