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==================================
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)
Simplicity
Use the same encoding for nearly all fields.
Simple decoding, so slow CPUs (and embedded systems) can handle it.
Extensibility
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.
Compactness
~0.2% overhead for normal bitrates.
The index is <100kb per hour.
A typical file header is about 100 bytes (audio + video headers together).
A packet header is about ~1-5 bytes.
Error resistance
Seeking / playback is possible without an index.
Headers & index can be repeated.
Damaged files can be played back with minimal data loss and fast
resynchronization times.
The specification is frozen. All files following the specification will be
compatible unless the specification 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 using 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
The time when a frame 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]
frame_res= reserved_count[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)
frame_res v
for(i=0; i<frame_res; 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)
maximum 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 maximum frame size without a checksum after the
frame header.
max_pts_distance (v)
Maximum absolute difference of the pts of the new frame from last_pts in
the timebase of the stream, without a checksum after the frame header.
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 96 bit 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 do not support B-frames normally use 0.
MPEG-1/MPEG-2-style codecs with B-frames use 1.
H.264-style B-pyramid uses 2.
H.264 and future codecs might need values >2.
Audio codecs generally use 0. (We are not aware of any, but it
is theoretically possible that a codec might need a value >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 specification.
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
useful to work around 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 stripped result is not less valid per codec spec as before stripping.
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, the frame is a keyframe.
1 FLAG_EOR If set, the 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 at the frame header,
otherwise data_size_msb is 0.
6 FLAG_CHECKSUM If set, 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)
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.
MUST be <16384 and >-16384.
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 values 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 nonzero.
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
The 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 up to 15 bytes before a syncpoint
startcode, relative to position of current syncpoint. The syncpoint
pointed to MUST be the closest syncpoint such that at least one keyframe
with a pts lower or equal to the current 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)
The offset from the beginning of the 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 the
whole file.
chapter_id (s)
The ID of the chapter this packet applies to. If zero, the packet applies
to the whole file. Positive chapter_id values represent real chapters and
MUST NOT overlap.
A negative chapter_id indicates a sub region of the 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 the same timebase as 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", ... (or more precise names)
"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"
An ISO 639-2 (three-letter) language code, optionally followed by an
ISO 3166-1 country code that is separated from the language
code by a hyphen. All codes defined in ISO 639-2 are allowed,
including "und" (Undetermined), "mul" (Multiple languages).
See http://www.loc.gov/standards/iso639-2/
and http://www.din.de/gremien/nas/nabd/iso3166ma/codlstp1/en_listp1.html
the language code
A demuxer MUST ignore unknown language and country codes instead of
treating them as an error.
"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.
Exceptions are 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 end of the file. 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 end of the file if there is no index.
Headers MUST be repeated at least twice (so they exist three times in a file).
There MUST be a syncpoint 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 an 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 (including a syncpoint) is
found.
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 ATM.
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)
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