MP4文件格式详解——结构概述
MP4文件格式详解（ISO-14496-12/14）
Author:Pirate Leo
Email:[email protected]
一、基本概念
1、文件，由许多Box和FullBox组成。
2、Box，每个Box由Header和Data组成。
3、FullBox，是Box的扩展，Box结构的基础上在Header中增加8bits version和24bits flags。
4、Header，包含了整个Box的长度size和类型type。当size==0时，代表这是文件中最后一个Box；当size==1时，意味着Box长度需要更多bits来描述，在后面会定义一个64bits的largesize描述Box的长度；当type是uuid时，代表Box中的数据是用户自定义扩展类型。
5、Data，是Box的实际数据，可以是纯数据也可以是更多的子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的结构用伪代码表示如下：
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;
} 结构如下图：
文件基本结构描述图
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，下表中列出了所有必选或可选的Box类型，√代表Box必选。
具体列表：
|
ftyp |
√ |
file type and compatibility |
|||||
|
pdin |
progressive download information |
||||||
|
moov |
√ |
container for all the metadata |
|||||
|
mvhd |
√ |
movie header, overall declarations |
|||||
|
trak |
√ |
container for an individual track or stream |
|||||
|
tkhd |
√ |
track header, overall information about the track |
|||||
|
tref |
track reference container |
||||||
|
edts |
edit list container |
||||||
|
elst |
an edit list |
||||||
|
mdia |
√ |
container for the media information in a track |
|||||
|
mdhd |
√ |
media header, overall information about the media |
|||||
|
hdlr |
√ |
handler, declares the media (handler) type |
|||||
|
minf |
√ |
media information container |
|||||
|
vmhd |
video media header, overall information (video track only) |
||||||
|
smhd |
sound media header, overall information (sound track only) |
||||||
|
hmhd |
hint media header, overall information (hint track only) |
||||||
|
nmhd |
Null media header, overall information (some tracks only) |
||||||
|
dinf |
√ |
data information box, container |
|||||
|
dref |
√ |
data reference box, declares source(s) of media data in track |
|||||
|
stbl |
√ |
sample table box, container for the time/space map |
|||||
|
stsd |
√ |
sample descriptions (codec types, initialization etc.) |
|||||
|
stts |
√ |
(decoding) time-to-sample |
|||||
|
ctts |
(composition) time to sample |
||||||
|
stsc |
√ |
sample-to-chunk, partial data-offset information |
|||||
|
stsz |
sample sizes (framing) |
||||||
|
stz2 |
compact sample sizes (framing) |
||||||
|
stco |
√ |
chunk offset, partial data-offset information |
|||||
|
co64 |
64-bit chunk offset |
||||||
|
stss |
sync sample table (random access points) |
||||||
|
stsh |
shadow sync sample table |
||||||
|
padb |
sample padding bits |
||||||
|
stdp |
sample degradation priority |
||||||
|
sdtp |
independent and disposable samples |
||||||
|
sbgp |
sample-to-group |
||||||
|
sgpd |
sample group description |
||||||
|
subs |
sub-sample information |
||||||
|
mvex |
movie extends box |
||||||
|
mehd |
movie extends header box |
||||||
|
trex |
√ |
track extends defaults |
|||||
|
ipmc |
IPMP Control Box |
||||||
|
moof |
movie fragment |
||||||
|
mfhd |
√ |
movie fragment header |
|||||
|
traf |
track fragment |
||||||
|
tfhd |
√ |
track fragment header |
|||||
|
trun |
track fragment run |
||||||
|
sdtp |
independent and disposable samples |
||||||
|
sbgp |
sample-to-group |
||||||
|
subs |
sub-sample information |
||||||
|
mfra |
movie fragment random access |
||||||
|
tfra |
track fragment random access |
||||||
|
mfro |
√ |
movie fragment random access offset |
|||||
|
mdat |
media data container |
||||||
|
free |
free space |
||||||
|
skip |
free space |
||||||
|
udta |
user-data |
||||||
|
cprt |
copyright etc. |
||||||
|
meta |
metadata |
||||||
|
hdlr |
√ |
handler, declares the metadata (handler) type |
|||||
|
dinf |
data information box, container |
||||||
|
dref |
data reference box, declares source(s) of metadata items |
||||||
|
ipmc |
IPMP Control Box |
||||||
|
iloc |
item location |
||||||
|
ipro |
item protection |
||||||
|
sinf |
protection scheme information box |
||||||
|
frma |
original format box |
||||||
|
imif |
IPMP Information box |
||||||
|
schm |
scheme type box |
||||||
|
schi |
scheme information box |
||||||
|
iinf |
item information |
||||||
|
xml |
XML container |
||||||
|
bxml |
binary XML container |
||||||
|
pitm |
primary item reference |
||||||
|
fiin |
file delivery item information |
||||||
|
paen |
partition entry |
||||||
|
fpar |
file partition |
||||||
|
fecr |
FEC reservoir |
||||||
|
segr |
file delivery session group |
||||||
|
gitn |
group id to name |
||||||
|
tsel |
track selection |
||||||
|
meco |
additional metadata container |
||||||
|
mere |
metabox relation |
正式开始前先对文件的几个重要部分宏观介绍一下，以便诸位在后续学习时心中有数：
1、ftypbox，在文件的开始位置，描述的文件的版本、兼容协议等；2、moovbox，这个box中不包含具体媒体数据，但包含本文件中所有媒体数据的宏观描述信息，moov box下有mvhd和trak box。
>>mvhd中记录了创建时间、修改时间、时间度量标尺、可播放时长等信息。
>>trak中的一系列子box描述了每个媒体轨道的具体信息。
3、moofbox，这个box是视频分片的描述信息。并不是MP4文件必须的部分，但在我们常见的可在线播放的MP4格式文件中（例如Silverlight Smooth Streaming中的ismv文件）确是重中之重。
4、mdatbox，实际媒体数据。我们最终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，每个Fragment中包含moof和mdat。这样的结构符合渐进式播放需求。（mdat及其描述信息逐步传输，收齐一个Fragment便可播放其中的mdat）。
下一篇： [Multimedia]ffplay