本文为转载，原作者：小新快跑/音视频小话

自从ffmpeg6.0应用enhance rtmp支持h265/av1的flv格式后，7.0迎来了flv的hdr能力。本文介绍ffmpeg7.0如何支持hdr in flv。

enhance rtmp关于hdr

文档[enhance-rtmp-v2.md](https://github.com/veovera/enhanced-rtmp/blob/main/docs/enhanced/enhanced-rtmp-v2.md)中，metadata-frame章节规定相关支持HDR部分。

定义一个新的VideoPacketType.Metadata，其内容为AMF格式的metadata(AMF是一种简便的描述格式，非常节省内存)，具体内部有HDR相关的colorInfo数据(每次hdr参数更新的时候，就发送该metadata)。

type ColorInfo = {

colorConfig: {

// number of bits used to record the color channels for each pixel

bitDepth:                 number, // SHOULD be 8, 10 or 12



//

// colorPrimaries, transferCharacteristics and matrixCoefficients are defined

// in ISO/IEC 23091-4/ITU-T H.273. The values are an index into

// respective tables which are described in “Colour primaries”,

// "Transfer characteristics" and "Matrix coefficients" sections.

// It is RECOMMENDED to provide these values.

//



// indicates the chromaticity coordinates of the source color primaries

colorPrimaries:           number, // enumeration [0-255]



// opto-electronic transfer characteristic function (ex. PQ, HLG)

transferCharacteristics:  number, // enumeration [0-255]



// matrix coefficients used in deriving luma and chroma signals

matrixCoefficients:       number, // enumeration [0-255]

},



hdrCll: {

//

// maximum value of the frame average light level

// (in 1 cd/m2) of the entire playback sequence

//

maxFall:  number,     // [0.0001-10000]



//

// maximum light level of any single pixel (in 1 cd/m2)

// of the entire playback sequence

//

maxCLL:   number,     // [0.0001-10000]

},



//

// The hdrMdcv object defines mastering display (i.e., where

// creative work is done during the mastering process) color volume (a.k.a., mdcv)

// metadata which describes primaries, white point and min/max luminance. The

// hdrMdcv object SHOULD be provided.

//

// Specification of the metadata along with its ranges adhere to the

// ST 2086:2018 - SMPTE Standard (except for minLuminance see

// comments below)

//

hdrMdcv: {

//

// Mastering display color volume (mdcv) xy Chromaticity Coordinates within CIE

// 1931 color space.

//

// Values SHALL be specified with four decimal places. The x coordinate SHALL

// be in the range [0.0001, 0.7400]. The y coordinate SHALL be

// in the range [0.0001, 0.8400].

//

redX:         number,

redY:         number,

greenX:       number,

greenY:       number,

blueX:        number,

blueY:        number,

whitePointX:  number,

whitePointY:  number,



//

// max/min display luminance of the mastering display (in 1 cd/m2 ie. nits)

//

// note: ST 2086:2018 - SMPTE Standard specifies minimum display mastering

// luminance in multiples of 0.0001 cd/m2.

//

// For consistency we specify all values

// in 1 cd/m2. Given that a hypothetical perfect screen has a peak brightness

// of 10,000 nits and a black level of .0005 nits we do not need to

// switch units to 0.0001 cd/m2 to increase resolution on the lower end of the

// minLuminance property. The ranges (in nits) mentioned below suffice

// the theoretical limit for Mastering Reference Displays and adhere to the

// SMPTE ST 2084 standard (a.k.a., PQ) which is capable of representing full gamut

// of luminance level.

//

maxLuminance: number,     // [5-10000]

minLuminance: number,     // [0.0001-5]

},

}

flv mux

在libavformat/flvenc.c中，新增flv_write_metadata_packet函数，用于添加metadata。

flv的数据结构:

typedef struct FLVContext {

.....

int metadata_pkt_written;//0: 还未写过，需要写；1: 写过了，不需要写。

} FLVContext;

在函数flv_write_metadata_packet中实现，见注释：

static void flv_write_metadata_packet(AVFormatContext *s, AVCodecParameters *par, unsigned int ts) {

FLVContext *flv = s->priv_data;



//不需要写入，就返回；可以通过设置这个标志变量来使能/去使能更新写metadata

if (flv->metadata_pkt_written)

return;

//支持h265, av1, vp9

if (par->codec_id == AV_CODEC_ID_HEVC || par->codec_id == AV_CODEC_ID_AV1 || par->codec_id == AV_CODEC_ID_VP9) {

......

