DV Data in the AVI File Format
Microsoft has specified the format for storage of digital video (DV) data in AVI files. Conforming to this specification will ensure that the AVI files authored in this format will be compatible with future versions of the Microsoft® DirectShow® digital video architecture for the Microsoft® Windows® platform.
This article provides background information to understand the format of Audio-Video Interleaved (AVI) files containing DV audio and video data, or information for programmers who use DV-AVI files on other platforms. Applications that read or write AVI files should use the File Source (Async) filter with the AVI Splitter filter and the AVI MUX filter with the File Writer filter and their associated interfaces provided in the DirectShow architecture, rather than developing the routines to perform these services. These filters simplify the programming requirements for accessing these files.
This article also describes the format of AVI files containing DV data. Specific FOURCCs (four-character codes) for interleaved DV data streams and DV compressor/decompressor stream handlers are defined. The stream format structure for DV data is defined. Specifications for two methods of storing DV data in the AVI file format are specified.
It is assumed that the reader is familiar with the DV data format. (This format is defined in the Specification of Consumer-use Digital VCRs, also called the Blue Book).
This article contains the following sections.
For more information about resource interchange file format (RIFF) files, see the Windows Software Development Kit (SDK) Multimedia Programmer's Guide and Multimedia Programmer's Reference.
For more information about AVI files, see Chapter 6 of the Microsoft Video for Windows Development Kit version 1.1 Programmer's Guide and version 1.02 of the OpenDML AVI File Format Extensions published by the OpenDML AVI M-JPEG File Format Subcommittee, February 28, 1996.
For more information on compressors and decompressors, see the Video Compression and Decompression Drivers section of the Windows DDK Documentation in the MSDN Library.
Types of DV AVI Files
There are two types of DV AVI files:
- AVI Files Containing One DV Data Stream
- AVI Files Containing DV Video as a 'vids' Stream and DV Audio as 'auds' Streams
AVI Files Containing One DV Data Stream
Interleaved DV data can be stored in its native format as a single stream within an AVI RIFF file. This has the advantage of using the minimum amount of data storage for DV. The primary disadvantage is that this file format is not backward-compatible with Video for Windows, because it doesn't contain either a video 'vids' or an audio 'auds' stream. Support is provided for the interleaved DV stream through the DV Muxer and DV Splitter filters provided with DirectShow.
DV data can be stored in a single stream within an AVI RIFF file by specifying the 'iavs' (interleaved audio and video stream) FOURCC (four-character code) in the fccType member and either of the 'dvsd', 'dvhd', or 'dvsl' FOURCCs in the fccHandler member of the 'strh' stream header chunk. The frames per second of the video stream must be specified in the dwRate and dwScale members and the total number of video blocks in the 'movi' chunk in the dwLength member.
The 'dvsd' stream handler FOURCC specifies that the DV data is as defined in Part 2 of the Specification of Consumer-use Digital VCRs. Video is in the format of 525 lines at 29.97 Hz (525-60) or 625 lines at 25.00 Hz (625-50).
The 'dvhd' stream handler FOURCC specifies that the DV data is as defined in Part 3 of the Specification of Consumer-use Digital VCRs. Video is in the format of 1125 lines at 30.00 Hz (1125-60) or 1250 lines at 25.00 Hz (1250-50).
The 'dvsl' stream handler FOURCC specifies that the DV data is as defined in Part 6 of Specification of Consumer-use Digital VCRs. Video is in the format of high-compression SD (SDL).
Note The remainder of this article provides definitions for 'dvsd' streams.
