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元數(shù)據(jù)moov（二）tkhd box（ISO-14496-12）
Author:Pirate Leo
Email:codeevoship@gmail.com
ISO 14496 - 12 定義了一種封裝媒體數(shù)據(jù)的基礎(chǔ)文件格式，mp4、3gp、ismv等我們常見媒體封裝格式都是以這種基礎(chǔ)文件格式為基礎(chǔ)衍生的。
如果從全局角度了解基礎(chǔ)文件格式，請看我之前的博文《MP4文件格式詳解——結(jié)構(gòu)概述》。
本系列文檔從MP4文件入手，對文件中重要的box進(jìn)行解析。
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本次繼續(xù)解析moov box，關(guān)于moov的解析推薦從我上一篇博文《MP4文件格式詳解——元數(shù)據(jù)moov（一）》看起。
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moov
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container for all the metadata
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mvhd
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movie header, overall declarations
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trak
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container for an individual track or stream
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tkhd
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track header, overall information about the track
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tref
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track reference container
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edts
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edit list container
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elst
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an edit list
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mdia
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container for the media information in a track
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mdhd
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media header, overall information about the media
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hdlr
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handler, declares the media (handler) type
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minf
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media information container
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vmhd
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video media header, overall information (video track only)
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smhd
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sound media header, overall information (sound track only)
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hmhd
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hint media header, overall information (hint track only)
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nmhd
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Null media header, overall information (some tracks only)
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dinf
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data information box, container
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dref
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data reference box, declares source(s) of media data in track
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stbl
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sample table box, container for the time/space map
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stsd
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sample descriptions (codec types, initialization etc.)
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stts
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(decoding) time-to-sample
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ctts
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(composition) time to sample
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stsc
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sample-to-chunk, partial data-offset
information
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stsz
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sample sizes (framing)
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stz2
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compact sample sizes (framing)
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stco
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chunk offset, partial data-offset information
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co64
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64-bit chunk offset
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stss
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sync sample table (random access points)
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stsh
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shadow sync sample table
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padb
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sample padding bits
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stdp
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sample degradation priority
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sdtp
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independent and disposable samples
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sbgp
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sample-to-group
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sgpd
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sample group description
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subs
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sub-sample information
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前面我們已經(jīng)知道每個文件是由多個Track（軌道）組成的，每個Track都對應(yīng)了自身trak box，其中存放了本track的元數(shù)據(jù)信息。
本次繼續(xù)解析trak box的一系列子box：
1）tkhd box
- aligned(8) class TrackHeaderBox extends FullBox(‘tkhd’, version, flags)
- {
- if (version==1)
- {
- unsigned int(64) creation_time;
- unsigned int(64) modification_time;
- unsigned int(32) track_ID;
- const unsigned int(32) reserved = 0;
- unsigned int(64) duration;
- }
- else
- { // version==0
- unsigned int(32) creation_time;
- unsigned int(32) modification_time;
- unsigned int(32) track_ID;
- const unsigned int(32) reserved = 0;
- unsigned int(32) duration;
- }
- const unsigned int(32)[2] reserved = 0;
- template int(16) layer = 0;
- template int(16) alternate_group = 0;
- template int(16) volume = {if track_is_audio 0x0100 else 0};
- const unsigned int(16) reserved = 0;
- template int(32)[9] matrix= { 0x00010000,0,0,0,0x00010000,0,0,0,0x40000000 };
- // unity matrix
- unsigned int(32) width;
- unsigned int(32) height;
- }<strong>
- </strong>
上圖是實(shí)際媒體中的tkhd的數(shù)據(jù)：
0x5c是tkhd box長度，0x746b6864是“tkhd”的ASCII碼。
0x00 00 00 0f是使用了Full box中的flag位（Full box 8bits version + 24bits flag，詳見我第一篇日志），這里flag= 0xf，即1111b。
這4位從低到高分別代表：
Track_enabled: Indicates that the track is enabled. 若此位為0，則該track內(nèi)容無需播放（比如我們用一些非線編軟件<如Sony Vegas>做視頻剪輯時，有些Track僅為我們參考與模仿用，在輸出時將該Track關(guān)掉）。
Track_in_movie: Indicates that the track is used in the presentation.
Track_in_preview: Indicates that the track is used when previewing the presentation.
Track_in_poster: Indicates that the track is used in movie's poster.
/* -------------------------------------------------------------------------------------------------------*/
important：我們知道，MP4文件格式是ISO-14496-12基礎(chǔ)文件格式的衍生品，14496-14中對-12協(xié)議進(jìn)行了擴(kuò)充與進(jìn)一步定義。
重要的是該“14496-12 基礎(chǔ)文件格式”協(xié)議如果認(rèn)祖歸宗，我們發(fā)現(xiàn)這種文件格式最初是由Apple公司的QuickTime媒體格式發(fā)展而來的。
即，mov格式發(fā)展出了“ISO 14496 - 12協(xié)議”，再由該協(xié)議衍生出了mp4,f4v,ismv,3gp等我們常見的媒體封裝格式。
因此上述標(biāo)志位的poster位，在14496-12中并沒有見到描述，而在Apple的協(xié)議中卻看到了準(zhǔn)確定義。
詳見 https://developer.apple.com/library/mac/#documentation/QuickTime/QTFF/QTFFChap2/qtff2.html
/* -------------------------------------------------------------------------------------------------------*/
兩個0xc5268eb6 是track的創(chuàng)建時間與最后修改時間；
緊隨其后的0x00000002，代表track ID =2，Track ID是非0的，唯一的，不可重復(fù)使用的標(biāo)識track的方式；
后面32bit全0是保留位；
0x0009d97c是本track的時長，需要配合mvhd box中的timescale 計算實(shí)際的持續(xù)時間。
后續(xù)一些寫死的字段不再分析，有些與mvhd重復(fù)，可以參見之前的文章。我們看兩個關(guān)鍵字段：
layer，類似photoshop中圖層的概念，數(shù)值小的在播放時更貼近用戶（上層圖層）。
alternate_group，track的備用分組ID，當(dāng)該值為0時，意味著本track內(nèi)容無備份；否則本track會可能會有零到多個備份track。當(dāng)播放時相同group ID的track只選擇一個進(jìn)行播放。
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