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yuv4mpeg - Man Page

video stream format used by pipe-based MJPEGtools


Many of the MJPEGtools communicate via pipes and act as filters (or sources or sinks).  The format of video data used in the pipes is referred to as "YUV4MPEG", or, more precisely, "YUV4MPEG2".  (The format was extended and codified during v1.5.x of the tools.)

The basic structure is a stream header followed by an unlimited number of frames.  Each frame itself consists of a header followed by video data. The headers are vaguely human-readable ASCII, but the video data is simple byte-size binary.

The MJPEGtools distribution has a C library (libmjpegutils) which contains functions for manipulating YUV4MPEG2 streams.  We recommend that you use this library rather than writing your own code if possible.  See the header file "yuv4mpeg.h" for a description of these functions.

Design Goals:



The precise description of the the YUV4MPEG2 stream format is as follows:

STREAM consists of

STREAM-HEADER consists of

FRAME consists of

FRAME-HEADER consists of

TAGGED-FIELD consists of

VALUE consists of

RATIO  consists of

Header tags fall into three categories:  optional, required and  has-default. Optional tags are completely optional and may be omitted from a header. Required tags must be present in a header.  Has-Default tags have a default value which is implied if the tag is not present. Independent of these categories, some tags allow an "unknown" value and some do not.

The supported tags for the STREAM-HEADER:

W[integer] - frame width in pixels, must be > 0 (required)

H[integer] - frame height in pixels, must be > 0 (required)

C[string] - chroma subsampling, image data format (has default)

420jpeg  - 4:2:0 with JPEG/MPEG-1 siting (default)
420mpeg2 - 4:2:0 with MPEG-2 siting
420paldv - 4:2:0 with PAL-DV siting
411      - 4:1:1, cosited
422      - 4:2:2, cosited
444      - 4:4:4 (no subsampling)
444alpha - 4:4:4 with an alpha channel
mono     - luma (Y') plane only

I[char] - interlacing specification:  (has default)

? - unknown (default)
p - progressive/none
t - top-field-first
b - bottom-field-first
m - mixed-mode: refer to 'I' tag in frame header

F[ratio] - frame-rate (has default of 0:0 == unknown)

A[ratio] - sample aspect ratio (has default of 0:0 == unknown)

X[string] - 'metadata' (optional; unparsed, but passed around)

The currently supported tags for the FRAME-HEADER:

I[string] - framing and sampling (required if-and-only-if Im is present in stream header).  Value is a string of three characters "xyz" which have the following meanings:

x: frame presentation
   t - top-field-first
   T - top-field-first and repeat
   b - bottom-field-first
   B - bottom-field-first and repeat
   1 - single progressive frame
   2 - double progressive frame (repeat)
   3 - triple progressive frame (repeat)
y: frame temporal sampling
   p - progressive (fields sampled at same time)
   i - interlaced (fields sampled at different times)
z: frame chroma-subsampling
   p - progressive (subsampling over whole frame)
   i - interlaced (each field subsampled independently)
   ? - unknown (allowed only for non-4:2:0 subsampling)

X[string] - 'metadata' (optional; unparsed, but passed around)

Note that a filter application must faithfully forward all "X" tags from  input pipe to output pipe (unless it uses one of those tags, of course). The supplied library will do this automatically if the functions y4m_copy_stream_info() and y4m_copy_frame_info() are used appropriately.

Notes on Image Data

All image data is in the CCIR-601 Y'CbCr colorspace, presented plane-by-plane in row-major order. Each sample within each plane is one octet (8-bits) in size. When all planes are present, they are transmitted in the order Y', Cb, Cr, potentially followed by an alpha/transparency mask plane (for the  444alpha chroma format).  The alpha channel data is follows the same range as the Y' luma channel:  full transparency is at 16 and full opacity is at 235.

All Y' and alpha planes consist of (height X width) octets. The size of the chroma planes depends on the subsampling mode:

Notes on Framing

For more details about reading and writing yuv4mpeg files read the sources - there are many utility programs, some simple, some complex that can serve as guides.

See Also

mjpegtools(1), yuv4mpeg.h


This manual page Copyright 2004 Matthew J. Marjanovic.


25 November 2002 MJPEG Tools Team MJPEG Tools Manual