The stream header chunk must be followed by DVINFO stream format chunk. The DVINFO stream format has the following data structure defined for it.
typedef struct tag_DVINFO {
DWORD dwDVAAuxSrc;
DWORD dwDVAAuxCtl;
DWORD dwDVAAuxSrc1;
DWORD dwDVAAuxCtl1;
DWORD dwDVVAuxSrc;
DWORD dwDVVAuxCtl;
DWORD dwDVReserved[2];
} DVINFO, *PDVINFO;
- dwDVAAuxSrc
- Specifies the Audio Auxiliary Data Source Pack for the first audio block (first 5 DV DIF sequences for 525-60 systems or 6 DV DIF sequences for 625-50 systems) of a frame. A DIF sequence is a data block that contains 150 DIF blocks. A DIF block consists of 80 bytes. The Audio Auxiliary Data Source Pack is defined in section D.7.1 of Part 2, Annex D, "The Pack Header Table and Contents of Packs" of the Specification of Consumer-use Digital VCRs.
- dwDVAAuxCtl
- Specifies the Audio Auxiliary Data Source Control Pack for the first audio block of a frame. The Audio Auxiliary Data Control Pack is defined in section D.7.2 of Part 2, Annex D, "The Pack Header Table and Contents of Packs" of the Specification of Consumer-use Digital VCRs.
- dwDVAAuxSrc1
- Specifies the Audio Auxiliary Data Source Pack for the second audio block (second 5 DV DIF sequences for 525-60 systems or 6 DV DIF sequences for 625-50 systems) of a frame.
- dwDVAAuxCtl1
- Specifies the Audio Auxiliary Data Source Control Pack for the second audio block of a frame.
- dwDVVAuxSrc
- Specifies the Video Auxiliary Data Source Pack as defined in section D.8.1 of Part 2, Annex D, "The Pack Header Table and Contents of Packs" of the Specification of Consumer-use Digital VCRs.
- dwDVVAuxCtl
- Specifies the Video Auxiliary Data Source Control Pack as defined in section D.8.2 of Part 2, Annex D, "The Pack Header Table and Contents of Packs" of the Specification of Consumer-use Digital VCRs.
- DwDVReserved[2]
- Reserved. Set this array to zero.
The actual DV data is stored as '##dc' chunks in the 'movi' chunk (the ## in the format represents the stream identifier). Each chunk contains one frame of data, either 10 or 12 DV DIF sequences for 525-60 or 625-50 systems, respectively. The DV SD ('dvsd') DIF sequence format is defined in Part 2 of the Specification of Consumer-use Digital VCRs.
The following diagram illustrates the playback of an AVI file with one DV data stream using a DirectShow filter graph (the DV Splitter and DV Video Decoder filters are included in DirectShow specifically to deal with DV data). The table that follows the diagram defines the media types.
| Media | Major type | Subtype | Format structure |
|---|---|---|---|
| A | MEDIATYPE_Stream | MEDIASUBTYPE_AVI | none |
| B | MEDIATYPE_iavs | MEDIASUBTYPE_dvsd, MEDIASUBTYPE_dvhd, or MEDIASUBTYPE_dvsl | DVINFO |
| C | MEDIATYPE_VIDEO | MEDIASUBTYPE_dvsd, MEDIASUBTYPE_dvhd, or MEDIASUBTYPE_dvsl | DVINFO |
| D | MEDIATYPE_AUDIO | NULL | WAVEFORMATEX |
| E | MEDIATYPE_VIDEO | standard video types | VIDEOINFO |
The following diagram illustrates the creation of an AVI file with one DV data stream by using a DirectShow filter graph (the DV Video Encoder and DV Muxer filters are included in DirectShow specifically to deal with DV data). The preceding table defines the media types. Upstream filters (not shown) can be of any combination to produce the proper media types, D and E.
The following diagram illustrates the creation of an AVI file with one DV data stream using a source filter that communicates through hardware device drivers with a DV device (such as a 1394-based DV camcorder) for DV data input, by using a DirectShow filter graph (the source filter is included in DirectShow specifically to deal with DV data). The preceding table defines the media types.
The following diagram illustrates the output of an AVI file with one DV data stream using a sink filter that communicates through hardware device drivers with a DV device (such as a 1394-based DV camcorder) for DV data output, by using a DirectShow filter graph (the sink filter is included in DirectShow specifically to deal with DV data). The preceding table defines the media types.
The following example shows the AIFF RIFF form for an AVI file with one DV data stream, expanded with completed header chunks.
