======================================== NUT Open Container Format DRAFT 20060312 ======================================== 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 4096) header_checksum u(32) packet_footer reserved_bytes checksum u(32) [Note: in index packet, reserved_bytes comes before index_ptr] 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 } Headers: main header: version v stream_count v max_distance v time_base_count v for(i=0; i0) 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>=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(; j65536 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 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. SHOULD NOT be higher than 1/timebase stream_id Stream identifier stream_id MUST be < stream_count stream_class 0 video 1 audio 2 subtiles 3 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 time_base_id id to time_base table 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 a muxer SHOULD mark 0x00 and 0xFF as invalid to improve error detection flags[frame_code], frame_flags Bit Name Description 1 FLAG_KEY if set, frame is keyframe 2 FLAG_EOR if set, stream has no relevance on presentation. (EOR) 32 FLAG_SIZE_MSB if set, data_size_msb is at frame header, otherwise data_size_msb is 0 64 FLAG_CHECKSUM if set then the frame header contains a checksum 4096 FLAG_CODED if set, coded_flags are stored in the frame header. 8192 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 2*max_distance 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< 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 both MUST be 0 if unknown otherwise both MUST be non zero 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 packet_header 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_div16 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 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 The highest pts in the entire file syncpoint_pos_div16 offset from begginning 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 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 timestamp of start of chapter 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 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 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. 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; ibuf_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)