//写入tag头，标识其为视频

avio_w8(pb, FLV_TAG_TYPE_VIDEO); //write video tag type

metadata_size_pos = avio_tell(pb);

avio_wb24(pb, 0 + flags_size);

put_timestamp(pb, ts); //ts = pkt->dts, gen

avio_wb24(pb, flv->reserved);



//根据enhance rtmp标志，写入FLV_IS_EX_HEADER标识，和fourCC的字段(hvc1 or av01 or vp09)

if (par->codec_id == AV_CODEC_ID_HEVC) {

avio_w8(pb, FLV_IS_EX_HEADER | PacketTypeMetadata| FLV_FRAME_VIDEO_INFO_CMD); // ExVideoTagHeader mode with PacketTypeMetadata

avio_write(pb, "hvc1", 4);

} else if (par->codec_id == AV_CODEC_ID_AV1 || par->codec_id == AV_CODEC_ID_VP9) {

avio_w8(pb, FLV_IS_EX_HEADER | PacketTypeMetadata| FLV_FRAME_VIDEO_INFO_CMD);

avio_write(pb, par->codec_id == AV_CODEC_ID_AV1 ? "av01" : "vp09", 4);

}

//下面为写入AMF格式的hdr相关的colorInfo数据

avio_w8(pb, AMF_DATA_TYPE_STRING);

put_amf_string(pb, "colorInfo");



avio_w8(pb, AMF_DATA_TYPE_OBJECT);



put_amf_string(pb, "colorConfig");  // colorConfig



avio_w8(pb, AMF_DATA_TYPE_OBJECT);



if (par->color_trc != AVCOL_TRC_UNSPECIFIED &&

par->color_trc < AVCOL_TRC_NB) {

put_amf_string(pb, "transferCharacteristics");  // color_trc

put_amf_double(pb, par->color_trc);

}



if (par->color_space != AVCOL_SPC_UNSPECIFIED &&

par->color_space < AVCOL_SPC_NB) {

put_amf_string(pb, "matrixCoefficients"); // colorspace

put_amf_double(pb, par->color_space);

}



if (par->color_primaries != AVCOL_PRI_UNSPECIFIED &&

par->color_primaries < AVCOL_PRI_NB) {

put_amf_string(pb, "colorPrimaries"); // color_primaries

put_amf_double(pb, par->color_primaries);

}



put_amf_string(pb, "");

avio_w8(pb, AMF_END_OF_OBJECT);



if (lightMetadata) {

put_amf_string(pb, "hdrCll");

avio_w8(pb, AMF_DATA_TYPE_OBJECT);



put_amf_string(pb, "maxFall");

put_amf_double(pb, lightMetadata->MaxFALL);



put_amf_string(pb, "maxCLL");

put_amf_double(pb, lightMetadata->MaxCLL);



put_amf_string(pb, "");

avio_w8(pb, AMF_END_OF_OBJECT);

}



if (displayMetadata && (displayMetadata->has_primaries || displayMetadata->has_luminance)) {

put_amf_string(pb, "hdrMdcv");

avio_w8(pb, AMF_DATA_TYPE_OBJECT);

if (displayMetadata->has_primaries) {

put_amf_string(pb, "redX");

put_amf_double(pb, av_q2d(displayMetadata->display_primaries[0][0]));



put_amf_string(pb, "redY");

put_amf_double(pb, av_q2d(displayMetadata->display_primaries[0][1]));



put_amf_string(pb, "greenX");

put_amf_double(pb, av_q2d(displayMetadata->display_primaries[1][0]));



put_amf_string(pb, "greenY");

put_amf_double(pb, av_q2d(displayMetadata->display_primaries[1][1]));



put_amf_string(pb, "blueX");

put_amf_double(pb, av_q2d(displayMetadata->display_primaries[2][0]));



put_amf_string(pb, "blueY");

put_amf_double(pb, av_q2d(displayMetadata->display_primaries[2][1]));



put_amf_string(pb, "whitePointX");

put_amf_double(pb, av_q2d(displayMetadata->white_point[0]));



put_amf_string(pb, "whitePointY");

put_amf_double(pb, av_q2d(displayMetadata->white_point[1]));