00000000 RIFF (0FAE35D4) 'AVI '
0000000C LIST (00000106) 'hdrl'
00000018 avih (00000038)
dwMicroSecPerFrame : 33367
dwMaxBytesPerSec : 3728000
dwPaddingGranularity : 0
dwFlags : 0x810 HASINDEX | TRUSTCKTYPE
dwTotalFrames : 2192
dwInitialFrames : 0
dwStreams : 1
dwSuggestedBufferSize : 120000
dwWidth : 720
dwHeight : 480
dwReserved : 0x0
00000058 LIST (0000006C) 'strl'
00000064 strh (00000038)
fccType : 'iavs'
fccHandler : 'dvsd'
dwFlags : 0x0
wPriority : 0
wLanguage : 0x0 undefined
dwInitialFrames : 0
dwScale : 100 (29.970 Frames/Sec)
dwRate : 2997
dwStart : 0
dwLength : 2192
dwSuggestedBufferSize : 120000
dwQuality : 0
dwSampleSize : 0
rcFrame : 0,0,720,480
000000A4 strf (00000020)
dwDVAAuxSrc : 0x........
dwDVAAuxCtl : 0x........
dwDVAAuxSrc1 : 0x........
dwDVAAuxCtl1 : 0x........
dwDVVAuxSrc : 0x........
dwDVVAuxCtl : 0x........
dwDVReserved[2] : 0,0
000000CC LIST (0FADAC00) 'movi'
0FADACD4 idx1 (00008900)
AVI Files Containing DV Video as a 'vids' Stream and DV Audio as 'auds' Streams
Interleaved DV data can be split into a video stream and one to four audio streams within an AVI RIFF file. This has the advantage of being backward-compatible with Video for Windows, because it contains a standard video 'vids' stream and at least one standard audio 'auds' stream The primary disadvantage is that this file format requires the audio data to be redundantly stored as audio streams. The "video" stream is actually the native interleaved DV data stream. However, as a standard 'vids' stream with a handler type of 'dvsd', the DV Video Decoder is used. This format also requires that "captured" files are split by using the DV Splitter filter before they are written as AVI files.
DV data can be stored as a video stream with a separate number of audio streams in an AVI RIFF file. The video stream is specified with a standard video stream header (the fccType member value is 'vids'). The fccHandler member is specified as 'dvsd', 'dvhd', or 'dvsl'. The frames per second of the video stream must be specified in the dwRate and dwScale members and the total number of video blocks in the 'movi' chunk in the dwLength member.
In this AVI file containing DV video as a 'vids' stream and DV audio as 'auds' streams form of DV, the video stream format chunk is a standard BITMAPINFOHEADER structure. The stream format chunk can be optionally extended to include the DVINFO structure, by increasing the stream format chunk size from 40 bytes (size of the BITMAPINFOHEADER structure) to 72 bytes (size of BITMAPINFOHEADER plus DVINFO structures) and immediately following the BITMAPINFOHEADER data structure with a DVINFO data structure.
The audio stream(s) is specified with a standard audio stream header (the fccType member value is 'auds'). The fccHandler member is not used for audio streams.
The DV video data is stored as '##dc' chunks, as defined in the preceding description of an AVI file with one DV data, and the audio data is stored as '##wb' chunks in the 'movi' chunk.
The following diagram illustrates the playback of an AVI file containing DV video as a 'vids' stream and DV audio as 'auds' streams, by using a DirectShow filter graph (the DV Video Decoder filter is included in DirectShow specifically to deal with DV data). The DV media types table defines the media types.
The following diagram illustrates the creation of an AVI file containing DV video as a 'vids' stream and DV audio as 'auds' streams, using a DirectShow filter graph (the DV Video Decoder is included in DirectShow specifically to deal with DV data). The DV media types table defines the media types. Upstream filters (not shown) can be of any combination to produce the proper media types, D and E.
