path: root/DOCS/tech/nut.txt

==================================
NUT Open Container Format 20061104
==================================



Intro:
======

NUT is a Free multimedia container format for storage of audio, video,
subtitles and related user defined streams, it provides exact timestamps for
synchronization and seeking, is simple, has low overhead and can recover
in case of errors in the stream

other common multimedia container formats are AVI, OGG, Matroska, MP4, MOV
ASF, MPEG-PS, MPEG-TS


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 <100kb per hour
    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

The spec is frozen. All files following spec will be compatible unless the
spec is unfrozen.


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

keyframe
    A keyframe is a frame from which you can start decoding, a more
    exact definition is below
    The nth frame is a keyframe if and only if frames n, n+1, ... in 
    presentation order (that are all frames with a pts >= frame[n].pts) can
    be decoded successfully without reference to frames prior n in storage
    order (that are all frames with a dts < frame[n].dts)
    if no such frames exist (for example due to use of overlapped transforms
    like the MDCT in an audio codec) then the definition shall be extended
    by dropping n out of the set of frames which must be decodable, if this
    is still insufficient then n+1 shall be dropped, and so on until there is
    a keyframe
    every frame which is marked as a keyframe MUST be a keyframe according to
    the definition above, a muxer MUST mark every frame it knows is a keyframe
    as such, a muxer SHOULD NOT analyze future frames to determine the
    keyframe status of the current frame but instead just set the frame as
    non keyframe
    (FIXME maybe move somewhere else?)
pts
    presentation time of the first frame/sample that is completed by decoding
    the coded frame.
dts
    of a frame is the time when it is input into a synchronous 1-in-1-out
    decoder


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]
    [Note: the character NUL (U+0000) is not legal within
    or at the end of a string.]

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

t (v coded universal timestamp)
    tmp                                 v
    id= tmp % time_base_count
    value= (tmp / time_base_count) * time_base[id]


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

file:
    file_id_string
    while(!eof){
        if(next_byte == 'N'){
            packet_header
            switch(startcode){
                case      main_startcode:  main_header; break;
                case    stream_startcode:stream_header; break;
                case      info_startcode:  info_packet; break;
                case     index_startcode:        index; break;
                case syncpoint_startcode:    syncpoint; break;
            }
            packet_footer
        }else
            frame
    }

the structure of an undamaged file should look like the following, but
demuxers should be flexible and be able to deal with damaged headers so the
above is a better loop in practice (not to mention it is simpler)
note, demuxers MUST be able to deal with new and unknown headers

file:
    file_id_string
    while(!eof){
        packet_header, main_header, packet_footer
        reserved_headers
        for(i=0; i<stream_count; i++){
            packet_header, stream_header, packet_footer
            reserved_headers
        }
        while(next_code == info_startcode){
            packet_header, info_packet, packet_footer
            reserved_headers
        }
        if(next_code == index_startcode){
            packet_header, index_packet, packet_footer
        }
        if (!eof) while(next_code != main_startcode){
            if(next_code == syncpoint_startcode){
                packet_header, syncpoint, packet_footer
            }
            frame
            reserved_headers
        }
    }


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

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]

packet_header
    startcode                           f(64)
    forward_ptr                         v
    if(forward_ptr > 4096)
        header_checksum                 u(32)

packet_footer
    checksum                            u(32)

reserved_headers
    while(next_byte == 'N' && next_code !=      main_startcode
                           && next_code !=    stream_startcode
                           && next_code !=      info_startcode
                           && next_code !=     index_startcode
                           && next_code != syncpoint_startcode){
        packet_header
        reserved_bytes
        packet_footer
    }

        Headers:

main_header:
    version                             v
    stream_count                        v
    max_distance                        v
    time_base_count                     v
    for(i=0; i<time_base_count; i++)
        time_base_num                   v
        time_base_denom                 v
        time_base[i]= time_base_num/time_base_denom
    tmp_pts=0
    tmp_mul=1
    tmp_stream=0
    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++){
            if (i == 'N') {
                flags[i]= FLAG_INVALID;
                j--;
                continue;
            }
            flags[i]= tmp_flag;
            stream_id[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

stream_header:
    stream_id                           v
    stream_class                        v
    fourcc                              vb
    time_base_id                        v
    msb_pts_shift                       v
    max_pts_distance                    v
    decode_delay                        v
    stream_flags                        v
    codec_specific_data                 vb
    if(stream_class == video){
        width                           v
        height                          v
        sample_width                    v
        sample_height                   v
        colorspace_type                 v
    }else if(stream_class == audio){
        samplerate_num                  v
        samplerate_denom                v
        channel_count                   v
    }
    reserved_bytes

