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nut format draft 0.02
Intro:
Features / goals:
(supported by the format, not necessary by a specific implementation)
Simple
use the same encoding for nearly all fields
simple decoding, so slow cpus can handle it
Extendible
no limit for the possible values for 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)
Error resistant
seeking / playback without an index
headers & index can be repeated
audio packet reshuffle
checksums to allow quick redownloading of damaged parts
Definitions:
MUST the specific part must be done to conform to this standard
SHOULD its recommanded to be done that way but its not strictly required
Syntax:
Type definitions:
v
value=0
do{
more_data u(1)
data u(7)
value= 128*value + data
}while(more_data)
b (binary data or string)
length v
for(i=0; i<length; i++){
data[i] u(8)
}
Note: strings MUST be encoded in utf8
f(x) n fixed bits
u(x) unsigned number encoded in x bits in MSB first order
Bitstream syntax:
packet header
forward ptr v
backward ptr v
align_byte
while(not byte aligned)
one f(1)
reserved_bytes
for(i=0; i<forward_ptr - length_of_non_reserved; i++)
reserved u(8)
main header:
packet header
main_startcode f(64)
version v
stream_count v
file_size v
length_in_msec v
reserved_bytes
checksum u(32)
stream_header:
packet_header
stream_startcode f(64)
stream_id v
stream_class v
fourcc b
average_bitrate v
language_code b
time_base_nom v
time_base_denom v
lsb_timestamp_length v
fixed_fps u(1)
index_flag u(1)
reserved u(6)
for(;;){
codec_specific_data_type v
if(codec_specific_data_type==0) break;
codec_specific_data b
}
video_stream_header:
stream_header
width v
height v
sample_width v
sample_height v
colorspace_type v
depth v
reserved_bytes
checksum u(32)
audio_stream_header:
stream_header
samplerate v
channel_count v
reserved_bytes
checksum u(32)
frame
packet header
if(keyframe){
keyframe_startcode f(64)
}
zero_bit f(1)
priority u(2)
checksum_flag u(1)
msb_timestamp_flag u(1)
subpacket_type u(2)
reserved u(1)
lsb_timestamp v
stream_id v
if(msb_timestamp_flag)
msb_timestamp v
if(sub_packet_type==00)
sub_packet[0]
else{
subpacket_count v
shuffle_type v
if(subpacket_type==10){
subpacket_base_size v
for(i=0; i<subpacket_count; i++)
subpacket_size_diff[i] v
}
for(i=0; i<subpacket_count; i++)
subpacket[i]
}
}
if(checksum_flag)
frame_checksum u(32)
Index:
packet header
index_startcode f(64)
stream_id v
index_length v
for(i=0; i<index_length; i++){
index_timestamp v
index_position v
}
reserved_bytes
checksum u(32)
info_packet: (optional)
packet header
info_startcode f(64)
start_time v
end_time v
start_stream_id v
end_stream_id v
for(;;){
type b
if(type=="") break;
name b
value b
}
reserved_bytes
checksum u(32)
stuffing_packet: (optional)
packet_header
stuffing_startcode f(64)
for(i=0; i<forward_ptr - length_of_non_reserved; i++)
stuffing f(8)
forward_ptr
backward_ptr
pointer to the next / previous packet
pointers are relative and backward pointer is implicitelly negative
Note: a frame with 0 bytes means that its skiped
Note: the forward pointer is equal to the size of this packet including
the header
the backward pointer is equal to the size of the previous packet
Example:
0
size1 (size of frame1 including header)
frame1
size1
size2
frame2
size2
size3
frame3
*_startcode
the first bit is allways set
version
0 for now
file_size
size in bytes, can be 0 if not meaningfull (realtime streams, ...)
length_in_msec
length of the file in milli seconds (can be 0 if realtime or such)
index_flag
1 indicates that this file has an index
Note, all files SHOULD have an index at the end except, (realtime) streams
stream_id
Note: streams with a lower relative class MUST have a lower relative id
so a stream with class 0 MUST allways have a id which is lower then any
stream with class > 0
streams should use low ids
stream_class
0 video
32 audio
64 subtiles
Note the remaining values are reserved and MUST NOT be used
a decoder MUST ignore streams with reserved classes
fourcc
identification for the codec
example: "H264"
MUST contain 4 bytes, note, this might be increasd in the future if
needed
language_code
ISO 639 and ISO 3166 for language/country code
something like "usen" (US english), can be 0
if unknown
see http://www.loc.gov/standards/iso639-2/englangn.html
and http://www.din.de/gremien/nas/nabd/iso3166ma/codlstp1/en_listp1.html
time_base_nom / time_base_denom = time_base
the number of timer ticks per second, this MUST be equal to the fps
if the fixed_fps is 1
time_base_denom MUST not be 0
time_base_nom and time_base_denom MUST be relative prime
time_base_nom MUST be < 2^15
examples:
fps time_base_nom time_base_denom
30 30 1
29.97 30000 1001
23.976 24000 1001
lsb_timestamp_length
length in bits of the lsb_timestamp
MUST be <16
fixed_fps
1 indicates that the fps is fixed
codec_specific_data_type
0 end
1 native
2 bitmapinfoheader
3 waveformatex
4 imagedesc
5 sounddesc
"native", means a simple api & container independanet storage form,
for example some mpeg4-es headers
codec_specific_data
private global data for a codec (could be huffman tables or ...)