The following diagram illustrates the creation of an AVI file containing DV video as a 'vids' stream and DV audio as 'auds' streams using a source filter that communicates through hardware device drivers with a DV device (such as a 1394-based DV camcorder) for DV data input, by using a DirectShow filter graph (the source and DV Splitter filters are included in DirectShow specifically to deal with DV data). The DV media types table defines the media types.
The following diagram illustrates the output of an AVI file containing DV video as a 'vids' stream and DV audio as 'auds' streams using a sink filter that communicates through hardware device drivers with a DV device (such as a 1394-based DV camcorder) for DV data output, by using a DirectShow filter graph (the DV Muxer and sink filters are included in DirectShow specifically to deal with DV data). The DV media types table defines the media types.
The following example shows the AIFF RIFF form for an AVI file containing DV video as a 'vids' stream and DV audio as 'auds' streams expanded with completed header chunks (including optional DVINFO data following the BITMAPINFO in the 'strf' sub-chunk for the 'vids' stream).
00000000 RIFF (103E2920) 'AVI '
0000000C LIST (00000146) 'hdrl'
00000018 avih (00000038)
dwMicroSecPerFrame : 33367
dwMaxBytesPerSec : 3728000
dwPaddingGranularity : 0
dwFlags : 0x810 HASINDEX | TRUSTCKTYPE
dwTotalFrames : 2192
dwInitialFrames : 0
dwStreams : 2
dwSuggestedBufferSize : 120000
dwWidth : 720
dwHeight : 480
dwReserved : 0x0
00000058 LIST (00000094) 'strl'
00000064 strh (00000038)
fccType : 'vids'
fccHandler : 'dvsd'
dwFlags : 0x0
wPriority : 0
wLanguage : 0x0 undefined
dwInitialFrames : 0
dwScale : 100 (29.970 Frames/Sec)
dwRate : 2997
dwStart : 0
dwLength : 2192
dwSuggestedBufferSize : 120000
dwQuality : 0
dwSampleSize : 0
rcFrame : 0,0,720,480
000000A4 strf (00000048)
biSize : 40
biWidth : 720
biHeight : 480
biPlanes : 1
biBitCount : 24
biCompression : 0x64737664 'dvsd'
biSizeImage : 120000
biXPelsPerMeter : 0
biYPelsPerMeter : 0
biClrUsed : 0
biClrImportant : 0
dwDVAAuxSrc : 0x........
dwDVAAuxCtl : 0x........
dwDVAAuxSrc1 : 0x........
dwDVAAuxCtl1 : 0x........
dwDVVAuxSrc : 0x........
dwDVVAuxCtl : 0x........
dwDVReserved[2] : 0,0
000000F4 LIST (0000005E) 'strl'
00000100 strh (00000038)
fccType : 'auds'
fccHandler : ' '
dwFlags : 0x0
wPriority : 0
wLanguage : 0x0 undefined
dwInitialFrames : 0
dwScale : 1 (32000.000 Samples/Sec)
dwRate : 32000
dwStart : 0
dwLength : 2340474
dwSuggestedBufferSize : 4272
dwQuality : 0
dwSampleSize : 4
rcFrame : 0,0,0,0
00000140 strf (00000012)
wFormatTag : 1 PCM
nChannels : 2
nSamplesPerSec : 32000
nAvgBytesPerSec : 128000
nBlockAlign : 4
wBitsPerSample : 16
cbSize : 0
00000814 LIST (103D0EF4) 'movi'
103D1710 idx1 (00011210)
AVI RIFF File Reference
The Microsoft AVI file format is a RIFF file specification used with applications that capture, edit, and play back audio-video sequences. In general, AVI files contain multiple streams of different types of data. Most AVI sequences use both audio and video streams. A simple variation for an AVI sequence uses video data and does not require an audio stream.
Modifications to the original AVI file specification made in the OpenDML AVI File Format Extensions are not discussed in this section. For further information on these extensions, see version 1.02 of the OpenDML AVI File Format Extensions published by the OpenDML AVI M-JPEG File Format Subcommittee, February 28, 1996.
This section contains the following topics.
