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mpv/DOCS/tech/mpcf.txt
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2006-03-10 02:49:29 +00:00

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========================================
NUT Open Container Format DRAFT 20060207
========================================
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 <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
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:
----------------
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:
version v
stream_count v
max_distance v
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_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;
}
}
stream_header:
stream_id v
stream_class v
fourcc vb
time_base_nom v
time_base_denom v
msb_pts_shift v
max_pts_distance v
max_frame_size v
decode_delay v
reserved u(7)
fixed_fps u(1)
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_nom v
samplerate_denom v
channel_count v
}
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
}
frame_flags= flags[frame_code]
if(frame_flags&FLAG_CODED){
coded_flags v
frame_flags ^= coded_flags
}
if(frame_flags&FLAG_SIZE_MSB){
data_size_msb v
}
for(i=0; i<reserved_count[frame_code]; i++)
reserved v
if(frame_flags&FLAG_CHECKSUM){
checksum u(32)
}
data
index:
max_pts v
syncpoints v
for(i=0; i<syncpoints; i++){
syncpoint_pos_div8 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
}
}
}
last 8 reserved_bytes of index:
index_ptr u(64)
info_packet:
stream_id_plus1 v
chapter_id v
chapter_start v
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<-3){
type= "r"
value.den= -value-3
value.num s
}else{
type= "v"
}
}
syncpoint:
coded_pts v
stream = coded_pts % stream_count
global_key_pts = coded_pts/stream_count
back_ptr_div8 v
Complete definition:
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 a 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 its simpler)
note, demuxers MUST be able to deal with new and unknown headers
packet_header
startcode f(64)
forward_ptr v
if(forward_ptr > 4096)
header_checksum u(32)
packet_footer
reserved_bytes
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
packet_footer
}
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
}
}
Tag description:
----------------
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)
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.
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)
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
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
max_pts_distance
max absoloute difference of pts of new frame from last_pts in the
timebase of the stream, without a checksum after the frameheader
Note that last_pts is not necessarily the pts of the last frame
on the same stream, as it is altered by syncpoint timestamps.
max_frame_size
max frame size without a checksum after the frameheader.
MUST be <=4*max_distance. If stored value is bigger then
max_frame_size MUST be set to 4*max_distance .
stream_id
Stream identifier
stream_id MUST be < stream_count
stream_class
0 video
1 audio
2 subtiles
4 userdata
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
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
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
decode_delay MUST NOT be set higher than necessary for a codec.
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], frame_flags
Bit Name Description
1 FLAG_SIZE_MSB if set, data_size_msb is at frame header,
otherwise data_size_msb is 0
2 FLAG_CHECKSUM if set then the frame header contains a checksum
4 FLAG_KEY if set, frame is keyframe
8 FLAG_EOR if set, stream has no relevance on
presentation. (EOR)
256 FLAG_CODED if set, coded_flags are stored in the frame header.
512 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 is larger than max_frame_size or
its pts differs by more then max_pts_distance from the last frame
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
reserved_count[frame_code]
must be <256
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. (EOR
frames are NOT exempt from this rule)
width/height
MUST be set to the coded width/height, MUST NOT be 0
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, MUST NOT be 0
crc32 checksum
Generator polynomial is 0x104C11DB7. Starting value is zero.
checksum
crc32 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).
for frame headers the checksum contains the framecode byte and all
following bytes upto the checksum itself
header_checksum
checksum over the startcode and forward pointer
Syncpoint tags:
---------------
back_ptr_div8
back_ptr = back_ptr_div8 * 8 + 7
back_ptr must point to a position within 8 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
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])
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
s = max_pts % stream_count
pts = max_pts / stream_count
The highest pts in the entire file in the timebase of stream 's' .
syncpoint_pos_div8
offset from begginning of file to up to 7 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
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
Stream this info packet applies to. If zero, packet applies to whole
file.
chapter_id
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
s= chapter_start % stream_count
timestamp= chapter_start / stream_count
timestamp of start of chapter in timebase of stream 's'.
chapter_len
Length of chapter in same timebase of chapter_start.
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
"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
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
demxuxers 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)
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)
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