        Basic Packets:

frame:
    frame_code                          f(8)
    frame_flags= flags[frame_code]
    if(frame_flags&FLAG_CODED){
        coded_flags                     v
        frame_flags ^= coded_flags
    }
    if(frame_flags&FLAG_STREAM_ID){
        stream_id                       v
    }
    if(frame_flags&FLAG_CODED_PTS){
        coded_pts                       v
    }
    if(frame_flags&FLAG_SIZE_MSB){
        data_size_msb                   v
    }
    if(frame_flags&FLAG_RESERVED)
        reserved_count[frame_code]      v
    for(i=0; i<reserved_count[frame_code]; i++)
        reserved                        v
    if(frame_flags&FLAG_CHECKSUM){
        checksum                        u(32)
    }
    data

index:
    max_pts                             t
    syncpoints                          v
    for(i=0; i<syncpoints; i++){
        syncpoint_pos_div16             v
    }
    for(i=0; i<stream_count; i++){
        last_pts= -1
        for(j=0; j<syncpoints; ){
            x                           v
            type= x & 1
            x>>=1
            n=j
            if(type){
                flag= x & 1
                x>>=1
                while(x--)
                    has_keyframe[n++][i]=flag
                has_keyframe[n++][i]=!flag;
            }else{
                while(x != 1){
                    has_keyframe[n++][i]=x&1;
                    x>>=1;
                }
            }
            for(; j<n && j<syncpoints; j++){
                if (!has_keyframe[j][i]) continue
                A                           v
                if(!A){
                    A                       v
                    B                       v
                    eor_pts[j][i] = last_pts + A + B
                }else
                    B=0
                keyframe_pts[j][i] = last_pts + A
                last_pts += A + B
            }
        }
    }
    reserved_bytes
    index_ptr                           u(64)

info_packet:
    stream_id_plus1                     v
    chapter_id                          s (Note due to a typo this was v
                                           until 2006-11-04)
    chapter_start                       t
    chapter_len                         v
    count                               v
    for(i=0; i<count; i++){
        name                            vb
        value                           s
        if (value==-1){
            type= "UTF-8"
            value                       vb
        }else if (value==-2){
            type                        vb
            value                       vb
        }else if (value==-3){
            type= "s"
            value                       s
        }else if (value==-4){
            type= "t"
            value                       t
        }else if (value<-4){
            type= "r"
            value.den= -value-4
            value.num                   s
        }else{
            type= "v"
        }
    }
    reserved_bytes

syncpoint:
    global_key_pts                      t
    back_ptr_div16                      v
    reserved_bytes

            Complete definition:


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

file_id_string
    "nut/multimedia container\0"
    the very first thing in every nut file, useful for identifying nut
    files

*_startcode (f(64))
    all startcodes start with 'N'

main_startcode (f(64))
    0x7A561F5F04ADULL + (((uint64_t)('N'<<8) + 'M')<<48)

stream_startcode (f(64))
    0x11405BF2F9DBULL + (((uint64_t)('N'<<8) + 'S')<<48)

syncpoint_startcode (f(64))
    0xE4ADEECA4569ULL + (((uint64_t)('N'<<8) + 'K')<<48)

index_startcode (f(64))
    0xDD672F23E64EULL + (((uint64_t)('N'<<8) + 'X')<<48)

info_startcode (f(64))
    0xAB68B596BA78ULL + (((uint64_t)('N'<<8) + 'I')<<48)

version (v)
    NUT version. The current value is 3. All lower values are pre-freeze

stream_count (v)
    number of streams in this file

time_base_count (v)
    number of different time bases in this file
    this MUST NOT be 0

forward_ptr (v)
    size of the packet data (exactly the distance from the first byte
    after the packet_header to the first byte of the next packet)
    every nut packet contains a forward_ptr immediately after its startcode
    with the exception of frame_code based packets, the forward pointer
    can be used to skip over the packet without decoding its contents

max_distance (v)
    max distance between startcodes. If p1 and p2 are the byte
    positions of the first byte of two consecutive startcodes, then
    p2-p1 MUST be less than or equal to max_distance unless the entire
    span from p1 to p2 comprises a single packet or a syncpoint
    followed by a single frame. This imposition places efficient upper
    bounds on seek operations and allows for the detection of damaged
    frame headers, should a chain of frame headers pass max_distance
    without encountering any startcode.

