AVI Stream Headers

Article
06/30/2006

The main header is followed by one or more 'strl' chunks. (A 'strl' chunk is required for each data stream.) These chunks contain information about the streams in the file. Each 'strl' chunk must contain a stream header and stream format chunk. Stream header chunks are identified by the FOURCC (four-character code) 'strh' and the stream format chunks are identified by the FOURCC 'strf'. In addition to the stream header and stream format chunks, the 'strl' chunk might also contain a stream-header data chunk and a stream name chunk. Stream-header data chunks are identified by the FOURCC 'strd'. Stream name chunks are identified by the FOURCC 'strn'.

The stream header structure contains header information for a single stream of a file.

typedef struct {
    FOURCC fccType;
    FOURCC fccHandler;
    DWORD  dwFlags;
    DWORD  dwPriority;
    DWORD  dwInitialFrames;
    DWORD  dwScale;
    DWORD  dwRate;
    DWORD  dwStart;
    DWORD  dwLength;
    DWORD  dwSuggestedBufferSize;
    DWORD  dwQuality;
    DWORD  dwSampleSize;
    RECT   rcFrame;
} AVIStreamHeader;

The stream header specifies the type of data the stream contains, such as audio or video, by means of a FOURCC.

fccType
Contains a FOURCC that specifies the type of the data contained in the stream. The following standard AVI values for video and audio are defined.

Value	Definition
'vids'	Indicates the stream contains video data. The stream format chunk contains a BITMAPINFO structure that can include palette information.
'auds'	Indicates the stream contains audio data. The stream format chunk contains a WAVEFORMATEX or PCWAVEFORMAT structure.
'txts'	Indicates the stream contains text data.

fccHandler
Optionally, contains a FOURCC that identifies a specific data handler. The data handler is the preferred handler for the stream. For audio and video streams, this specifies the installable compressor or decompressor.

dwFlags
Contains any flags for the data stream. The bits in the high-order word of these flags are specific to the type of data contained in the stream. The following standard flags are defined.

Flag	Definition
AVISF_DISABLED	Indicates this stream should not be enabled by default.
AVISF_VIDEO_PALCHANGES	Indicates this video stream contains palette changes. This flag warns the playback software that it will need to animate the palette.

dwPriority
Specifies priority of a stream type. For example, in a file with multiple audio streams, the one with the highest priority might be the default stream.
dwInitialFrames
Specifies how far audio data is skewed ahead of the video frames in interleaved files. Typically, this is about 0.75 seconds. If you are creating interleaved files, specify the number of frames in the file prior to the initial frame of the AVI sequence in this member. For more information about the contents of this member, see "Special Information for Interleaved Files" in the Video for Windows Programmer's Guide.
dwScale
Used with dwRate to specify the time scale that this stream will use. Dividing dwRate by dwScale gives the number of samples per second. For video streams, this rate should be the frame rate. For audio streams, this rate should correspond to the time needed for nBlockAlign bytes of audio, which for PCM audio simply reduces to the sample rate.
dwRate
See dwScale.
dwStart
Specifies the starting time of the AVI file. The units are defined by the dwRate and dwScale members in the main file header. Usually, this is zero, but it can specify a delay time for a stream that does not start concurrently with the file.
dwLength
Specifies the length of this stream. The units are defined by the dwRate and dwScale members of the stream's header.
dwSuggestedBufferSize
Specifies how large a buffer should be used to read this stream. Typically, this contains a value corresponding to the largest chunk present in the stream. Using the correct buffer size makes playback more efficient. Use zero if you do not know the correct buffer size.
dwQuality
Specifies an indicator of the quality of the data in the stream. Quality is represented as a number between 0 and 10,000. For compressed data, this typically represents the value of the quality parameter passed to the compression software. If set to –1, drivers use the default quality value.
dwSampleSize
Specifies the size of a single sample of data. This is set to zero if the samples can vary in size. If this number is nonzero, then multiple samples of data can be grouped into a single chunk within the file. If it is zero, each sample of data (such as a video frame) must be in a separate chunk. For video streams, this number is typically zero, although it can be nonzero if all video frames are the same size. For audio streams, this number should be the same as the nBlockAlign member of the WAVEFORMATEX structure describing the audio.
rcFrame
Specifies the destination rectangle for a text or video stream within the movie rectangle specified by the dwWidth and dwHeight members of the AVI main header structure. The rcFrame member is typically used in support of multiple video streams. Set this rectangle to the coordinates corresponding to the movie rectangle to update the whole movie rectangle. Units for this member are pixels. The upper-left corner of the destination rectangle is relative to the upper-left corner of the movie rectangle.

The last eight members describe the playback characteristics of the stream. These factors include the playback rate (dwScale and dwRate), the starting time of the sequence (dwStart), the length of the sequence (dwLength), the size of the playback buffer (dwSuggestedBuffer), an indicator of the data quality (dwQuality), and the sample size (dwSampleSize).

Some of the members in the stream header structure are also present in the main header structure. The data in the main header applies to the whole file, while the data in the stream header structure applies only to a stream.

A stream format ('strf') chunk must follow a stream header ('strh') chunk. The stream format chunk describes the format of the data in the stream. For video streams, the information in this chunk is a BITMAPINFO structure (including palette information if appropriate). For audio streams, the information in this chunk is a WAVEFORMATEX or PCMWAVEFORMAT structure. (The WAVEFORMATEX structure is an extended version of the WAVEFORMAT structure.) For more information about this structure and other stream types, see the New Multimedia Data Types and Data Techniques Standards Update.

The 'strl' chunk might also contain an additional stream-header data ('strd') chunk. If used, this chunk follows the stream format chunk. The format and content of this chunk is defined by installable compression or decompression drivers. Typically, drivers use this information for configuration. Applications that read and write RIFF files do not need to decode this information. They transfer this data to and from a driver as a memory block.

The optional 'strn' stream name chunk provides a zero-terminated text string describing the stream. (The AVI file functions can use this chunk to let applications identify the streams they want to access by their names.)

An AVI player associates the stream headers in the LIST 'hdrl' chunk with the stream data in the LIST 'movi' chunk by using the order of the 'strl' chunks. The first 'strl' chunk applies to stream 0, the second applies to stream 1, and so forth.

For example, if the first 'strl' chunk describes the wave audio data, the wave audio data is contained in stream 0. Similarly, if the second 'strl' chunk describes video data, then the video data is contained in stream 1.

Last updated on Tuesday, May 18, 2004

AVI Stream Headers

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