- AVI RIFF Form
- AVI Main Header
- AVI Stream Headers
- Stream Data (LIST 'movi' Chunk)
- BITMAPINFOHEADER Structure
- WAVEFORMATEX Structure
AVI RIFF Form
AVI files use the AVI RIFF form. The AVI RIFF form is identified by the FOURCC (four-character code) 'AVI '. All AVI files include two mandatory LIST chunks. These chunks define the format of the stream and stream data. AVI files might also include an index chunk. This optional chunk specifies the location of data chunks within the file. An AVI file with these components has the following form:
RIFF ('AVI '
LIST ('hdrl'
.
.
.
)
LIST ('movi'
.
.
.
)
['idx1'<AVI Index>]
)
The LIST chunks and the index chunk are subchunks of the RIFF 'AVI ' chunk. The 'AVI ' chunk identifies the file as an AVI RIFF file. The LIST 'hdrl' chunk defines the format of the data and is the first required LIST chunk. The LIST 'movi' chunk contains the data for the AVI sequence and is the second required LIST chunk. The 'idx1' chunk is the index chunk. AVI files must keep these three components in the proper sequence.
The LIST 'hdrl' and LIST 'movi' chunks use subchunks for their data. The following example shows the AVI RIFF form expanded with the chunks needed to complete the LIST 'hdrl' and LIST 'movi' chunks:
RIFF ('AVI '
LIST ('hdrl'
'avih'(<Main AVI Header>)
LIST ('strl'
'strh'(<Stream header>)
'strf'(<Stream format>)
'strd'(<additional header data>)
'strn'(<Stream name>)
...
)
.
.
.
)
LIST ('movi'
{SubChunk | LIST ('rec '
SubChunk1
SubChunk2
.
.
.
)
.
.
.
}
.
.
.
)
['idx1'<AVI Index>]
)
AVI Main Header
This and following sections describe the chunks contained in the LIST 'hdrl' and LIST 'movi' chunks. The 'idx1' chunk is not described in this document. For more information on the 'idx1' chunk and indexes in AVI files, see version 1.02 of the OpenDML AVI File Format Extensions published by the OpenDML AVI M-JPEG File Format Subcommittee, February 28, 1996.
The file begins with the main header. In the AVI file, this header is identified by the 'avih' FOURCC (four-character code). The header contains global information for the entire AVI file, such as the number of streams within the file and the width and height of the AVI sequence. The AVI main header structure is defined as follows:
typedef struct {
DWORD dwMicroSecPerFrame;
DWORD dwMaxBytesPerSec;
DWORD dwReserved1;
DWORD dwFlags;
DWORD dwTotalFrames;
DWORD dwInitialFrames;
DWORD dwStreams;
DWORD dwSuggestedBufferSize;
DWORD dwWidth;
DWORD dwHeight;
DWORD dwReserved[4];
} MainAVIHeader;
- dwMicroSecPerFrame
- Specifies the number of microseconds between frames. This value indicates the overall timing for the file.
- dwMaxBytesPerSec
- Specifies the approximate maximum data rate of the file. This value indicates the number of bytes per second the system must handle to present an AVI sequence as specified by the other parameters contained in the main header and stream header chunks.
- dwReserved1
- Reserved. Set this to zero.
- dwFlags
- Contains any flags for the file. The following flags are defined.
AVIF_HASINDEX Indicates the AVI file has an 'idx1' chunk containing an index at the end of the file. For good performance, all AVI files should contain an index. AVIF_MUSTUSEINDEX Indicates that the index, rather than the physical ordering of the chunks in the file, should be used to determine the order of presentation of the data. For example, you could use this to create a list of frames for editing. AVIF_ISINTERLEAVED Indicates the AVI file is interleaved. AVIF_WASCAPTUREFILE Indicates the AVI file is a specially allocated file used for capturing real-time video. Applications should warn the user before writing over a file with this flag set because the user probably defragmented this file. AVIF_COPYRIGHTED Indicates the AVI file contains copyrighted data and software. When this flag is used, software should not permit the data to be duplicated. - dwTotalFrames
- Specifies the total number of frames of data in the file.
