Working with Video Frames

 
Microsoft DirectShow 9.0

Working with Video Frames

Uncompressed video is a sequence of bitmaps played in rapid succession, typically at a rate of about 30 frames per second. Because most video enters a DirectShow filter graph in a compressed format, the video stream generally goes through a decoder for decompression. Many decoders output data in a YUV format and leave the final conversion to RGB for the video hardware just prior to rendering. If a decoder uses DirectX Video Acceleration, the video hardware performs additional work to decode the image. Thus, final decompression of the bitmaps may not be performed until the data reaches the video hardware.

But to perform many types of video analysis, processing, or editing, it is often necessary to work on uncompressed bitmaps in some type of RGB or YUV format before they are rendered or written to file. This work is typically done within a transform filter based on the CTransformFilter base class, specifically in the Transform method. This method receives a pointer to an IMediaSample object that encapsulates the video data. The IMediaSample::GetPointer method returns a pointer to the first byte of the raw data. For uncompressed frames, this data consists of pixels that can be accessed or modified directly by the filter. The following sections provide background information that will help you work effectively with DIB data in this manner.

  • Note   You can also modify the bits by using GDI, GDI+, DirectDraw or Direct3D functions, but these techniques are beyond the scope of this article.

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