}

if (displayMetadata->has_luminance) {

put_amf_string(pb, "maxLuminance");

put_amf_double(pb, av_q2d(displayMetadata->max_luminance));



put_amf_string(pb, "minLuminance");

put_amf_double(pb, av_q2d(displayMetadata->min_luminance));

}

put_amf_string(pb, "");

avio_w8(pb, AMF_END_OF_OBJECT);

}

put_amf_string(pb, "");

avio_w8(pb, AMF_END_OF_OBJECT);



flv->metadata_pkt_written = 1;//标识写过了

}

}

其中HDR的数据来源为AVCodecParameters *par数据结构中的内容：

typedef struct AVCodecParameters {

....

/**

* Video only. Additional colorspace characteristics.

*/

enum AVColorRange                  color_range;

enum AVColorPrimaries              color_primaries;

enum AVColorTransferCharacteristic color_trc;

enum AVColorSpace                  color_space;

enum AVChromaLocation              chroma_location;



/*

* 类型: AV_PKT_DATA_CONTENT_LIGHT_LEVEL, 数据: AVContentLightMetadata* lightMetadata

* 类型: AV_PKT_DATA_MASTERING_DISPLAY_METADATA, 数据: AVMasteringDisplayMetadata *displayMetadata

*/

AVPacketSideData *coded_side_data;

....

}

flv demux

解析函数在libavformat/flvdec.c文件中，函数amf_parse_object中。

static int amf_parse_object(AVFormatContext *s, AVStream *astream,

AVStream *vstream, const char *key,

int64_t max_pos, int depth)
{

FLVMetaVideoColor *meta_video_color = flv->metaVideoColor;

......

if (meta_video_color) {

if (amf_type == AMF_DATA_TYPE_NUMBER ||

amf_type == AMF_DATA_TYPE_BOOL) {

if (!strcmp(key, "colorPrimaries")) {

meta_video_color->primaries = num_val;

} else if (!strcmp(key, "transferCharacteristics")) {

meta_video_color->transfer_characteristics = num_val;

} else if (!strcmp(key, "matrixCoefficients")) {

meta_video_color->matrix_coefficients = num_val;

} else if (!strcmp(key, "maxFall")) {

meta_video_color->max_fall = num_val;

} else if (!strcmp(key, "maxCLL")) {

meta_video_color->max_cll = num_val;

} else if (!strcmp(key, "redX")) {

meta_video_color->mastering_meta.r_x = num_val;

} else if (!strcmp(key, "redY")) {

meta_video_color->mastering_meta.r_y = num_val;

} else if (!strcmp(key, "greenX")) {

meta_video_color->mastering_meta.g_x = num_val;

} else if (!strcmp(key, "greenY")) {

meta_video_color->mastering_meta.g_y = num_val;

} else if (!strcmp(key, "blueX")) {

meta_video_color->mastering_meta.b_x = num_val;

} else if (!strcmp(key, "blueY")) {

meta_video_color->mastering_meta.b_y = num_val;

} else if (!strcmp(key, "whitePointX")) {

meta_video_color->mastering_meta.white_x = num_val;

} else if (!strcmp(key, "whitePointY")) {

meta_video_color->mastering_meta.white_y = num_val;

} else if (!strcmp(key, "maxLuminance")) {

meta_video_color->mastering_meta.max_luminance = num_val;

} else if (!strcmp(key, "minLuminance")) {

meta_video_color->mastering_meta.min_luminance = num_val;

}

}

}

......

}