关于c++:SkeyeRTMPClient扩展支持HEVC解决方案之HEVCDecoderConfigurationRecord结构详解

在上一篇 SkeyeRTMPClient 拉取 RTMP 流扩大反对 HEVC(H.265) 解决方案 中对于 HEVCDecoderConfigurationRecord 构造解析的解说存在一些表述上不分明的中央，本文为之续篇，重点对 HEVC 格局的 MetaData 构造的解析进行解说。
在 SkeyeRTMPPusher 扩大反对 H265 的解决方案讲述时。咱们对 Metadata 构造进行过详解，大家能够回顾一下这篇文章 RTMP 推送扩大反对 HEVC(H265) 之 Metadata 构造填写详解，重点来了，因为失常状况下，咱们只须要从 MetaData 中取出对咱们解码有用的数据头（即 VPS,SPS 和 PPS），所以咱们对 HEVCDecoderConfigurationRecord 填充的 MetaData 其余数据并不关怀，然而，在解析时，咱们须要对该构造所有数据都解析进去，以保障能准确无误的获取到咱们所须要的数据头信息。

再次回顾 HEVCDecoderConfigurationRecord 构造：

typedef struct HEVCDecoderConfigurationRecord {
    uint8_t  configurationVersion;
    uint8_t  general_profile_space;
    uint8_t  general_tier_flag;
    uint8_t  general_profile_idc;
    uint32_t general_profile_compatibility_flags;
    uint64_t general_constraint_indicator_flags;
    uint8_t  general_level_idc;
    uint16_t min_spatial_segmentation_idc;
    uint8_t  parallelismType;
    uint8_t  chromaFormat;
    uint8_t  bitDepthLumaMinus8;
    uint8_t  bitDepthChromaMinus8;
    uint16_t avgFrameRate;
    uint8_t  constantFrameRate;
    uint8_t  numTemporalLayers;
    uint8_t  temporalIdNested;
    uint8_t  lengthSizeMinusOne;
    uint8_t  numOfArrays;
    HVCCNALUnitArray *array;
} HEVCDecoderConfigurationRecord;

而事实上，该构造如果间接填入到 MetaData 中是不正确的，咱们看 ffmpeg 中 hevc.c 文件中的实现，该构造具体申明如下：

// The CodecPrivate syntax shall follow the
// syntax of HEVCDecoderConfigurationRecord
// defined in ISO/IEC 14496-15.
//
// The number zero (0) shall be written to
// the configurationVersion variable until
// official finalization of 14496-15, 3rd ed.
//
// After its finalization, this field and the
// following CodecPrivate structure shall
// follow the definition of the
// HEVCDecoderConfigurationRecord in 14496-15.

unsigned int(8)  configurationVersion;
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;
  }
}

从上代码段咱们能够看出，以 general_constraint_indicator_flags 这个参数为例，构造体申明位宽 64，而理论位宽是 48，, 所以构造体申明的参数位宽和理论位宽可能是不对等的，这就将导致解析 MetaData 时产生错位，从而解析产生谬误，从而，咱们从新意识 HEVCDecoderConfigurationRecord，并申明其构造如下：

// RTMP 扩大反对 HEVC(H.265) [4/18/2019 SwordTwelve]
typedef struct  _Parser_HEVCDecoderConfigurationRecord {
    uint8_t  configurationVersion;
    uint8_t  general_profile_space:2;
    uint8_t  general_tier_flag:1;
    uint8_t  general_profile_idc:5;
    uint32_t general_profile_compatibility_flags;//6
    uint8_t general_constraint_indicator_flags[6];//12
    uint8_t  general_level_idc;
    uint8_t reserved1:4;// bit(4) reserved =‘1111’b;
    uint8_t min_spatial_segmentation_idc_L:4;//12 位之低 4 位
    uint8_t min_spatial_segmentation_idc_H;//12 位之高 8 位
    uint8_t reserved2:6;//bit(6) reserved =‘111111’b;
    uint8_t  parallelismType:2;
    uint8_t reserved3:6;//bit(6) reserved =‘111111’b;
    uint8_t  chromaFormat:2;
    uint8_t reserved4:5;//bit(5) reserved =‘11111’b;
    uint8_t  bitDepthLumaMinus8:3;
    uint8_t reserved5:5;//bit(5) reserved =‘11111’b;
    uint8_t  bitDepthChromaMinus8:3;
    uint16_t avgFrameRate;
    uint8_t  constantFrameRate:2;
    uint8_t  numTemporalLayers:3;
    uint8_t  temporalIdNested:1;
    uint8_t  lengthSizeMinusOne:2;
    uint8_t  numOfArrays;
    //Parser_HVCCNALUnitArray *array;
} Parser_HEVCDecoderConfigurationRecord;

当初位宽曾经对齐，咱们能够间接从 MetaData 外面讲该构造拷贝进去，从而获取到正确的参数和值，如下代码所示：

        Parser_HEVCDecoderConfigurationRecord *decoder_header = (Parser_HEVCDecoderConfigurationRecord*)((char*)parser_config);
        parser_offset += sizeof(Parser_HEVCDecoderConfigurationRecord);
        int nNumOfArrays = decoder_header->numOfArrays;
        for (int i=0; i<nNumOfArrays; i++)
        {......}

同理，咱们从 MetaData 中拷贝出 nal 单元数据头也是须要思考这个问题，这里咱们申明 Parser_HVCCNALUnitArray 构造如下：

// RTMP 扩大反对 HEVC(H.265) [4/18/2019 SwordTwlve]
typedef struct _Parser_HVCCNALUnitArray {
    uint8_t  NAL_unit_type;
//    uint16_t numNalus;
//     uint16_t *nalUnitLength;
//     uint8_t  **nalUnit;
} Parser_HVCCNALUnitArray;

如上代码所示，为了保障字节对齐，咱们只保留 NAL_unit_type 参数，通过字节拷贝运行，计算出 nalu 的数量 numNalus，而后再从 nalunit 数组中获得咱们须要的头信息。

    Parser_HVCCNALUnitArray* pNALUnit = (Parser_HVCCNALUnitArray*)((char *)parser_config+parser_offset);
    parser_offset += sizeof(Parser_HVCCNALUnitArray);
    int numNalus = ntohs(*(unsigned short *)((char *)parser_config + parser_offset));  
    parser_offset += 2;
    for (int nI=0; nI<numNalus; nI++)
    {......}