msb_timestamp_flag
indicates that the msb_timestamp is coded
MUST be 1 for keyframes
subpacket_type
00 1 subpacket per packet (normally used for video)
01 subpacket_count fixed length subpackets per packet
10 subpacket_count variable length subpackets per packet
11 reserved
Note, if there are multiple subpackets then the timestamp of the packet
is the timestamp of the first subpacket
Note, if multiple subpackets are stored in one frame then the resulting
framesize SHOULD be < 16kbyte and not more then 0.5 sec of data SHOULD
be put in a single packet
subpacket_base_size
an offset which should be added to the subpacket_size_diff of each
subpacket to get the actual size, so its normally the size of the
smallest subpacket
for fixed length subpackets, the size is calculated from the
subpacket_count
subpacket_count
the number of subpackets, if not pressent then 1
subpacket_size_diff
the (allways positive) difference from the subpacket_base_size to the
actual size of the current subpacket, if its not coded
(subpacket_type != 10) then its 0
Note a subpacket MUST be in exactly one packet, it cannot be split
msb_timestamp
most significant bits of the timestamp, SHOULD be 0 for the first frame
lsb_timestamp
most significant bits of the timestamp in time_base precission, with
lsb_timestamp_length bits
Example: IBBP display order
keyframe msb_timestamp=0 lsb_timestamp=0 -> timestamp=0
frame lsb_timestamp=3 -> timestamp=3
frame lsb_timestamp=1 -> timestamp=1
frame lsb_timestamp=2 -> timestamp=2
...
keyframe msb_timestamp=1 lsb_timestamp=1 -> timestamp=257
frame msb_timestamp=0 lsb_timestamp=255->timestamp=255
frame msb_timestamp=1 lsb_timestamp=0 -> timestamp=256
frame lsb_timestamp=4 -> timestamp=260
frame lsb_timestamp=2 -> timestamp=258
frame lsb_timestamp=3 -> timestamp=259
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 relative prime if not zero
MUST be 0 if unknown
depth
for compatibility with some win32 codecs
zero_bit
MUST be 0, its there to distinguish non keyframes from other packets,
Note: all packets have a 64-bit startcode except non-keyframes to reduce
their size, and all startcodes start with a 1 bit
priority
if 0 then the frame isnt used as reference (b frame) and can be droped
MUST be > 0 for keyframes
shuffle_type
audio is often encoded in small subpackets, and to increase the
error robustness these can be shuffled
0 -> no shuffle
1-16 -> interleave packets by 2^n
checksum
crc32 checksum using the generator polynomial 0x104c11db7 (same as ogg)
checksum_flag
indicates that the frame_checksum is coded
must be 1 for the last non keyframe before a keyframe
frame_checksum
identical to checksum, but instead of covering just the current
packet, it covers all frames of the same stream id since the last
frame_checksum
this field is only coded if checksum_flag=1
index_timestamp
value in time_base precission, relative to the last index_timestamp
index_position
position in bytes of the first byte of the keyframe header, relative
to the last index_position
start_time, stop_time
the time range in msecs to which the info applies
Note: can be used to mark chapters
start_stream_id / end_stream_id
the stream(s) to which the info packet applies
type
the fourcc of the type
for example: "UTF8" -> String or "JPEG" -> jpeg image
0 length means end
name
the name of the info entry, valid names are
"Author"
"Description"
"Copyright"
"Encoder" the name & version of the software used for encoding
"Title"
"Cover" an image of the (cd,dvd,vhs,..) cover (preferable 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: dont forget the timezone
"Keywords"
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-"
value
stuffing
0xFF
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 repated, but if they are then they MUST all be repeated together
and repeated headers MUST be identical
headers MUST be repeated every 10sec at least ? FIXME
