【多媒体封装格式详解】---MP4【3】

2.2.2  Media Box

Box Type: ‘mdia’

mdia box 结构十分复杂。来个例子。

2.2.2.1 Media Header Box

Box Type: ‘mdhd’

aligned(8) class MediaHeaderBox extends FullBox(‘mdhd’, version, 0) {
if (version==1) {
unsigned int(64) creation_time;
unsigned int(64) modification_time;
unsigned int(32) timescale;
unsigned int(64) duration;
} else { // version==0
unsigned int(32) creation_time;
unsigned int(32) modification_time;
unsigned int(32) timescale;
unsigned int(32) duration;
}
bit(1) pad = 0;
unsigned int(5)[3] language; // ISO-639-2/T language code
unsigned int(16) pre_defined = 0;
}

Field	Type	Comment
box size	4	box大小
box type	4	box类型
version	1	box版本，0或1，一般为0。
creation time	4	创建时间（相对于UTC时间1904-01-01零点的秒数）
modification time	4	修改时间
time scale	4	文件媒体在1秒时间内的刻度值，可以理解为1秒长度的时间单元数 一般情况下视频的 都是90000
duration	4	该track的时间长度
language	2	媒体语言码
pre-defined	2

2.2.2.2 Handler Reference Box

Box Type: ‘hdlr’

从hdlr 这个box里面，我们可以获得这个track的类型信息。

aligned(8) class HandlerBox extends FullBox(‘hdlr’, version = 0, 0) {
unsigned int(32) pre_defined = 0;
unsigned int(32) handler_type;
const unsigned int(32)[3] reserved = 0;
string name;
}

Field	Type	Comment
box size	4	box大小
box type	4	box类型
version	1	box版本，0或1，一般为0。
flags	3
pre-defined	4
handler_type	4	‘vide’ Video track ‘soun’ Audio track ‘hint’ Hint track
reserved	12	0
name	string	字符串 track type name

例子 

 00 00 00 2168 64 6C 7200 00 00 00 00 00 00 00 ; ...!hdlr........

76 69 64 65 00 00 00 00 00 00 00 00 00 00 00 00 ; vide............

00

上面的例子 可知track 的类型是 ‘vide’ 这是个video track

2.2.2.3 Media Information Box

Box Type: ‘minf’

minf 里包含着一系列的box。里面是track有关的特征信息。

一般情况minf 包含：Media Information Header Boxes、Data Information Box(dinf)、Sample Table Box。

Media Information Header Boxes 根据类型分‘vmhd’, ‘smhd’, ’hmhd’, ‘nmhd’

2.2.2.3.1 Media Information Header Boxes

Box Types: ‘vmhd’, ‘smhd’, ’hmhd’, ‘nmhd’

Video Media Header Box(vmhd)

aligned(8) class VideoMediaHeaderBox
extends FullBox(‘vmhd’, version = 0, 1) {
template unsigned int(16) graphicsmode = 0; // copy, see below
template unsigned int(16)[3] opcolor = {0, 0, 0};
}

Field	Type	Comment
box size	4	box大小
box type	4	box类型
version	1	box版本，0或1，一般为0。
flags	3	flags
graphicsmode	2	specifies a composition mode for this video track, from the following enumerated set, which may be extended by derived specifications: copy = 0 copy over the existing image
opcolor	2*3	is a set of 3 colour values (red, green, blue) available for use by graphics modes

例子：

00 00 00 14 76 6D 68 640000 00 01 00 00 00 00 ; ....vmhd........

00 00 00 00                                     ; ....

Sound Media Header Box(smhd)

aligned(8) class SoundMediaHeaderBox
extends FullBox(‘smhd’, version = 0, 0) {
template int(16) balance = 0;
const unsigned int(16) reserved = 0;
}

Field	Type	Comment
box size	4	box大小
box type	4	box类型
version	1	box版本，0或1，一般为0。
flags	3	flags
balance	2	立体声平衡，[8.8]格式值，一般为0，-1.0表示全部左声道，1.0表示全部右声道
reserved	2	0

例子：

 00 00 00 10 73 6D 68 64 00 00 00 0000 0000 00 ; ....smhd........

Hint Media Header Box(hmhd)

aligned(8) class HintMediaHeaderBox
extends FullBox(‘hmhd’, version = 0, 0) {
unsigned int(16) maxPDUsize;
unsigned int(16) avgPDUsize;
unsigned int(32) maxbitrate;
unsigned int(32) avgbitrate;
unsigned int(32) reserved = 0;
}

2.2.2.3.2 Data Information Box

Box Type: ‘dinf’

dinf 里面包含的是Data Reference Box 可能的类型：‘url ‘, ‘urn ‘, ‘dref’

aligned(8) class DataEntryUrlBox (bit(24) flags)
extends FullBox(‘url ’, version = 0, flags) {
string location;
}
aligned(8) class DataEntryUrnBox (bit(24) flags)
extends FullBox(‘urn ’, version = 0, flags) {
string name;
string location;
}
aligned(8) class DataReferenceBox
extends FullBox(‘dref’, version = 0, 0) {
unsigned int(32) entry_count;
for (i=1; i • entry_count; i++) {
DataEntryBox(entry_version, entry_flags) data_entry;
}
}

未完待续。。。。。

接下来要把里面的真实数据流搞出来。