MP4文件格式详解——结构概述

最新推荐文章于 2025-04-12 22:28:30 发布

最新推荐文章于 2025-04-12 22:28:30 发布 · 174 阅读

本文详细阐述了MP4文件的组成结构，包括文件的基本概念、Box类型及其作用，以及MP4文件格式的概述，帮助读者理解文件的内部运作。

MP4文件格式详解（ISO-14496-12/14）

Author:Pirate Leo

Email:codeevoship@gmail.com

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