path: root/DOCS/tech/mpcf.txt

========================================
NUT Open Container Format DRAFT 20060105
========================================



Intro:
======

Features / goals:
    (supported by the format, not necessarily by a specific implementation)

Simple
    use the same encoding for nearly all fields
    simple decoding, so slow CPUs (and embedded systems) can handle it

Extendible
    no limit for the possible values of all fields (using universal vlc)
    allow adding of new headers in the future
    allow adding more fields at the end of headers

Compact
    ~0.2% overhead, for normal bitrates
    index is <10kb per hour (1 keyframe every 3sec)
    a usual header for a file is about 100 bytes (audio + video headers together)
    a packet header is about ~1-5 bytes

Error resistant
    seeking / playback without an index
    headers & index can be repeated
    damaged files can be played back with minimal data loss and fast
    resync times



Definitions:
============

MUST    the specific part must be done to conform to this standard
SHOULD  it is recommended to be done that way, but not strictly required



Syntax:
=======

Since NUT heavily uses variable length fields, the simplest way to describe it
is using a pseudocode approach.



Conventions:
============

The data types have a name, used in the bitstream syntax description, a short
text description and a pseudocode (functional) definition, optional notes may
follow:

name    (text description)
    functional definition
    [Optional notes]

The bitstream syntax elements have a tagname and a functional definition, they
are presented in a bottom up approach, again optional notes may follow and
are reproduced in the tag description:

name:    (optional note)
    functional definition
    [Optional notes]

The in-depth tag description follows the bitstream syntax.
The functional definition has a C-like syntax.



Type definitions:
=================

f(n)    (n fixed bits in big-endian order)
u(n)    (unsigned number encoded in n bits in MSB-first order)

v   (variable length value, unsigned)
    value=0
    do{
        more_data                       u(1)
        data                            u(7)
        value= 128*value + data
    }while(more_data)

s   (variable length value, signed)
    temp                                v
    temp++
    if(temp&1) value= -(temp>>1)
    else       value=  (temp>>1)

b   (binary data or string, to be use in vb, see below)
    for(i=0; i<length; i++){
        data[i]                         u(8)
    }
    [Note: strings MUST be encoded in UTF-8]

vb  (variable length binary data or string)
    length                              v
    value                               b



Bitstream syntax:
=================

Common elements:
----------------

packet header:
    forward ptr                         v

reserved_bytes:
    for(i=0; i<forward_ptr - length_of_non_reserved; i++)
        reserved                        u(8)
    [a demuxer MUST ignore any reserved bytes
    a muxer MUST NOT write any reserved bytes, as this would make it
    impossible to add new fields at the end of packets in the future
    in a compatible way]

        Headers:

main header:
    main_startcode                      f(64)
    packet header
    version                             v
    stream_count                        v
    max_distance                        v
    max_index_distance                  v
    for(i=0; i<256; ){
        tmp_flag                        v
        tmp_fields                      v
        if(tmp_fields>0) tmp_pts        s
        if(tmp_fields>1) tmp_mul        v
        if(tmp_fields>2) tmp_stream     v
        if(tmp_fields>3) tmp_size       v
        else tmp_size=0
        if(tmp_fields>4) tmp_res        v
        else tmp_res=0
        if(tmp_fields>5) count          v
        else count= tmp_mul - tmp_size
        for(j=6; j<tmp_fields; j++){
            tmp_reserved[i]             v
        }
        for(j=0; j<count && i<256; j++, i++){
            flags[i]= tmp_flag;
            stream_id_plus1[i]= tmp_stream;
            data_size_mul[i]= tmp_mul;
            data_size_lsb[i]= tmp_size + j;
            pts_delta[i]= tmp_pts;
            reserved_count[i]= tmp_res;
        }
    }
    reserved_bytes
    checksum                            u(32)

stream_header:
    stream_startcode                    f(64)
    packet_header
    stream_id                           v
    stream_class                        v
    fourcc                              vb
    time_base_nom                       v
    time_base_denom                     v
    msb_pts_shift                       v
    decode_delay                        v
    fixed_fps                           u(1)
    reserved                            u(7)
    codec_specific_data                 vb

video_stream_header:
    stream_header
    width                               v
    height                              v
    sample_width                        v
    sample_height                       v
    colorspace_type                     v
    reserved_bytes
    checksum                            u(32)

audio_stream_header:
    stream_header
    samplerate_nom                      v
    samplerate_denom                    v
    channel_count                       v
    reserved_bytes
    checksum                            u(32)

other_stream_header:
    stream_header
    reserved_bytes
    checksum                            u(32)

