音視訊八股文（10）-- mp4結構
2023-05-05 21:00:25
介紹
mp4⽂件格式⼜被稱為MPEG-4 Part 14，出⾃MPEG-4標準第14部分 。它是⼀種多媒體格式容器，⼴泛⽤於包裝視訊和⾳頻資料流、海報、字幕和後設資料等。（順便⼀提，⽬前流⾏的視訊編碼格式AVC/H264
定義在MPEG-4 Part 10）。
概述
mp4⽂件由box組成，每個box分為Header和Data。其中Header部分包含了box的型別和⼤⼩，Data包含了⼦box或者資料，box可以巢狀⼦box。
下圖是⼀個典型mp4⽂件的基本結構：
MP4⽂件的基本組成單元是box，也就是說MP4⽂件是由各種各樣的box組成的，有parent box，還有children box。因此，這些boxes之間存在⼀定的層次關係，總結如下表所示，表中標記出了各個box必選或可選特性，√代表Box必選。
| ftyp | √ | file type and compatibility ⽂件型別和相容性 | |||||
| pdin | progressive download information | ||||||
| moov | √ | container for all the metadata 所有後設資料的容器 | |||||
| mvhd | √ | movie header, overall declarations 電影頭，整體宣告 | |||||
| trak | √ | container for an individual track or stream 單個軌或流的容器 | |||||
| tkhd | √ | track header, overall information about the track 軌的頭部，關於該軌的概括資訊，⽐如視訊寬⾼ | |||||
| tref | track reference container | ||||||
| edts | edit list container | ||||||
| elst | an edit list | ||||||
| mdia | √ | container for the media information in a track 軌媒體資訊的容器 | |||||
| mdhd | √ | media header, overall information about the media 媒體頭，關於媒體的總體資訊 | |||||
| hdlr | √ | handler, declares the media (handler) type 媒體的播放過程資訊 | |||||
| minf | √ | media information container 媒體資訊容器 | |||||
| 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 資料資訊box，容器 | |||||
| dref | √ | data reference box, declares source(s) of media data in track 如何定位媒體資訊 | |||||
| stbl | √ | sample table box, container for the time/space map 包含了track中的sample的所有時間和位置資訊，以及sample的編解碼等資訊。利⽤這個表可以解析sample的時序、型別、⼤⼩以及在各⾃儲存容器中的位置。 | |||||
| stsd | √ | sample descriptions (codec types,initialization etc.) 如果是視訊，包含：編碼型別、寬⾼、⻓度等資訊；如果是⾳頻，包含：聲道、取樣率等資訊 | |||||
| stts | √ | (decoding) time-to-sample描述了sample時序的對映⽅法，我們可以通過它找到任何時間的sample。 | |||||
| ctts | (composition) time to sample。 | ||||||
| stsc | √ | sample-to-chunk, partial data-offset information ⽤chunk組織sample可以⽅便優化資料獲取，⼀個chunk包含⼀個或多個sample。 | |||||
| stsz | sample sizes (framing) 每個sample的⼤⼩。雖然這⾥沒有打勾，但對於mp4還是⾮常必要的。 | ||||||
| stz2 | compact sample sizes (framing) | ||||||
| stco | √ | chunk offset, partial data-offset information 定義了每個chunk在媒體流中的偏移位置 | |||||
| co64 | 64-bit chunk offset | ||||||
| stss | sync sample table (random access points) ⽤於確定media中的關鍵幀 | ||||||
| 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 |
本⽂使⽤mediainfo和mp4box進⾏分析
圖中看到mp4⽂件由⼏個主要組成部分，下⾯以