Author: diego Date: Thu Mar 8 11:26:00 2007 New Revision: 22488 Modified: trunk/DOCS/tech/nut.txt Log: spelling/punctuation/grammar/wording improvements, no semantic changes Modified: trunk/DOCS/tech/nut.txt ============================================================================== --- trunk/DOCS/tech/nut.txt (original) +++ trunk/DOCS/tech/nut.txt Thu Mar 8 11:26:00 2007 @@ -7,73 +7,72 @@ NUT Open Container Format 20061104 Intro: ====== -NUT is a Free multimedia container format for storage of audio, video, +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 +in case of errors in the stream. -other common multimedia container formats are AVI, OGG, Matroska, MP4, MOV -ASF, MPEG-PS, MPEG-TS +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 +Simplicity + 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 +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. -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 +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 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 +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 spec is frozen. All files following spec will be compatible unless the -spec is unfrozen. +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 +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 + 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 + 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 + 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 + non-keyframe. (FIXME maybe move somewhere else?) pts - presentation time of the first frame/sample that is completed by decoding + 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 + The time when a frame is input into a synchronous 1-in-1-out decoder. Syntax: @@ -96,7 +95,7 @@ name (text description) [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 presented in a bottom-up approach, again optional notes may follow and are reproduced in the tag description: name: (optional note) @@ -166,10 +165,10 @@ file: frame } -the structure of an undamaged file should look like the following, but +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 +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 @@ -203,10 +202,10 @@ 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 + [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] + in a compatible way.] packet_header startcode f(64) @@ -368,8 +367,8 @@ index: info_packet: stream_id_plus1 v - chapter_id s (Note due to a typo this was v - until 2006-11-04) + chapter_id s (Note: Due to a typo this was v + until 2006-11-04.) chapter_start t chapter_len v count v @@ -411,8 +410,7 @@ Tag description: file_id_string "nut/multimedia container\0" - the very first thing in every nut file, useful for identifying nut - files + The very first thing in every NUT file, useful for identifying NUT files. *_startcode (f(64)) all startcodes start with 'N' @@ -433,24 +431,24 @@ 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 + 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 + 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 + 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 + 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 @@ -459,25 +457,25 @@ max_distance (v) 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 + 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 + 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 + 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. + This is also half the maximum frame size without a checksum after the + frame header. 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. + 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 + SHOULD NOT be higher than 1/timebase. stream_id (v) Stream identifier @@ -488,16 +486,16 @@ stream_class (v) 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 + 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 + 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 @@ -515,7 +513,7 @@ time_base_num (v) / time_base_denom (v) time_base_id (v) index into the time_base table - MUST be < time_base_count + MUST be < time_base_count. convert_ts To switch from 2 different timebases, the following calculation is @@ -526,8 +524,8 @@ convert_ts 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 + 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, @@ -549,16 +547,16 @@ compare_ts msb_pts_shift (v) amount of bits in lsb_pts - MUST be <16 + 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) + 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) @@ -566,50 +564,50 @@ stream_flags (v) 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) + 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 - usefull to workaround possible encoder bugs + 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 striped result is not less valid per codec spec as before striping + 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 + 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 + 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 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. + 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 @@ -622,52 +620,52 @@ flags[frame_code], frame_flags (v) 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 + 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 + 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 + 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 + 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 + 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 + 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 + difference between the current pts and last_pts. + MUST be <16384 and >-16384. reserved_count[frame_code] (v) - MUST be <256 + MUST be <256. data_size - the size of the following frame - data_size= data_size_lsb + data_size_msb*data_size_mul; + 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) + 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 + 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 + 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 @@ -680,33 +678,33 @@ lsb_pts 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 + 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 + 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 + 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 + 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 + 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 + 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 + 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 + 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 @@ -716,21 +714,21 @@ colorspace_type (v) 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 + 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 + 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 + 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 + Checksum over the startcode and forward pointer. Syncpoint tags: --------------- @@ -738,7 +736,7 @@ 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 + 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. @@ -756,23 +754,23 @@ Index tags: ----------- max_pts (t) - The highest pts in the entire file + 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 + 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 + 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) + 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 @@ -790,22 +788,23 @@ Info tags: ---------- stream_id_plus1 (v) - Stream this info packet applies to. If zero, packet applies to whole - file. + Stream this info packet applies to. If zero, packet applies to the + 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 + 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_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. + Length of chapter in the same timebase as chapter_start. count (v) number of name/value pairs in this info packet @@ -815,94 +814,95 @@ type "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) + 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 + The name of the info entry. Valid names are "Author" "Description" "Copyright" "Encoder" - the name & version of the software used for encoding + 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 + 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 + 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) + "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 + 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/ + 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 + 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 + 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 + 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) +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 +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 +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) +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) +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 sync point immediately before the first frame after any headers +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 +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. @@ -911,55 +911,56 @@ 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 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 +the file. -demuxers SHOULD NOT search the whole file for info packets +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 +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) +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 + 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 + 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 +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 +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 +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 +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) +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 +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 +is available after seeking. Semantic requirements: @@ -971,9 +972,9 @@ It often highly improves usability and i 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 +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. @@ -1029,7 +1030,7 @@ static inline int put_v(BufferContext *b if(space_left(bc) < 9) return -1; - val &= 0x7FFFFFFFFFFFFFFFULL; // FIXME can only encode up to 63 bits currently + val &= 0x7FFFFFFFFFFFFFFFULL; // FIXME: Can only encode up to 63 bits ATM. for(i=7; ; i+=7){ if(val>>i == 0) break; }