- dwInitialFrames
- Specifies the initial frame for interleaved files. Noninterleaved files should specify zero. 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.
- dwStreams
- Specifies the number of streams in the file. For example, a file with audio and video has two streams.
- dwSuggestedBufferSize
- Specifies the suggested buffer size for reading the file. Generally, this size should be large enough to contain the largest chunk in the file. If set to zero, or if it is too small, the playback software will have to reallocate memory during playback, which will reduce performance. For an interleaved file, the buffer size should be large enough to read an entire record, and not just a chunk.
- dwWidth
- Specifies the width of the AVI file in pixels.
- dwHeight
- Specifies the height of the AVI file in pixels.
- dwReserved[4]
- Reserved. Set this array to zero.
AVI Stream Headers
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.
'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 PCMWAVEFORMAT 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.
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.
Stream Data (LIST 'movi' Chunk)
Following the header information is a LIST 'movi' chunk that contains chunks of the actual data in the streams - that is, the pictures and sounds themselves. The data chunks can reside directly in the LIST 'movi' chunk or they might be grouped into 'rec' chunks. The 'rec' grouping implies that the grouped chunks should be read from disk all at once. This is used only for files specifically interleaved to play from CD-ROM.
Like any RIFF chunk, the data chunks contain a FOURCC (four-character code) to identify the chunk type. A FOURCC is a 32-bit quantity represented as a sequence of one to four ASCII alphanumeric characters, padded on the right with blank characters. The FOURCC that identifies each chunk consists of the stream number and a two-character code that defines the type of information encapsulated in the chunk. For example, a waveform chunk is identified by a two-character code of 'wb'. If a waveform chunk corresponded to the second LIST 'hdrl' stream description, it would have a FOURCC of '01wb'.
Note While two-character codes are a convenient way to describe a stream, do not expect them to be recognized by other applications. Use FOURCCs when creating a stream or transferring the information to other applications.
Because all the format information is in the header, the audio data contained in these data chunks does not contain any information about its format. An audio data chunk has the following format (the ## in the format represents the stream identifier):
WAVE Bytes '##wb'
BYTE abBytes[];
Video data can be compressed or uncompressed DIBs. An uncompressed DIB has BI_RGB specified for the biCompression member in its associated BITMAPINFO structure. A compressed DIB has a value other than BI_RGB specified in the biCompression member. For more information about compression formats, see the description of the BITMAPINFOHEADER data structure in the Microsoft Windows Programmer's Reference.
A data chunk for an uncompressed DIB contains RGB video data. These chunks are identified by a two-character code of 'db' (db is an abbreviation for DIB bits). Data chunks for a compressed DIB are identified by a two-character code of 'dc' (dc is an abbreviation for DIB compressed). Neither data chunk will contain any header information about the DIBs. The data chunk for an uncompressed DIB has the following form:
DIB Bits '##db'
BYTE abBits[];
The data chunk for a compressed DIB has the following form.
Compressed DIB Bits '##dc'
BYTE abBits[];
Video data chunks can also define new palette entries used to update the palette during an AVI sequence. For more information on specifying palette information, see Video for Windows Programmer's Guide.
Text streams can use arbitrary two-character codes.
BITMAPINFOHEADER Structure
The BITMAPINFOHEADER structure contains information for the video stream of an AVI RIFF file. This structure has the following members.
typedef struct tagBITMAPINFOHEADER {
DWORD biSize;
LONG biWidth;
LONG biHeight;
WORD biPlanes;
WORD biBitCount;
DWORD biCompression;
DWORD biSizeImage;
LONG biXPelsPerMeter;
LONG biYPelsPerMeter;
DWORD biClrUsed;
DWORD biClrImportant;
} BITMAPINFOHEADER;
- biSize
- Specifies the number of bytes required by the structure.
- biWidth
- Specifies the width of the bitmap, in pixels.