        Basic Packets:

frame:
    frame_code                          f(8)
    if(stream_id_plus1[frame_code]==0){
        stream_id                       v
    }
    if(pts_delta[frame_code]==0){
        coded_pts                       v
    }
    if(flags[frame_code]&1){
        data_size_msb                   v
    }
    for(i=0; i<reserved_count[frame_code]; i++)
        reserved                        v
    data

index:
    index_startcode                     f(64)
    packet header
    stream_id                           v
    max_pts                             v
    index_length                        v
    for(i=0; i<index_length; i++){
        index_pts                       v
        index_position                  v
    }
    reserved_bytes
    checksum                            u(32)

info_frame: (optional)
    for(;;){
        id                              v
        if(id==0) break
        name= info_table[id][0]
        type= info_table[id][1]
        if(type==NULL)
            type                        vb
        if(name==NULL)
            name                        vb
        if(type=="v")
            value                       v
        else if(type=="s")
            value                       s
        else
            value                       vb
    }
    reserved_bytes
        checksum                        u(32)

info_packet: (optional)
    info_startcode                      f(64)
    packet header
    info_frame

syncpoint:
    syncpoint_startcode                 f(64)
    coded_pts                           v
    stream = coded_pts % stream_count
    global_key_pts = coded_pts/stream_count
    back_ptr                            v

            Complete definition:

file:
    file_id_string
    while(!eof && next_code != index_startcode){
        main_header
        for(i=0; i<stream_count; i++){
            if(next_packet==video_stream_header)
                video_stream_header
            else if(next_packet==audio_stream_header)
                audio_stream_header
            else
                other_stream_header
        }
        while(next_code == info_startcode){
            info_packet
        }
        while(next_code != main_startcode){
            if(next_code == syncpoint_startcode)
                syncpoint
            frame
        }
    }
    if (next_code == index_startcode){
        while(!eof){
            index
        }
        index_ptr                       u(64)
    }


Tag description:
----------------

forward_ptr
    size of the packet data (exactly the distance from the first byte
    after the forward_ptr to the first byte of the next packet)

back_ptr
    real_back_ptr = back_ptr * 8 + 7
    real_back_ptr must point to a position such that a syncpoint
    startcode begins within the next 8 bytes, and such that at least
    one keyframe for each stream lies between the syncpoint to which
    real_back_ptr points, and the current syncpoint.

global_key_pts
    After a syncpoint, last_pts of each stream is to be set to:
    last_pts[i] = convert_ts(global_key_pts, timebase[stream], timebase[i])

file_id_string
    "nut/multimedia container\0"

*_startcode
        all startcodes start with 'N'

main_startcode
    0x7A561F5F04ADULL + (((uint64_t)('N'<<8) + 'M')<<48)

stream_starcode
    0x11405BF2F9DBULL + (((uint64_t)('N'<<8) + 'S')<<48)

syncpoint_startcode
    0xE4ADEECA4569ULL + (((uint64_t)('N'<<8) + 'K')<<48)

    syncpoint_startcodes SHOULD be placed immediately before a keyframe if the
    previous frame of the same stream was a non-keyframe, unless such
    non-keyframe - keyframe transitions are very frequent

index_startcode
    0xDD672F23E64EULL + (((uint64_t)('N'<<8) + 'X')<<48)

info_startcode
    0xAB68B596BA78ULL + (((uint64_t)('N'<<8) + 'I')<<48)

version
    NUT version. The current value is 2.

max_distance
    max distance of syncpoints, the distance may only be larger if
    there is no more than a single frame between the two syncpoints. This can
    be used by the demuxer to detect damaged frame headers if the damage
    results in too long of a chain

