yuv4mpeg; man page

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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

          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)

              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




MJPEG Linux Square             25 November 2002                    yuv4mpeg(5)
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