====== MP4 ======

==== 参考 ====

  * https://www.cnblogs.com/chyingp/p/mp4-file-format.html
  * https://lucius0.github.io/2018/01/16/archivers/media-study-08/
  * ffmpeg\libavformat\movenc.c

==== 文档 ====

  * 参考：https://www.cnblogs.com/codertian/p/8277965.html
  * 文档下载：https://standards.iso.org/ittf/PubliclyAvailableStandards/index.html
  * 14496-12  14496-14 14496-15 下载要收费了， csdn  解决
    * {{ :public:it:iso_iec_14496-12_2015.pdf |}}
    * {{ :public:it:iso_iec_14496-14-2020.pdf |}}
    * {{ :public:it:iso_iec_14496-15_2013_avcff.pdf |}}
    * {{ :public:it:标准iso-iec14496-15-2017.docx |}} 这个是在WPS里用图片型PDF经ORC扫描生成的，可能有错漏。查看时可比对2013版
  * 14496-15 涉及h264 h265 流  的 stsd  box 封装
    * 14496-15-2017  5.4.2 讲述 h264 的 avc1-avcC  box 结构， ''avcC'' 主要是 ''AVCDecoderConfigurationRecord'',  这个见  5.3.3.1.2
    * avc1 是 ''AVCSampleEntry''  ， 继承 ''VisualSampleEntry''
    * ''VisualSampleEntry''  结构见 14496-12_2015 12.1.3.2<code c>
	class VisualSampleEntry(codingname) extends SampleEntry (codingname){ 
		 unsigned int(16) pre_defined = 0; 
		 const unsigned int(16) reserved = 0; 
		 unsigned int(32)[3] pre_defined = 0; 
		 unsigned int(16) width; 
		 unsigned int(16) height; 
		 template unsigned int(32) horizresolution = 0x00480000; // 72 dpi 
		 template unsigned int(32) vertresolution = 0x00480000; // 72 dpi 
		 const unsigned int(32) reserved = 0; 
		 template unsigned int(16) frame_count = 1; 
		 string[32] compressorname; 
		 template unsigned int(16) depth = 0x0018; 
		 int(16) pre_defined = -1; 
		 // other boxes from derived specifications 
		 CleanApertureBox clap; // optional 
		 PixelAspectRatioBox pasp; // optional 
	}

	aligned(8) abstract class SampleEntry (unsigned int(32) format) 
	 extends Box(format){ 
	 const unsigned int(8)[6] reserved = 0; 
	 unsigned int(16) data_reference_index; 
	}
</code>
  * H265 的 hev1 - hvcC box 结构  14496-15_2013 就有提及，在 8.4.1 章节， hvcC box 包含 `HEVCDecoderConfigurationRecord` 结构，具体见 8.3.3.1 章节
  * ''Compressorname'' in the base class VisualSampleEntry indicates the name of the compressor used with the value "\013HEVC Coding" being recommended (\013 is 11, the length of the string in 
bytes).
  * ''HEVCDecoderConfigurationRecord'' 结构：<code c>
	aligned(8) class HEVCDecoderConfigurationRecord { 
		 unsigned int(8) configurationVersion = 1; 
		 unsigned int(2) general_profile_space; 
		 unsigned int(1) general_tier_flag; 
		 unsigned int(5) general_profile_idc; 
		 unsigned int(32) general_profile_compatibility_flags; 
		 unsigned int(48) general_constraint_indicator_flags; 
		 unsigned int(8) general_level_idc; 
		 bit(4) reserved = ‘1111’b; 
		 unsigned int(12) min_spatial_segmentation_idc; 
		 bit(6) reserved = ‘111111’b; 
		 unsigned int(2) parallelismType; 
		 bit(6) reserved = ‘111111’b; 
		 unsigned int(2) chromaFormat; 
		 bit(5) reserved = ‘11111’b; 
		 unsigned int(3) bitDepthLumaMinus8; 
		 bit(5) reserved = ‘11111’b; 
		 unsigned int(3) bitDepthChromaMinus8; 
		 bit(16) avgFrameRate; 
		 bit(2) constantFrameRate; 
		 bit(3) numTemporalLayers; 
		 bit(1) temporalIdNested; 
		 unsigned int(2) lengthSizeMinusOne; 
		 unsigned int(8) numOfArrays; 
		 for (j=0; j < numOfArrays; j++) { 
			 bit(1) array_completeness; 
			 unsigned int(1) reserved = 0; 
			 unsigned int(6) NAL_unit_type; 
			 unsigned int(16) numNalus; 
			 for (i=0; i< numNalus; i++) { 
				 unsigned int(16) nalUnitLength; 
				 bit(8*nalUnitLength) nalUnit; 
			 } 
		 } 
	}
</code>


==== 一些分析 ====

  * mp4 的主要box：
  * {{:public:it:426ff9a882f84a3e8503e94c5fe6d7c9.png?|}}

  * fmp4 的moov box:
  * {{:public:it:a23fa461c3944c798e5e51355a976832.png|}}


  * 组装fmp4的 moov box 必须获取视频流的 width, height,  pps, sps

  * 获取 pps sps:
  * avc1 参考 ffmpeg 的  mov_write_video_tag 函数

==== 哥伦布编码 ====
  * https://www.cnblogs.com/wangguchangqing/p/6297792.html
  * Golomb编码，及其两个变种：Golomb-Rice和Exp-Golomb