headers MUST be repeated at least twice (so they exist 3 times in a file)
Index
the index can be repeated but there SHOULD be at least one at the end
Note: in case of realtime streaming there is no end, so no index there either
Info packets
the info_packet can be repeated, it can also contain different names & values
each time but only if allso the time is different
Info packets can be used to describe the file or some part of it (chapters)
info packets, SHOULD be placed at the begin of the file at least
for realtime streaming info packets will normally be transmitted when they apply
for example, the current song title & artist of the currently shown music video
Stuffing packets
can be used as a filler, for example to leave some empty space at the begin for
a copy of the index
Unknown packets
MUST be ignored by the decoder
Sample code (GPL, & 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(;;){
int tmp= *(bc->buf_ptr++);
if(tmp&0x80)
val= (val<<7) + tmp - 0x80;
else
return (val<<7) + tmp;
}
}
static inline void put_v(BufferContext *bc, uint64_t val){
int i;
assert(val);
for(i=56;; i-=8){
if(val>>i) break;
}
for(;i>0; i-=8){
*(bc->buf_ptr++)= 0x80 | (val>>i);
}
*(bc->buf_ptr++)= val&0x7F;
}
Example stream
main header
video_stream_header (stream 0, video jpjp, timebase 30, lsb_timestamp_length=8)
video_stream_header (stream 1 subtitle usen, timebase 30, lsb_timestamp_length=8)
video_stream_header (stream 2 subtitle atde, timebase 30, lsb_timestamp_length=8)
audio_stream_header (stream 3, audio jpjp, timebase 1 , lsb_timestamp_length=8)
audio_stream_header (stream 4, audio usen, timebase 1 , lsb_timestamp_length=8)
index (stream 0)
keyframe (stream 0, msb_timestamp=0, lsb_timestamp=0)
keyframe (stream 1, msb_timestamp=0, lsb_timestamp=0)
keyframe (stream 2, msb_timestamp=0, lsb_timestamp=0)
keyframe (stream 3, msb_timestamp=0, lsb_timestamp=0)
keyframe (stream 4, msb_timestamp=0, lsb_timestamp=0)
frame (stream 0, lsb_timestamp=1)
frame (stream 0, lsb_timestamp=2)
...
frame (stream 0, lsb_timestamp=30)
keyframe (stream 3, msb_timestamp=0, lsb_timestamp=1)
keyframe (stream 4, msb_timestamp=0, lsb_timestamp=1)
frame (stream 0, lsb_timestamp=31)
frame (stream 0, lsb_timestamp=32)
...
frame (stream 0, lsb_timestamp=60)
frame (stream 1, lsb_timestamp=60)
frame (stream 2, lsb_timestamp=60)
keyframe (stream 3, msb_timestamp=0, lsb_timestamp=2)
keyframe (stream 4, msb_timestamp=0, lsb_timestamp=2)
frame (stream 0, lsb_timestamp=61)
frame (stream 0, lsb_timestamp=62)
...
main header
video_stream_header (stream 0, video jpjp, timebase 30, lsb_timestamp_length=8)
video_stream_header (stream 1 subtitle usen, timebase 30, lsb_timestamp_length=8)
video_stream_header (stream 2 subtitle atde, timebase 30, lsb_timestamp_length=8)
audio_stream_header (stream 3, audio jpjp, timebase 1 , lsb_timestamp_length=8)
audio_stream_header (stream 4, audio usen, timebase 1 , lsb_timestamp_length=8)
frame (stream 0, lsb_timestamp=255)
frame (stream 0, msb_timestamp=1 lsb_timestamp=0)
frame (stream 0, lsb_timestamp=1)
frame (stream 0, lsb_timestamp=2)
frame (stream 1, msb_timestamp=1 lsb_timestamp=2)
frame (stream 2, msb_timestamp=1 lsb_timestamp=2)
frame (stream 0, lsb_timestamp=3)
frame (stream 0, lsb_timestamp=4)
...
keyframe (stream 3, msb_timestamp=0, lsb_timestamp=9)
keyframe (stream 4, msb_timestamp=0, lsb_timestamp=9)
main header
video_stream_header (stream 0, video jpjp, timebase 30, lsb_timestamp_length=8)
video_stream_header (stream 1 subtitle usen, timebase 30, lsb_timestamp_length=8)
video_stream_header (stream 2 subtitle atde, timebase 30, lsb_timestamp_length=8)
audio_stream_header (stream 3, audio jpjp, timebase 1 , lsb_timestamp_length=8)
audio_stream_header (stream 4, audio usen, timebase 1 , lsb_timestamp_length=8)
index (stream 0)
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