媒体文件格式分析之FMP4 MP4 中最基本的单元就是Box,它内部是通过一个一个独立的Box拼接而成的。所以,这里,我们先从 Box 的讲解开始,每个 Box 是由 Header 和 Data 组成的,FullBox 是 Box 的扩展,Box 结构的基础上在 Header 中增加 8bits version 和 24bits flags
1. 名词解释
2. 最小单元Box
2.1 常见的mp4文件结构(简化版)
3. Mp4文件整体结构
这里,我们按照 MP4 box 的划分来进行相关的阐述。先看一张 MP4 给出的结构图:
一般来说,解析媒体文件,最关心的部分是视频文件的宽高、时长、码率、编码格式、帧列表、关键帧列表,以及所对应的时戳和在文件中的位置,这些信息,在mp4中,是以特定的算法分开存放在stbl box下属的几个box中的,需要解析stbl下面所有的box,来还原媒体信息。下表是对于以上几个重要的box存放信息的说明
3.1 File Type Box (ftyp)
通常放在MP4文件的开头,告诉解码器基本的解码版本和兼容格式。
- 基本格式如下:
aligned(8) class FileTypeBox
extends Box(‘ftyp’) {
unsigned int(32) major_brand;
unsigned int(32) minor_version;
unsigned int(32) compatible_brands[];
}
- 字段说明:
- Nginx模块实现
ngx_int_t
ngx_rtmp_mp4_write_ftyp(ngx_buf_t *b)
{
u_char *pos;
pos = ngx_rtmp_mp4_start_box(b, "ftyp");
/* major brand */
ngx_rtmp_mp4_box(b, "iso6");
/* minor version */
ngx_rtmp_mp4_field_32(b, 1);
/* compatible brands */
ngx_rtmp_mp4_box(b, "isom");
ngx_rtmp_mp4_box(b, "iso6");
ngx_rtmp_mp4_box(b, "dash");
ngx_rtmp_mp4_update_box_size(b, pos);
return NGX_OK;
}
3.2 Movie Box (moov)
作为容器盒子,存放相关的trak及meta信息.
- 基本格式如下:
-
aligned(8) class MovieExtendsBox extends Box(‘mvex’){ }
文章最后扫码可领取音视频免费学习资料,资料包括(C/C++,Linux,FFmpeg webRTC rtmp hls rtsp ffplay srs 等等)
3.2.1 Movie Header Box (mvhd)
mvhd 是 moov 下的第一个 box,用来描述 media 的相关信息:
- 基本格式如下:
aligned(8) class MovieHeaderBox extends FullBox(‘mvhd’, 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; } template int(32) rate = 0x00010000; // typically 1.0 template int(16) volume = 0x0100; // typically, full volume const bit(16) reserved = 0; const unsigned int(32)[2] reserved = 0; template int(32)[9] matrix = { 0x00010000,0,0,0,0x00010000,0,0,0,0x40000000 }; // Unity matrix bit(32)[6] pre_defined = 0; unsigned int(32) next_track_ID; }
- 字段说明:
- Nginx模块实现
static ngx_int_t
ngx_rtmp_mp4_write_mvhd(ngx_buf_t *b)
{
u_char *pos;
pos = ngx_rtmp_mp4_start_box(b, "mvhd");
/* version */
ngx_rtmp_mp4_field_32(b, 0);
/* creation time */
ngx_rtmp_mp4_field_32(b, 0);
/* modification time */
ngx_rtmp_mp4_field_32(b, 0);
/* timescale */
ngx_rtmp_mp4_field_32(b, 1000);
/* duration */
ngx_rtmp_mp4_field_32(b, 0);
/* reserved */
ngx_rtmp_mp4_field_32(b, 0x00010000);
ngx_rtmp_mp4_field_16(b, 0x0100);
ngx_rtmp_mp4_field_16(b, 0);
ngx_rtmp_mp4_field_32(b, 0);
ngx_rtmp_mp4_field_32(b, 0);
ngx_rtmp_mp4_write_matrix(b, 1, 0, 0, 1, 0, 0);
/* reserved */
ngx_rtmp_mp4_field_32(b, 0);
ngx_rtmp_mp4_field_32(b, 0);
ngx_rtmp_mp4_field_32(b, 0);
ngx_rtmp_mp4_field_32(b, 0);
ngx_rtmp_mp4_field_32(b, 0);
ngx_rtmp_mp4_field_32(b, 0);
/* next track id */
ngx_rtmp_mp4_field_32(b, 1);
ngx_rtmp_mp4_update_box_size(b, pos);
return NGX_OK;
}
3.2.2 Movie Extends Box (mvex)(fMP4专有)
mvex 是 fMP4 的标准盒子。它的作用是告诉解码器这是一个fMP4的文件,具体的 samples 信息内容不再放到 trak 里面,而是在每一个 moof 中。基本格式为:
aligned(8) class MovieExtendsHeaderBox extends FullBox(‘mehd’, version, 0) { if (version==1) {
unsigned int(64) fragment_duration;
} else { // version==0
unsigned int(32) fragment_duration;
}
}
3.2.2.1 Track Extends Box (trex)(fMP4专有)
trex 是 mvex 的子一级 box 用来给 fMP4 的 sample 设置默认值。基本内容为
aligned(8) class TrackExtendsBox extends FullBox(‘trex’, 0, 0){
unsigned int(32) track_ID;
unsigned int(32) default_sample_description_index;
unsigned int(32) default_sample_duration;
unsigned int(32) default_sample_size;
unsigned int(32) default_sample_flags
}
3.2.3 Track Box (trak)
trak box 就是主要存放相关 media stream 的内容。
3.2.3.1 Track Header Box (tkhd)
tkhd 是 trak box 的子一级 box 的内容。主要是用来描述该特定 trak 的相关内容信息。其主要内容为:
- 基本格式如下:
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;
}
- 字段说明:
3.2.3.2 Media Box (media)
mdia 主要用来包裹相关的 media 信息。
(1) Media Header Box (mdhd)
- 基本格式如下:
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;
}
- 字段说明:
(2) Handler Reference Box(hdlr)
- 基本格式如下:
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;
}
- 字段说明:
- handler_type 类型如下:
-
vide : Video track soun : Audio track hint : Hint track meta : Timed Metadata track auxv : Auxiliary Video track
3.2.3.3 Media Information Box (minf)
minf 是子属内容中,重要的容器 box,用来存放当前 track 的基本描述信息。
(1) Video Media Header Box(vmhd)
- 基本格式如下:
-
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}; }
(2) Sound Media Header Box(smhd)
- 基本格式如下:
-
aligned(8) class SoundMediaHeaderBox extends FullBox(‘smhd’, version = 0, 0) { template int(16) balance = 0; const unsigned int(16) reserved = 0; }
(3) Data Information Box(dinf)
dinf 是用来说明在 trak 中,media 描述信息的位置。其实本身就是一个容器,没啥内容:
- 基本格式如下:
-
aligned(8) class SoundMediaHeaderBox extends FullBox(‘smhd’, version = 0, 0) { template int(16) balance = 0; const unsigned int(16) reserved = 0; }
(4) Data Reference Box(dref)
dref 是用来设置当前Box描述信息的 data_entry。
- 基本格式如下:
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; }
}
- 字段说明: