MP4文件格式具体解释（ISO-14496-12/14）
Author:Pirate Leo
Email:[email protected]
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：
|
ftyp |
√ |
file type and compatibility |
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|
pdin |
progressive download information |
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|
moov |
√ |
container for all the metadata |
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|
mvhd |
√ |
movie header, overall declarations |
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|
trak |
√ |
container for an individual track or stream |
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|
tkhd |
√ |
track header, overall information about the track |
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|
tref |
track reference container |
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|
edts |
edit list container |
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|
elst |
an edit list |
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|
mdia |
√ |
container for the media information in a track |
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|
mdhd |
√ |
media header, overall information about the media |
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|
hdlr |
√ |
handler, declares the media (handler) type |
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|
minf |
√ |
media information container |
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|
vmhd |
video media header, overall information (video track only) |
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|
smhd |
sound media header, overall information (sound track only) |
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|
hmhd |
hint media header, overall information (hint track only) |
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|
nmhd |
Null media header, overall information (some tracks only) |
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|
dinf |
√ |
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 |
√ |
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 |
(composition) time to sample |
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|
stsc |
√ |
sample-to-chunk, partial data-offset information |
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|
stsz |
sample sizes (framing) |
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|
stz2 |
compact sample sizes (framing) |
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|
stco |
√ |
chunk offset, partial data-offset information |
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|
co64 |
64-bit chunk offset |
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|
stss |
sync sample table (random access points) |
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|
stsh |
shadow sync sample table |
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|
padb |
sample padding bits |
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|
stdp |
sample degradation priority |
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|
sdtp |
independent and disposable samples |
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|
sbgp |
sample-to-group |
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|
sgpd |
sample group description |
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|
subs |
sub-sample information |
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|
mvex |
movie extends box |
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|
mehd |
movie extends header box |
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|
trex |
√ |
track extends defaults |
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|
ipmc |
IPMP Control Box |
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|
moof |
movie fragment |
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|
mfhd |
√ |
movie fragment header |
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|
traf |
track fragment |
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|
tfhd |
√ |
track fragment header |
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|
trun |
track fragment run |
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|
sdtp |
independent and disposable samples |
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|
sbgp |
sample-to-group |
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|
subs |
sub-sample information |
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|
mfra |
movie fragment random access |
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|
tfra |
track fragment random access |
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|
mfro |
√ |
movie fragment random access offset |
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|
mdat |
media data container |
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|
free |
free space |
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|
skip |
free space |
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|
udta |
user-data |
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|
cprt |
copyright etc. |
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|
meta |
metadata |
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|
hdlr |
√ |
handler, declares the metadata (handler) type |
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|
dinf |
data information box, container |
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|
dref |
data reference box, declares source(s) of metadata items |
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|
ipmc |
IPMP Control Box |
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|
iloc |
item location |
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|
ipro |
item protection |
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|
sinf |
protection scheme information box |
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|
frma |
original format box |
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|
imif |
IPMP Information box |
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|
schm |
scheme type box |
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|
schi |
scheme information box |
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|
iinf |
item information |
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|
xml |
XML container |
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|
bxml |
binary XML container |
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|
pitm |
primary item reference |
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|
fiin |
file delivery item information |
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|
paen |
partition entry |
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|
fpar |
file partition |
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|
fecr |
FEC reservoir |
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|
segr |
file delivery session group |
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|
gitn |
group id to name |
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|
tsel |
track selection |
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|
meco |
additional metadata container |
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|
mere |
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）。