    syncpoints 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

    SHOULD be set to <=32768
    if the stored value is >65536 then max_distance MUST be set to 65536

    This is also half the max frame size without a checksum after the
    frameheader.


max_pts_distance (v)
    max absolute difference of pts of new frame from last_pts in the
    timebase of the stream, without a checksum after the frameheader.
    A frame header MUST include a checksum if abs(pts-last_pts) is
    strictly greater than max_pts_distance.
    Note that last_pts is not necessarily the pts of the last frame
    on the same stream, as it is altered by syncpoint timestamps.
    SHOULD NOT be higher than 1/timebase

stream_id (v)
    Stream identifier
    stream_id MUST be < stream_count

stream_class (v)
    0    video
    1    audio
    2    subtitles
    3    userdata
    Note: the remaining values are reserved and MUST NOT be used
          a demuxer MUST ignore streams with reserved classes

fourcc (vb)
    identification for the codec
    example: "H264"
    MUST contain 2 or 4 bytes, note, this might be increased in the future
    if needed
    the id values used are the same as in avi, so if a codec uses a specific
    fourcc in avi then the same fourcc MUST be used here

time_base_num (v) / time_base_denom (v) = time_base
    the length of a timer tick in seconds, this MUST be equal to the 1/fps
    if FLAG_FIXED_FPS is set
    time_base_num and time_base_denom MUST NOT be 0
    time_base_num and time_base_denom MUST be relatively prime
    time_base_denom MUST be < 2^31
    examples:
        fps       time_base_num    time_base_denom
        30        1                30
        29.97     1001             30000
        23.976    1001             24000
    There MUST NOT be 2 identical timebases in a file.
    There SHOULD NOT be more timebases than streams.

time_base_id (v)
    index into the time_base table
    MUST be < time_base_count

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

    ln        = from_time_base_num*to_time_base_denom
    sn        = from_timestamp
    d1        = from_time_base_denom
    d2        = to_time_base_num
    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

compare_ts
    Compares timestamps from 2 different timebases,
    if a is before b then compare_ts(a, b) = -1
    if a is after  b then compare_ts(a, b) =  1
    else                  compare_ts(a, b) =  0

    Care must be taken that this is done exactly with no rounding errors,
    simply casting to float or double and doing the obvious
    a*timebase > b*timebase is not compliant or correct, neither is the
    same with integers, and
    a*a_timebase.num*b_timebase.den > b*b_timebase.num*a_timebase.den
    will overflow. One possible implementation which shouldn't overflow
    within the range of legal timestamps and timebases is:

    if (convert_ts(a, a_timebase, b_timebase) < b) return -1;
    if (convert_ts(b, b_timebase, a_timebase) < a) return  1;
    return 0;

msb_pts_shift (v)
    amount of bits in lsb_pts
    MUST be <16

decode_delay (v)
    size of the reordering buffer used to convert pts to dts
    codecs which don't support b frames normally use 0
    mpeg1/mpeg2 style codecs with b frames use 1
    h264 style b pyramid uses 2
    h264 and future codecs might need values >2
    audio codecs generally use 0 (we aren't aware of any which doesn't
    but its theoretically possible that one exists which needs it >0)
    decode_delay MUST NOT be set higher than necessary for a codec.

stream_flags (v)
     Bit  Name            Description
       1  FLAG_FIXED_FPS  indicates that the fps is fixed

codec_specific_data (vb)
    private global data for a codec (could be huffman tables or ...)
    if a codec has a global header it SHOULD be placed in here instead of
    at the start of every keyframe
    the exact format is specified in the codec spec
    for H.264 the NAL units MUST be formatted as in a bytestream 
    (with 00 00 01 prefixes)
    codec_specific_data SHOULD contain exactly the essential global packets
    needed to decode a stream, more specifically it SHOULD NOT contain packets
    which contain only non essential metadata like author, title, ...
    it also MUST NOT contain normal packets which cause the reference decoder
    to generate any specific decoded samples
    the encoder name and version, shall be considered essential as it is very
    usefull to workaround possible encoder bugs
    The global headers MUST consist of the normal
    sequence of header packets required for codec initialization, in the
    order defined in the codec spec. An implementation MAY strip metadata and
    other redundant information not necessary for correct playback from the
    global headers as long as no incorrect values are stored and as long as
    the striped result is not less valid per codec spec as before striping

