| Microsoft DirectX 8.1 (C++) |
The CVideoTransformFilter class 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::Receive member function is called, it calls CVideoTransformFilter::ShouldSkipFrame to determine whether to start, continue, or stop skipping frames. This member function starts skipping samples only if all the following conditions are true.
After the class starts to skip frames, it will skip all frames until a key frame appears, at which time it resets the m_bSkipping flag 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::Receive if the call to the derived class Transform member function fails. (Receive returns 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. Logs 1 for key frames; logs 2 for nonkey frames. |
| m_idLate | Performance identifier for measuring lateness (available if PERF is defined). |
| m_idSkip | Performance identifier for measuring frame skipping. |
| m_idTimeTillKey | Performance identifier that represents an estimate of the time in milliseconds until the next key frame arrives. |
| 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 Quality structure's Late member 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 nonkey 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 CVideoTransformFilter object. |
| 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::StartStreaming to reset the quality control information when streaming starts or flushing starts. |