The Metadata in RIFF (.avi) files (Under construction)¶

Please note: Four bytes words in caps have special significance in this document, which are called Chunk headers, and appear exactly same in the original RIFF file.

This page brief about the Riff(Resource Interchange File Format) type video file container, popular video files using Riff container being AVI files.

Why RIFF?¶

Since video files most likely to have multiple data chunks like video,audio,subtitle,metadata which essentially describe about the video,it's been convenient to have structured data rather than in a random order.

What is RIFF?¶

RIFF is a structured video container format Issued as a joint design by IBM Corporation and Microsoft Corporation in 1991.

How it is structured?¶

Everything in RIFF is structured in Chunk, sub-chunk hierarchy. Every chunk contains ChunkID, ChunkSize, and ChunkData. First four bytes of a RIFF file type must be ASCII values corresponding to letters RIFF. This is the ChunkId parent of every other sub-chunk. Rules dominating RIFF structure which describe how a typical RIFF file are:

• Every chunk should have ChunkId(four bytes), ChunkSize(four bytes). Do not include size of ChunkId and ChunkSize in ChunkSize.
• ChunkData having odd number of bytes should be padded with one more dummy byte,and ChunkSize is always even.
• ChunkData can be simple data like video frames (encoded or plane), audio samples or another Chunk having similar structure as parent Chunk. Hence ChunkData can have sub-chunks and it is recursive, which means LIST chunk can contain another LIST chunk.
• Every chunk will have data which has to be decoded according to either predefined schema(Fixed size structured metadata like AVIH,*STRH* primitive chunks) or variable size metadata chunks like INFO main Chunk.

Following picture is a typical RIFF file.

Here Tags RIFF,AVI (notice single space after AVI),LIST,hdrl,movi are four byte character codes,and are present right in the beginning of the chunks and appear exactly same(corresponding ASCII) in the file.Following are the some of the predefined Primitive chunk Structure.

AVIH primitive Chunk (only child of hdrl main Header Chunk)¶

Byte Number	Metadata	Description
0 to 3	avih	Primitive Header tag Signifying start of avih chunk
4 to 7	size	56(fixed)
8 to 11	Microseconds Per Frame	Video related Data
12 to 15	Maximum Bytes Per Second	Video related Data
16 to 19	Padding Granularity	Video related Data
20 to 23	Flags	Video related Data
24 to 27	Total Frames	Video related Data
28 to 31	Initial Frames	Video related Data
32 to 35	Streams	Video related Data
36 to 39	Suggested Buffer Size	Video related Data
40 to 43	Width	Video Dimention
44 to 47	Height	Video Dimention
48 to 63	Reserved	Reserved

For More Detail refer http://msdn.microsoft.com/en-us/library/windows/desktop/dd318183(v=vs.85).aspx

STRH Primitive Chunk (First child of STRL main Header Chunk)¶

Byte Number	Metadata	Description
0 to 3	strh	Stream Header Chunk
4 to 7	size	56
8 to 11	Type,'auds' 'mids','txts' 'vids'	Define Data about either of Audio,Midi text or Video
12 to 15	Handler	For Audio and Video, This will be codec name which will be used by Audio/Video Players
16 to 19	Flags	Defines whether Stream should be enabled/disabled
20 to 23	Priority	Stream priority
24 to 27	Language	Stream Language
28 to 31	Initial Frames	Significant data for interleaved Files,specifies Audio position relative to Video
32 to 35	Scale	Used internally for calculations
36 to 39	Rate	Stream Rate
40 to 43	Start	Specifies Starting Time of the Stream
44 to 47	Length	Stream Length
48 to 51	Suggested Buffer Size	Used by Video Players
52 to 55	Quality	Stream Quality
56 to 59	Sample Size	Size of Single sample
60 to 63	Frame	Rectangle position ,upper Left corner

For More Detail refer http://msdn.microsoft.com/en-us/library/windows/desktop/dd318183(v=vs.85).aspx

STRF Primitive Chunk (Second child of STRL main Header Chunk located right after STRH)¶

-------This is Brother of 'vids' type STRH Header--------

Byte Number	Metadata	Description
0 to 3	STRF	Stream format Header Flag
4 to 7	size 40	size for Video
8 to 11	Width	Number of Horizontal pixels
12 to 15	Height	Number of Vertical pixels
16 to 19	Bit Planes(1)	Number of planes for the target device
20 to 23	Bits Per Pixel	Average bits per Pixels for compressed
24 to 27	Bits Compression	Image compression
28 to 31	Image Size	Image Size
32 to 35	X Pixels per Meter	Horizontal resolution
36 to 39	Y Pixels per Meter	Vertical Resolution
40 to 43	Color indices	Number of color indices in the color table
44 to 49	Number of Imp. Color	Number of Important color Indices in Bitmap

-------This is Brother of 'auds' type STRH Header--------

Byte Number	Metadata	Description
0 to 3	STRF	Stream format Header Flag
4 to 7	size(16/20)	For Audio
8 to 9	Compressor	Predefined Code of Compressor used
10 to 11	Channels	Number of channels Present in Audio Stream
12 to 13	Sample Rate	Number of Sample to be played in One second
14 to 15	Bytes Per Sec	Average Bytes Per One Second
16 to 17	Block Align	How Audio Stream Blocks are aligned
18 to 19	Bits per Sample	Size of one Sample in Bits

IDIT Main Header¶

IDIT tags can Exist independently inside a LIST Chunk

Byte Number	Metadata	Description
0 to 3	IDIT	IDIT Header Flag
4 to 7	size	Size of IDIT Chunk
8 to size	<Data>	Data which can be decoded as String Value

INFO Main Header Chunk¶

Byte Number	Metadata	Description
0 to 3	INFO	Info Header Tag
4 to 7	Info Primitive Tag	Predefined Tag Header
8 to 11	size	Size of Info Primitive Chunk
12 to 12+size	Info Primitive Data	String Array of Metadata
12+size+1 to 12+size+4	Info Primitive Chunk	Optional Second Info Primitive Chunk
...	size	Size of Second Info Primitive Chunk

Info is slightly special Metadata Chunk,because we can store variable sized metadata in here.These are basically user related data and video subjects related information.Here we have flexibility to add tags,having standard tag Ids like Comment,Copyright,Location Description,Genre,Country and more.

In Exiv2, all video files are treated as Image Files and the RIFF read flow is as follows:¶

Write-Flow¶