    SHOULD be set to <=32768 or at least <=65536 unless there is a very
    good reason to set it higher, otherwise reasonable error recovery will
    be impossible

max_index_distance
    max distance of keyframes which are represented in the index, the
    distance between consecutive entries A and B may only be larger if
    there are no keyframes within this stream between A and B
    SHOULD be set to <=32768 or at least <=65536 unless there is a very
    good reason to set it higher

stream_id
    Stream identifier
    stream_id MUST be < stream_count

stream_class
    0    video
    1    audio
    2    subtiles
    3    metadata
    4    userdata
    in metadata streams each frame contains exactly one info frame
    Note: the remaining values are reserved and MUST NOT be used
          a demuxer MUST ignore streams with reserved classes

fourcc
    identification for the codec
    example: "H264"
    MUST contain 2 or 4 bytes, note, this might be increased in the future
    if needed

time_base_nom / time_base_denom = time_base
    the length of a timer tick in seconds, this MUST be equal to the 1/fps
    if fixed_fps is 1
    time_base_nom and time_base_denom MUST NOT be 0
    time_base_nom and time_base_denom MUST be relatively prime
    time_base_denom MUST be < 2^31
    examples:
        fps       time_base_nom    time_base_denom
        30        1                30
        29.97     1001             30000
        23.976    1001             24000

convert_ts
    To switch from 2 different timebases, the following calculation is
    defined:

    ln        = from_time_base_nom*to_time_base_denom
    sn        = from_timestamp
    d1        = from_time_base_denom
    d2        = to_time_base_nom
    timestamp = (ln/d1*sn + ln%d1*sn/d1)/d2
    Note: this calculation MUST be done with unsigned 64 bit integers, and
    is equivalent to (ln*sn)/(d1*d2) but this would require a 96bit integer

msb_pts_shift
    amount of bits in lsb_pts
    MUST be <16

decode_delay
    maximum time between input and output for a codec, used to generate
    dts from pts
    is set to 0 for streams without B-frames, and set to 1 for streams with
    B-frames, may be larger for future codecs

fixed_fps
    1 indicates that the fps is fixed

codec_specific_data
    private global data for a codec (could be huffman tables or ...)

frame_code
    the meaning of this byte is stored in the main header
    the value 78 ('N') is forbidden to ensure that the byte is always
    different from the first byte of any startcode

flags[frame_code]
    first of the flags from MSB to LSB are called KD
    if D is 1 then data_size_msb is coded, otherwise data_size_msb is 0
    K is the keyframe_type
        0 -> no keyframe,
        1 -> keyframe,
    flags=4 can be used to mark illegal frame_code bytes
    frame_code=78 must have flags=4
    Note: frames MUST NOT depend(1) upon frames prior to the last
          frame_startcode
    Important: depend(1) means dependency on the container level (NUT) not
    dependency on the codec level

stream_id_plus1[frame_code]
    must be <250
    if it is 0, then the stream_id is coded in the frame

data_size_mul[frame_code]
    must be <16384

data_size_lsb[frame_code]
    must be <16384

pts_delta[frame_code]
    must be <16384 and >-16384

data_size
    data_size= data_size_lsb + data_size_msb*data_size_mul;

coded_pts
    if coded_pts < (1<<msb_pts_shift) then it is an lsb
    pts, otherwise it is a full pts + (1<<msb_pts_shift)
    lsb pts is converted to a full pts by:
    mask  = (1<<msb_pts_shift)-1;
    delta = last_pts - mask/2
    pts   = ((pts_lsb-delta)&mask) + delta

lsb_pts
    least significant bits of the pts in time_base precision
        Example: IBBP display order
        keyframe pts=0                       -> pts=0
        frame                    lsb_pts=3   -> pts=3
        frame                    lsb_pts=1   -> pts=1
        frame                    lsb_pts=2   -> pts=2
        ...
        keyframe msb_pts=257                 -> pts=257
        frame                    lsb_pts=255 -> pts=255
        frame                    lsb_pts=0   -> pts=256
        frame                    lsb_pts=4   -> pts=260
        frame                    lsb_pts=2   -> pts=258
        frame                    lsb_pts=3   -> pts=259
    all pts's of keyframes of a single stream MUST be monotone

dts
    dts is calculated by using a decode_delay+1 sized buffer for each
    stream, into which the current pts is inserted and the element with
    the smallest value is removed, this is then the current dts
    this buffer is initalized with decode_delay -1 elements

