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MP4文件格式具体解释（ISO-14496-12/14）
Author:Pirate Leo
Email:codeevoship@gmail.com
一、基本概念
1、 文件，由很多Box和FullBox组成。
2、 Box，每一个Box由Header和Data组成。
3、 FullBox，是Box的扩展，Box结构的基础上在Header中添加8bits version和24bits flags。
4、 Header，包括了整个Box的长度size和类型type。当size==0时，代表这是文件里最后一个Box；当size==1时，意味着Box长度须要很多其它bits来描写叙述，在后面会定义一个64bits的largesize描写叙述Box的长度；当type是uuid时，代表Box中的数据是用户自己定义扩展类型。
5、 Data，是Box的实际数据，能够是纯数据也能够是很多其它的子Boxes。
6、 当一个Box的Data中是一系列子Box时，这个Box又可成为Container Box。
Box的结构用伪代码表演示样例如以下：
aligned(8) class Box (unsigned int(32) boxtype,optional unsigned int(8)[16] extended_type)
{
unsigned int(32) size;
unsigned int(32) type = boxtype;
if (size==1)
{
unsigned int(64) largesize;
}
else if (size==0)
{
// box extends to end of file
}
if (boxtype==‘uuid’)
{
unsigned int(8)[16] usertype = extended_type;
}
结构例如以下图：
文件基本结构描写叙述图
二、MP4文件格式（ISO-14496-12/14）
MP4文件概述
MP4文件就是由各式各样的Box组成的，下表中列出了全部必选或可选的Box类型，√代表Box必选。
详细列表：
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ftyp |
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√ |
file type and compatibility |
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pdin |
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progressive download information |
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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 |
√ |
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 |
√ |
sample descriptions (codec types, initialization etc.) |
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stts |
√ |
(decoding) time-to-sample |
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ctts |
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(composition) time to sample |
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stsc |
√ |
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 |
√ |
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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mvex |
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movie extends box |
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mehd |
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movie extends header box |
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trex |
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√ |
track extends defaults |
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ipmc |
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IPMP Control Box |
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moof |
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movie fragment |
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mfhd |
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√ |
movie fragment header |
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traf |
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track fragment |
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tfhd |
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√ |
track fragment header |
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trun |
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track fragment run |
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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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subs |
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sub-sample information |
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mfra |
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movie fragment random access |
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tfra |
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track fragment random access |
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mfro |
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√ |
movie fragment random access offset |
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mdat |
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media data container |
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free |
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free space |
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skip |
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free space |
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udta |
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user-data |
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cprt |
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copyright etc. |
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meta |
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metadata |
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hdlr |
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√ |
handler, declares the metadata (handler) type |
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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 metadata items |
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ipmc |
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IPMP Control Box |
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iloc |
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item location |
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ipro |
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item protection |
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sinf |
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protection scheme information box |
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frma |
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original format box |
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imif |
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IPMP Information box |
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schm |
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scheme type box |
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schi |
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scheme information box |
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iinf |
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item information |
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xml |
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XML container |
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bxml |
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binary XML container |
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pitm |
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primary item reference |
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fiin |
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file delivery item information |
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paen |
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partition entry |
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fpar |
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file partition |
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fecr |
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FEC reservoir |
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segr |
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file delivery session group |
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gitn |
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group id to name |
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tsel |
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track selection |
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meco |
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additional metadata container |
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mere |
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metabox relation |
正式開始前先对文件的几个重要部分宏观介绍一下，以便诸位在兴许学习时心中有数：
1、 ftypbox，在文件的開始位置，描写叙述的文件的版本号、兼容协议等；2、 moovbox，这个box中不包括详细媒体数据，但包括本文件里全部媒体数据的宏观描写叙述信息，moov box下有mvhd和trak box。
>>mvhd中记录了创建时间、改动时间、时间度量标尺、可播放时长等信息。
>>trak中的一系列子box描写叙述了每一个媒体轨道的详细信息。
3、 moofbox，这个box是视频分片的描写叙述信息。并非MP4文件必须的部分，但在我们常见的可在线播放的MP4格式文件里（比如Silverlight Smooth Streaming中的ismv文件）确是重中之重。
4、 mdatbox，实际媒体数据。我们终于解码播放的数据都在这里面。
5、 mfrabox，一般在文件末尾，媒体的索引文件，可通过查询直接定位所需时间点的媒体数据。
附：Smooth Streaming中ismv文件结构，文件分为了多个Fragments，每一个Fragment中包括moof和mdat。这种结构符合渐进式播放需求。（mdat及其描写叙述信息逐步传输，收齐一个Fragment便可播放当中的mdat）。
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原文地址：http://www.cnblogs.com/mengfanrong/p/4199148.html