frame_code (f(8))
    frame_code is an 8-bit field which exists before every frame, it can
    store part of the size of the frame, the stream number, the timestamp
    and some flags amongst other things, what is not directly stored
    in it but is needed is stored in various fields immediately after it
    the values stored in it can be found 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
    a muxer SHOULD mark 0x00 and 0xFF as invalid to improve error
    detection

flags[frame_code], frame_flags (v)
     Bit  Name             Description
       0  FLAG_KEY         if set, frame is keyframe
       1  FLAG_EOR         if set, stream has no relevance on
                           presentation. (EOR)
       3  FLAG_CODED_PTS   if set, coded_pts is in the frame header
       4  FLAG_STREAM_ID   if set, stream_id is coded in the frame header
       5  FLAG_SIZE_MSB    if set, data_size_msb is at frame header,
                           otherwise data_size_msb is 0
       6  FLAG_CHECKSUM    if set then the frame header contains a checksum
       7  FLAG_RESERVED    if set, reserved_count is coded in the frame header
      12  FLAG_CODED       if set, coded_flags are stored in the frame header.
      13  FLAG_INVALID     if set, frame_code is invalid.

    EOR frames MUST be zero-length and must be set keyframe.
    All streams SHOULD end with EOR, where the pts of the EOR indicates the
    end presentation time of the final frame.
    An EOR set stream is unset by the first content frames.
    EOR can only be unset in streams with zero decode_delay .
    FLAG_CHECKSUM MUST be set if the frame's data_size is strictly greater than
    2*max_distance or the difference abs(pts-last_pts) is strictly greater than
    max_pts_distance (where pts represents this frame's pts and last_pts is
    defined as below).

last_pts
    the timestamp of the last frame with the same stream_id as the current
    if there is no such frame between the last syncpoint and the current
    frame then the syncpoint timestamp is used, see global_key_pts

stream_id[frame_code] (v)
    if FLAG_STREAM_ID is not set then this is the stream number for the
    frame following this frame_code
    if FLAG_STREAM_ID is set then this value has no meaning
    MUST be <250

data_size_mul[frame_code] (v)
    if FLAG_SIZE_MSB is set then data_size_msb which is stored after the
    frame code is multiplied with it and forms the more significant part
    of the size of the following frame
    if FLAG_SIZE_MSB is not set then this field has no meaning
    MUST be <16384

data_size_lsb[frame_code] (v)
    the less significant part of the size of the following frame
    this added together with data_size_mul*data_size_msb is the size of
    the following frame
    MUST be <16384

pts_delta[frame_code] (s)
    MUST be <16384 and >-16384
    if FLAG_CODED_PTS is set in the flags of the current frame then this
    value MUST be ignored, if FLAG_CODED_PTS is not set then pts_delta is the
    difference between the current pts and last_pts

reserved_count[frame_code] (v)
    MUST be <256

data_size
    the size of the following frame
    data_size= data_size_lsb + data_size_msb*data_size_mul;

coded_pts (v)
    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
    decoding timestamp
    the dts of a frame is the timestamp of the first sample which is
    output by a decoder when it is fed with the frame, note that the
    data output is not necessarily what is coded in the frame, but may
    be data from previous frames
    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 initialized 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. (EOR
    frames are NOT exempt from this rule)
    Dts of all frames MUST be bigger or equal to dts of all previous frames
    in the same stream

width (v) / height (v)
    width and height of the video in pixels
    MUST be set to the coded width/height, MUST NOT be 0

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

colorspace_type (v)
     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_num (v) / samplerate_denom (v) = samplerate
    the number of samples per second, MUST NOT be 0

crc32 checksum
    Generator polynomial is 0x104C11DB7. Starting value is zero.

checksum (u(32))
    crc32 checksum
    checksum is calculated for the area pointed to by forward_ptr not
    including the checksum itself (from first byte after the
    packet_header until last byte before the checksum).
    for frame headers the checksum contains the framecode byte and all
    following bytes up to the checksum itself

header_checksum (u(32))
    checksum over the startcode and forward pointer

Syncpoint tags:
---------------

back_ptr_div16 (v)
    back_ptr = back_ptr_div16 * 16 + 15
    back_ptr must point to a position within 16 bytes of a syncpoint
    startcode. This syncpoint MUST be the closest syncpoint such that at
    least one keyframe with a pts lower or equal to the original syncpoint's
    global_key_pts for all streams lies between it and the current syncpoint.

    A stream where EOR is set is to be ignored for back_ptr.

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

    global_key_pts MUST be bigger or equal to dts of all past frames across
    all streams, and smaller or equal to pts of all future frames.