    Pts of all frames in all streams MUST be bigger or equal to dts of all
    previous frames in all streams, compared in common timebase.

width/height
    MUST be set to the coded width/height

sample_width/sample_height (aspect ratio)
    sample_width is the horizontal distance between samples
    sample_width and sample_height MUST be relatively prime if not zero
    MUST be 0 if unknown

colorspace_type
     0    unknown
     1    ITU Rec 624 / ITU Rec 601 Y range: 16..235 Cb/Cr range: 16..240
     2    ITU Rec 709               Y range: 16..235 Cb/Cr range: 16..240
    17    ITU Rec 624 / ITU Rec 601 Y range:  0..255 Cb/Cr range:  0..255
    18    ITU Rec 709               Y range:  0..255 Cb/Cr range:  0..255

samplerate_nom / samplerate_denom = samplerate
    the number of samples per second

checksum
    adler32 checksum
    checksum is calculated for the area pointed to by forward_ptr not
    including the checksum itself (from first byte after the
    forward_ptr until last byte before the checksum).

max_pts
    The highest pts in the stream.

index_pts
    value of the pts of a keyframe relative to the last keyframe
    stored in this index

index_position
    position in bytes of the first byte of a keyframe, relative to the
    last keyframe stored in this index
    there MUST be no keyframe with the same stream_id as this index between
    two consecutive index entries if they are more than max_index_distance
    apart

index_ptr
    Length in bytes from the first byte of the first index startcode
    to the first byte of the index_ptr. If there is no index, index_ptr
    MUST NOT be written.

id
    the ID of the type/name pair, so it is more compact
    0 means end

type
    for example: "UTF8" -> string or "JPEG" -> JPEG image
    Note: nonstandard fields should be prefixed by "X-"
    Note: MUST be less than 6 byte long (might be increased to 64 later)

info packet types
    the name of the info entry, valid names are
    "StreamId"
        the stream(s) to which the info packet applies
    "Author"
    "Description"
    "Copyright"
    "Encoder"
        the name & version of the software used for encoding
    "Title"
    "Cover"
        image of the (CD, DVD, VHS, ..) cover (preferably PNG or JPEG)
    "Source"
        "DVD", "VCD", "CD", "MD", "FM radio", "VHS", "TV", "LD"
        Optional: appended PAL, NTSC, SECAM, ... in parentheses
    "CaptureDevice"
        "BT878", "BT848", "webcam", ... (more exact names are fine too)
    "CreationTime"
        "2003-01-20 20:13:15Z", ...
        (ISO 8601 format, see http://www.cl.cam.ac.uk/~mgk25/iso-time.html)
        Note: do not forget the timezone
    "Keywords"
    "TotalTime"
        total length of the stream in msecs
    "Language"
        ISO 639 and ISO 3166 for language/country code
        something like "eng" (US english), can be 0 if unknown
        and "multi" if several languages
        see http://www.loc.gov/standards/iso639-2/englangn.html
        and http://www.din.de/gremien/nas/nabd/iso3166ma/codlstp1/en_listp1.html
        the language code
    "Disposition"
        "original", "dub" (translated), "comment", "lyrics", "karaoke"
        Note: if someone needs some others, please tell us about them, so we
              can add them to the official standard (if they are sane)
        Note: nonstandard fields should be prefixed by "X-"
        Note: MUST be less than 64 bytes long

value
    value of this name/type pair

stuffing
    0x80 can be placed in front of any type v entry for stuffing purposes

info_table[][2]={
    {NULL            ,  NULL }, // end
    {NULL            ,  NULL },
    {NULL            , "UTF8"},
    {NULL            , "v"},
    {NULL            , "s"},
    {"StreamId"      , "v"},
    {"Author"        , "UTF8"},
    {"Title"         , "UTF8"},
    {"Language"      , "UTF8"},
    {"Description"   , "UTF8"},
    {"Copyright"     , "UTF8"},
    {"Encoder"       , "UTF8"},
    {"Keyword"       , "UTF8"},
    {"Cover"         , "JPEG"},
    {"Cover"         , "PNG"},
    {"Disposition"   , "UTF8"},
};


