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DOC file for AVI Installable compressorsToddLa----------------------------------------------------------------------------
This document covers what happens (from the drivers point ofview) when AVI loads, saves, edits, and plays a AVI file.
FORMAT OF A AVI FILE -------------------------------------------------------
AVI files are RIFF files with the following structure:
RIFF('AVI' LIST('hdrl' avih(<MainAVIHeader>) LIST ('strl' strh(<Stream header>) strf(<Stream format>) strd(<Stream data>) ) ) LIST('movi' 00??(<driver Data>) . . . 00??(<driver Data>) ) indx(<AVIIndex>) )
OPENING a file for decompression -------------------------------------------
this is what the driver see's when a AVI file is opened for playbackor editing.
1. The AVI runtime driver gets the FOURCC driver name from the stream header and tries to open the named driver. drivers are listed in SYSTEM.INI
[Installable Compressors] VIDC.SAMP = icsample.drv
This example lists a video driver ('VIDC') with the name 'SAMP'.
The first message a device driver receives is DRV_OPEN. Your device driver should allocate any instance data it uses when it processes this message. (Your driver will receive DRV_OPEN each time it is opened.)
2. If a 'strd' chunk exists for this stream, the AVI runtime driver will send your driver an ICM_SETSTATE message. The <MI>lParam1<D> parameter will contain a pointer to the data from the file.
3. Depending on how the AVI runtime driver will use the decompressed data, it will send either ICM_DECOMPRESS_GET_FORMAT or ICM_DECOMPRESS_QUERY to determine the decompression format.
ICM_DECOMPRESS_GET_FORMAT:
When your driver gets ICM_DECOMPRESS_GET_FORMAT, <MI>lParam1<D> points to the 'strf' data, and <MI>lParam2<D> points to a <B>BITMAPINFOHEADER<D> structure.
Your driver should fill in the <B>BITMAPINFOHEADER<D> with information about the format it will use for decompressing the data.
If your device driver cannot handle the input format, or the input format 'strf' is incorrect, your driver should return ICMERR_BADFORMAT to fail the message.
ICM_DECOMPRESS_QUERY:
When your driver gets ICM_DECOMPRESS_QUERY, <MI>lParam1<D> points to the 'strf' data, and <MI>lParam2<D> points to a <B>BITMAPINFOHEADER<D> structure. For this message, the AVI runtime driver filled the structure with a description of the output format it wants to use.
The sugested format will be the native or best format for the decompressed data. For example, if playback is on an 8-bit device, the AVI runtime driver will suggest an 8-bit DIB.
Your driver does not have to accept the suggested format. If you fail the message, the AVI runtime driver will suggest alternate formats until your driver accepts one. If your driver exhausts the list of formats normally used, the AVI runtime driver will request a format with ICM_DECOMPRESS_GET_FORMAT.
4. When the AVI runtime driver is ready, it sends the ICM_DECOMPRESS_BEGIN message to the device driver. The AVI runtime driver sets <MI>lParam1<D> to the input format ('strf' data) and <MI>lParam2<D> to the decompressed format of the DIB. If either of the formats is incorrect, your device driver should return ICERR_BADFORMAT.
5. The AVI runtime driver sends ICM_DECOMPRESS each time it has an image to decompress. The <B>ICDECOMPRESS<D> structure specified in <MI>lParam1<D> contains the decompression parameters. The AVI runtime driver uses the flags in the AVI index to ensure decompression will start on key frame boundries,
6. Your device driver receives ICM_DECOMPRESS_END when the AVI runtime driver no longer needs data decompressed. When the device driver is no longer needed the system will close it by sending DRV_CLOSE.
COMPRESSING a file ------------------------------------------------------
This is the sequence of messages that occur for compressing video data.
NOTE: When an AVI file is recompressed, each frame is decompressed to a full frame and passed to the decompressor.
1. The first message a device driver receives is DRV_OPEN. Your device driver should allocate any instance data it uses when it processes this message. (Your driver will receive DRV_OPEN each time it is opened.)
2. The AVI runtime driver restores the device driver state by sending ICM_SET_STATE. (The state is recalled from the strd data chunk of the AVI file. The state information was obtained with the ICM_GET_STATE message.)
3. The AVI runtime driver determines the size of the buffer needed to hold the compressed data format by sending ICM_COMPRESS_GET_FORMAT. When requesting the buffer size, the AVI runtime driver uses <MI>lParam1<D> to point to a <B>BITMAPINFOHEADER<D> structure and sets <MI>lParam2<D> to NULL. Your device driver should return the size of the buffer in bytes.
4. The AVI runtime driver requests the output format by sending ICM_COMPRESS_GET_FORMAT with valid pointers in both <MI>lParam1<D> and <MI>lParam2<D>. For this case, your device driver should return the output format in the buffer pointed to by <MI>lParam2<D>.
This data becomes the <B>strf<D> chunk in the AVI file. The data must start out like a <B>BITMAPINFOHEADER<D> data structure. A color table (if used) follows this information. The last part of the data is any additional information required to decompress the file.
5. When the AVI runtime driver is ready to start compressing data it sends the ICM_COMPRESS_BEGIN message. The AVI runtime driver uses <MI>lParam1<D> to point to the format of the data being compressed, and uses <MI>lParam2<D> to point to format for the compressed data. If your device driver cannot handle the formats, or if they are incorrect, your driver should return ICERR_BADFORMAT to fail the message.
6. Before the AVI runtime driver starts compressing data, it sends ICM_COMPRESS_GET_SIZE. For this message the AVI runtime driver uses <MI>lParam1<D> to point to the input format and uses <MI>lParam2<D> to point to the output format.
Your driver should return the worst case size (in bytes) that any frame can compress to. The AVI runtime driver uses this size value when it allocates buffers for the compression.
7. The AVI runtime driver sends ICM_COMPRESS for each frame it wants compressed. It uses <MI>lParam1<D> to point to an <B>ICCOMPRESS<D> structure containing the parameters used for compression.
Your driver uses the buffers pointed to by the fields of <B>ICCOMPRESS<D> for returning information about the compressed data.
Your driver returns the actual size of the compressed data in the <B>biSizeImage<D> field in the <B>BITMAPINFO<D> data structure pointed to by the <B>lpbiOutput<D> field of <B>ICCOMPRESS<D>.
Your driver returns the chunk ID in the buffer specified in the <B>lpckid<D> that it assigns to the data.
Your driver also returns a DWORD of flags in the buffer pointed to by <B>lpdwFlags<D>. The flags will be placed in the AVI index for this chunk. The following flags are defined:
AVIIF_TWOCC Specifies ckid is a TWOCC.
AVIIF_KEYFRAME Specifies this frame is a key frame.
Your driver can define its own flags but they must be set in the high word only.
If your driver handles temporal compression (that is, the compression is based on the difference between two frames), it needs to save the previous frame(s) for temporal compresssion.
8. Your device driver receives ICM_DECOMPRESS_END when the AVI runtime driver no longer needs data compressed, or when the AVI runtime driver is changing the format or palette. If the AVI runtime driver will continue compressing data, it will start the sequence by sending ICM_COMPRESS_BEGIN. When the device driver is no longer needed the system will close it by sending DRV_CLOSE.
The driver might also receive a ICM_COMPRESS_QUERY message to determine if it supports the input or output format.
COMPRESSING A BUNCH OF FRAMES -------------------------------------------
TBD
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