- biHeight
- Specifies the height of the bitmap, in pixels. If biHeight is positive, the bitmap is a bottom-up DIB (device-independent bitmap) and its origin is the lower left corner. If biHeight is negative, the bitmap is a top-down DIB and its origin is the upper left corner.
- biPlanes
- Specifies the number of planes for the target device. This value must be set to 1.
- biBitCount
- Specifies the number of bits per pixel. Some compression formats need this information to properly decode the colors in the pixel.
- biCompression
- Specifies the type of compression used or requested. Both existing and new compression formats use this member.
- biSizeImage
- Specifies the size, in bytes, of the image. This can be set to 0 for uncompressed RGB bitmaps.
- biXPelsPerMeter
- Specifies the horizontal resolution, in pixels per meter, of the target device for the bitmap. An application can use this value to select a bitmap from a resource group that best matches the characteristics of the current device.
- biYPelsPerMeter
- Specifies the vertical resolution, in pixels per meter, of the target device for the bitmap.
- biClrUsed
- Specifies the number of color indices in the color table that are actually used by the bitmap. If this value is zero, the bitmap uses the maximum number of colors corresponding to the value of the biBitCount member for the compression mode specified by biCompression.
- biClrImportant
- Specifies the number of color indices that are considered important for displaying the bitmap. If this value is zero, all colors are important.
When the value in the biBitCount member is set to greater than eight, video drivers can assume bitmaps are true color and they do not use a color table.
When the value in the biBitCount member is set to less than or equal to eight, video drivers can assume the bitmap uses a palette or color table defined in the BITMAPINFO data structure. This data structure has the following members:
typedef struct tagBITMAPINFO {
BITMAPINFOHEADER bmiHeader;
RGBQUAD bmiColors[1];
} BITMAPINFO;
The BITMAPINFO bmiheader member specifies a BITMAPINFOHEADER structure. The BITMAPINFO bmiColors member specifies an array of RGBQUAD data types that define the colors in the bitmap.
WAVEFORMATEX Structure
The WAVEFORMATEX structure contains information for the audio stream(s) of an AVI RIFF file. This structure has the following members.
typedef struct waveformat_extended_tag {
WORD wFormatTag;
WORD nChannels;
DWORD nSamplesPerSec;
DWORD nAvgBytesPerSec;
WORD nBlockAlign;
WORD wBitsPerSample;
WORD cbSize;
} WAVEFORMATEX;
- wFormatTag
- Defines the audio waveform type of the audio stream. A complete list of format tags can be found in the Mmreg.h header file included with Microsoft® Visual C++® and other Microsoft products.
- nChannels
- Specifies the number of channels in the audio stream, 1 for mono, 2 for stereo.
- nSamplesPerSec
- Specifies the frequency of the sample rate of the audio stream in samples/second (Hz). Examples are 11,025, 22,050, or 44,100.
- nAvgBytesPerSec
- Specifies the average data rate. Playback software can estimate the buffer size by using this value.
- nBlockAlign
- Specifies the block alignment of the data, in bytes. Playback software must process a multiple of nBlockAlign bytes of data at a time, so that the value of nBlockAlign can be used for buffer alignment.
- wBitsPerSample
- Specifies the number of bits per sample per channel data. Each channel is assumed to have the same sample resolution. If this field is not needed, then you should set it to zero.
- cbSize
- Specifies the size, in bytes, of the extra information in the format header, not including the size of the WAVEFORMATEX structure. For example, in the wave format corresponding to the wFormatTag WAVE_FORMAT_IMA_ADPCM, cbSize is calculated as sizeof(IMAADPCMWAVEFORMAT) - sizeof(WAVEFORMATEX), which yields two.
AVI 2.0 File Format Extensions
DirectShow currently supports the following AVI 2.0 file format extensions.
- Increased AVI file size (greater than 1 GB)
- Hierarchical indexing
See the specification in version 1.02 of the OpenDML AVI File Format Extensions published by the OpenDML AVI M-JPEG File Format Subcommittee, February 28, 1996.
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