Structure:
----------

the headers MUST be in exactly the following order (to simplify demuxer design)
main header
stream_header (id=0)
stream_header (id=1)
...
stream_header (id=n)

headers may be repeated, but if they are, then they MUST all be repeated
together and repeated headers MUST be identical
headers MAY only repeat at the closest possible positions after 2^x where x is
an integer and the file end, so the headers may be repeated at 4102 if that is
the closest position after 2^12=4096 at which the headers can be placed

headers MUST be placed at least at the start of the file and immediately before
the index or at the file end if there is no index
headers MUST be repeated at least twice (so they exist three times in a file)

there MUST be a sync point immediately before the first frame after any headers


Index:
------

Note: with realtime streaming, there is no end, so no index there either
An index SHOULD be written for every stream. Indices MUST be placed at end
of file. Indices MAY be repeated for a stream.


Info frames:
------------

Info frames can be used to describe the file or some part of it (chapters)


Unknown packets:
----------------

MUST be ignored by the demuxer


demuxer (non-normative):
------------------------

in the absence of a valid header at the beginning, players SHOULD search for
backup headers starting at offset 2^x; for each x players SHOULD end their
search at a particular offset when any startcode is found (including syncpoint)



Semantic requirements:
======================

If more than one stream of a given stream class is present, each one SHOULD
have info tags specifying disposition, and if applicable, language.
It often highly improves usability and is therefore strongly encouraged.

A demuxer MUST NOT demux a stream which contains more than one stream, or which
is wrapped in a structure to facilitate more than one stream or otherwise
duplicate the role of a container. any such file is to be considered invalid.



Sample code (Public Domain, & untested):
========================================

typedef BufferContext{
    uint8_t *buf;
    uint8_t *buf_ptr;
}BufferContext;

static inline uint64_t get_bytes(BufferContext *bc, int count){
    uint64_t val=0;

    assert(count>0 && count<9);

    for(i=0; i<count; i++){
        val <<=8;
        val += *(bc->buf_ptr++);
    }

    return val;
}

static inline void put_bytes(BufferContext *bc, int count, uint64_t val){
    uint64_t val=0;

    assert(count>0 && count<9);

    for(i=count-1; i>=0; i--){
        *(bc->buf_ptr++)= val >> (8*i);
    }

    return val;
}

static inline uint64_t get_v(BufferContext *bc){
    uint64_t val= 0;

    for(; space_left(bc) > 0; ){
        int tmp= *(bc->buf_ptr++);
        if(tmp&0x80)
            val= (val<<7) + tmp - 0x80;
        else
            return (val<<7) + tmp;
    }

    return -1;
}

static inline int put_v(BufferContext *bc, uint64_t val){
    int i;

    if(space_left(bc) < 9) return -1;

    val &= 0x7FFFFFFFFFFFFFFFULL; // FIXME can only encode upto 63 bits currently
    for(i=7; ; i+=7){
        if(val>>i == 0) break;
    }

    for(i-=7; i>0; i-=7){
        *(bc->buf_ptr++)= 0x80 | (val>>i);
    }
    *(bc->buf_ptr++)= val&0x7F;

    return 0;
}

static int64_t get_dts(int64_t pts, int64_t *pts_cache, int delay, int reset){
    if(reset) memset(pts_cache, -1, delay*sizeof(int64_t));

    while(delay--){
        int64_t t= pts_cache[delay];
        if(t < pts){
            pts_cache[delay]= pts;
            pts= t;
        }
    }

    return pts;
}



Authors:
========

Folks from the MPlayer developers mailing list (http://www.mplayerhq.hu/).
Authors in alphabetical order: (FIXME! Tell us if we left you out)
    Beregszaszi, Alex        (alex@fsn.hu)
    Bunkus, Moritz           (moritz@bunkus.org)
    Diedrich, Tobias         (ranma+mplayer@tdiedrich.de)
    Felker, Rich             (dalias@aerifal.cx)
    Franz, Fabian            (FabianFranz@gmx.de)
    Gereoffy, Arpad          (arpi@thot.banki.hu)
    Hess, Andreas            (jaska@gmx.net)
    Niedermayer, Michael     (michaelni@gmx.at)
    Shimon, Oded             (ods15@ods15.dyndns.org)