mp4的box信息——avcC，mdat

来源：互联网发布：雅思资料知乎编辑：程序博客网时间：2024/05/16 10:35

前面忽略了avcC的box信息和mdat的格式信息，后面遇到了问题，现在在这里补充上这两个box的信息。

这2个box的信息在iso_iec-14496-15中。

//————avcC——————-//
5.3.4.1
Sample description name and format
5.3.4.1.1 Definition
Box Types:‘avc1’, ‘avcC’, ‘m4ds’,’btrt’
Container:Sample Table Box (‘stbl’)
Mandatory:The avc1 box is mandatory
Quantity:One or more sample entries may be present

5.3.4.1.2 Syntax

// Visual Sequencesclass AVCConfigurationBox extends Box(‘avcC’) {    AVCDecoderConfigurationRecord() AVCConfig;}class MPEG4BitRateBox extends Box(‘btrt’){    unsigned int(32) bufferSizeDB;    unsigned int(32) maxBitrate;    unsigned int(32) avgBitrate;}class MPEG4ExtensionDescriptorsBox extends Box(‘m4ds’) {    Descriptor Descr[0 .. 255];}class AVCSampleEntry() extends VisualSampleEntry (‘avc1’){    AVCConfigurationBox config;    MPEG4BitRateBox ();    // optional    MPEG4ExtensionDescriptorsBox (); // optional}

关于这个：AVCDecoderConfigurationRecord的说明：

aligned(8) class AVCDecoderConfigurationRecord {    unsigned int(8) configurationVersion = 1;    unsigned int(8) AVCProfileIndication;    unsigned int(8) profile_compatibility;    unsigned int(8) AVCLevelIndication;    bit(6) reserved = ‘111111’b;    unsigned int(2) lengthSizeMinusOne;    bit(3) reserved = ‘111’b;    unsigned int(5) numOfSequenceParameterSets;    for (i=0; i< numOfSequenceParameterSets; i++) {        unsigned int(16) sequenceParameterSetLength ;        bit(8*sequenceParameterSetLength) sequenceParameterSetNALUnit;    }    unsigned int(8) numOfPictureParameterSets;    for (i=0; i< numOfPictureParameterSets; i++) {        unsigned int(16) pictureParameterSetLength;        bit(8*pictureParameterSetLength) pictureParameterSetNALUnit;    }}

分析一段avcC的数据：

字段16进制 2进制含义 00 00 00 2E size大小 61 76 63 43 ‘a’ ‘v’ ‘c’ ‘C’ 0x01 configurationVersion 0x4D AVCProfileIndication。10进制的77，对照profile的情况，知道是main profile 00 profile_compatibility .一般0 1F AVCLevelIndication.10进制的31 FF 1111 1111 前面6位为reserved，后面2位（0b11）为：lengthSizeMinusOne，表示3，那么用来表示size的字节就有3+1=4个 E1 1110 0001 前面3位是reserved,后面5bit是numOfSequenceParameterSets，表示有1个。 00 17 sequenceParameterSetLength 67 4D 00 1F 95 A8 14 01 6E 9B 80 80 80 A0 00 00 7D 00 00 0E A6 10 80 这0x17=23个字节就是sps的内容 01 numOfPictureParameterSets，有1个pps 00 04 pictureParameterSetLength 68 EE 3C 80 PPS的内容

回到mdat那里看：
00 20 FA 68 6D 64 61 74 00 00 00 17 67 4D 00 1F
95 A8 14 01 6E 9B 80 80 80 A0 00 00 7D 00 00 0E
A6 10 80 00 00 00 04 68 EE 3C 80 00 00 00 05 06

字段含义 00 20 FA 68 6D 64 61 74 00 00 00 17 4个字节的长度信息，由上面的lengthSizeMinusOne+1决定 67 4D 00 1F 95 A8 14 01 6E 9B 80 80 80 A0 00 00 7D 00 00 0E A6 10 80 sps内容

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