媒體檔案格式分析之FMP4

號外：金山雲全線已經支援dash和HLS Fmp4直播協議，歡迎大家試用！！

MP4 中最基本的單元就是Box，它內部是通過一個一個獨立的Box拼接而成的。所以，這裡，我們先從 Box 的講解開始,每個 Box 是由 Header 和 Data 組成的，FullBox 是 Box 的擴充套件，Box 結構的基礎上在 Header 中增加 8bits version 和 24bits flags

1. 名詞解釋

2. 最小單元Box

3. Mp4檔案整體結構

這裡，我們按照 MP4 box 的劃分來進行相關的闡述。先看一張 MP4 給出的結構圖:

3.1 File Type Box (ftyp)

通常放在MP4檔案的開頭，告訴解碼器基本的解碼版本和相容格式。

• 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[];
}

• 欄位說明:

• Nginx模組實現

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)

作為容器盒子，存放相關的trak及meta資訊.

• 基本格式如下:

aligned(8) class MovieExtendsBox extends Box(‘mvex’){ }

3.2.1 Movie Header Box (mvhd)

mvhd 是 moov 下的第一個 box，用來描述 media 的相關資訊:

• 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;
}

• 欄位說明:

• Nginx模組實現

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)

mvex 是 fMP4 的標準盒子。它的作用是告訴解碼器這是一個 fMP4 的檔案，具體的 samples 資訊內容不再放到 trak 裡面，而是在每一個 moof 中。基本格式為：

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)

trex 是 mvex 的子一級 box 用來給 fMP4 的 sample 設定預設值。基本內容為

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 就是主要存放相關 media stream 的內容。

3.2.3.1 Track Header Box (tkhd)

tkhd 是 trak box 的子一級 box 的內容。主要是用來描述該特定 trak 的相關內容資訊。其主要內容為：

• 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;
}

• 欄位說明:

3.2.3.2 Media Box (media)

mdia 主要用來包裹相關的 media 資訊。

(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;
}

• 欄位說明:

(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;
}

• 欄位說明:

• handler_type 型別如下：

	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 是子屬內容中，重要的容器 box，用來存放當前 track 的基本描述資訊。

######（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 是用來說明在 trak 中，media 描述資訊的位置。其實本身就是一個容器，沒啥內容：

• 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 是用來設定當前Box描述資訊的 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; }
 }

3.3 Moof Box

3.4 Sidxs Box