yuv4mpeg(5) MJPEG Tools Manual yuv4mpeg(5)NAME
YUV4MPEG2 - video stream format used by pipe-based MJPEGtools
DESCRIPTION
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 for‐
mat 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 con‐
tains 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:
o Easy to parse both via C or sh.
o Extensible; easy to add new parameters while maintaining back‐
wards compatibility.
o Simple upgrade from original "YUV4MPEG" format.
Drawbacks:
o Frame headers do not have constant size, so streams are not
seekable.
GRAMMAR
The precise description of the the YUV4MPEG2 stream format is as fol‐
lows:
STREAM consists of
- one STREAM-HEADER
- unlimited number of FRAMEs
STREAM-HEADER consists of
- magic string "YUV4MPEG2"
- unlimited number of TAGGED-FIELDs, each preceeded by a ' ' (single
space) separator
- single '\n' line terminator
FRAME consists of
- one FRAME-HEADER
- "length" octets of planar YCbCr 4:2:0 image data (If the stream is
interlaced, then the two fields per frame are interleaved, with
proper spatial ordering.)
FRAME-HEADER consists of
- magic string "FRAME"
- unlimited number of TAGGED-FIELDs, each preceeded by a ' ' (single
space) separator
- single '\n' line terminator
TAGGED-FIELD consists of
- single ASCII character tag
- VALUE (which does not contain whitespace)
VALUE consists of
- RATIO, or
- integer (base 10 ASCII representation), or
- single ascii character, or
- string (multiple ASCII characters)
RATIO consists of
- numerator (base 10 ASCII integer)
- ':' (a colon)
- denominator (base 10 ASCII integer)
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. Inde‐
pendent 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 func‐
tions y4m_copy_stream_info() and y4m_copy_frame_info() are used appro‐
priately.
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 trans‐
mitted in the order Y', Cb, Cr, potentially followed by an alpha/trans‐
parency mask plane (for the 444alpha chroma format). The alpha channel
data is follows the same range as the Y' luma channel: full trans‐
parency 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:
- 4:4:4 - (height X width) octets
- 4:2:2 - (height X width) / 2 octets
- 4:1:1 - (height X width) / 4 octets
- 4:2:0 - (height X width) / 4 octets
NOTES ON FRAMING
(More to come here.)
SEE ALSOmjpegtools(1), yuv4mpeg.h
AUTHOR
This manual page Copyright 2004 Matthew J. Marjanovic.
MJPEG Linux Square 25 November 2002 yuv4mpeg(5)