Index tags:
-----------

max_pts (t)
    The highest pts in the entire file

syncpoints (v)
    number of indexed syncpoints

syncpoint_pos_div16 (v)
    offset from beginning of file to up to 15 bytes before the syncpoint
    referred to in this index entry. Relative to position of last
    syncpoint.

has_keyframe
    indicates whether this stream has a keyframe between this syncpoint and
    the last syncpoint.

keyframe_pts
    The pts of the first keyframe for this stream in the region between the
    2 syncpoints, in the stream's timebase. (EOR frames are also keyframes)

eor_pts
    Coded only if EOR is set at the position of the syncpoint. The pts of
    that EOR. EOR is unset by the first keyframe after it.

index_ptr (u(64))
    Length in bytes of the entire index, from the first byte of the
    startcode until the last byte of the checksum.
    Note: A demuxer can use this to find the index when it is written at
    EOF, as index_ptr will always be 12 bytes before the end of file if
    there is an index at all.


Info tags:
----------

stream_id_plus1 (v)
    Stream this info packet applies to. If zero, packet applies to whole
    file.

chapter_id (s)
    Id of chapter this packet applies to. If zero, packet applies to whole
    file. Positive chapter_id's are real chapters and MUST NOT overlap.
    Negative chapter_id indicate a sub region of file and not a real
    chapter. chapter_id MUST be unique to the region it represents.
    chapter_id n MUST not be used unless there are at least n chapters in the
    file

chapter_start (t)
    timestamp of start of chapter

chapter_len (v)
    Length of chapter in same timebase of chapter_start.

count (v)
    number of name/value pairs in this info packet

type
    for example: "UTF8" -> string or "JPEG" -> JPEG image
    "v" -> unsigned integer
    "s" -> signed integer
    "r" -> rational
    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
    "Author"
    "Description"
    "Copyright"
    "Encoder"
        the name & version of the software used for encoding
    "Title"
    "Cover" (allowed types are "PNG" and "JPEG")
        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
    "SourceContainer"
        "nut", "mkv", "mov", "avi", "ogg", "rm", "mpeg-ps", "mpeg-ts", "raw"
    "SourceCodecTag"
        the source codec id like a fourcc which was used to store a specific
        stream in its SourceContainer
    "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"
    "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: names of fields SHOULD be in English if a word with the same
              meaning exists in English
        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
    except the forward_ptr and all fields in the frame header where a
    maximum of 8 stuffing bytes per field are allowed


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

Each set of repeated headers not at the beginning or end of the file SHOULD
be stored at the earliest possible position 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

Note: this allows an implementation reading the file to locate backup
headers in O(log filesize) time as opposed to O(filesize)

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
Index MAY only be repeated after main headers.
If an index is written anywhere in the file, it MUST be written at end of
file as well.


Info:
-----

If a info packet is stored anywhere then a muxer MUST also store an identical
info packet after every main-stream-header set

If a demuxer has seen several info packets with the same chapter_id and
stream_id then it MUST ignore all but the one with the highest position in
the file

demuxers SHOULD NOT search the whole file for info packets

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)


Seeking without an index (non-normative):
-----------------------------------------
A. backward seeking
    1. perform a binary search on the syncpoint timestamps finding the one
    which is largest and <= the target timestamp
B. forward seeking
    1a. perform a binary search on the syncpoint timestamps finding the one
    which is smallest and >= the target timestamp
    1b. perform a binary search on the syncpoint back pointers finding the
    smallest one which has a back ptr >= the position of what was found in 1.
2. follow the back pointer to the corresponding syncpoint

Seeking with an index (non-normative):
--------------------------------------
The demuxer only has to find the appropriate keyframe in the index and
start demuxing from the previous syncpoint

Note, more complicated seeking methods exist which are capable of quickly
seeking to the optimal point in the presence of an index even if only a
subset of all streams is active

A muxer SHOULD place syncpoints so that that simple low complexity seeking
works with fine granularity, that is syncpoints should be placed prior
to keyframes instead of non keyframes and with high enough frequency 
(once per second unless there are no keyframes between this and the previous
syncpoint)

encoders SHOULD place keyframes so that the number of points where all
streams have a keyframe at the same time is maximized, this ensures that
seeking (complicated or not) does not need to demux and decode significant
amounts of data to reach a point where a presentable frame for each stream
is available after seeking


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.
for example vorbis in ogg in nut is invalid, as is 
mpegvideo+mpegaudio in mpeg-ps/ts in nut or dvvideo + dvaudio in dv in nut



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 up to 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)