H264之NALU解析

IDR1 P4 B2 B3 P7 B5 B6 I10 B8 B9 P13 B11 B12 P16 B14 B15 
这⾥的B8可以跨过I10去参考P7(解码的时候哈)

原始图像：IDR1 B2 B3 P4 B5 B6 P7 B8 B9 I10

IDR1 P4 B2 B3 P7 B5 B6 IDR8 P11 B9 B10 P14 B11 B12 
这⾥的B9就只能参照IDR8和P11，不可以 参考IDR8前⾯的帧

⼀个NALU = ⼀组对应于视频编码的NALU头部信息 + 
⼀个原始字节序列负荷(RBSP,
Raw Byte Sequence Payload).

0x00 00 00 01 67 … 
0x00 00 00 01 68 …
0x00 00 00 01 65 …

67：
⼆进制：0110 0111
00111 = 7（⼗进制）

 const AVBitStreamFilter *bsfilter = av_bsf_get_by_name("h264_mp4toannexb"); 
 
 AVBSFContext *bsf_ctx = NULL; 
 // 2 初始化过滤器上下⽂ 
 av_bsf_alloc(bsfilter, &bsf_ctx); //AVBSFContext;
  // 3 添加解码器属性
  avcodec_parameters_copy(bsf_ctx->par_in, ifmt_ctx->streams[videoindex]->cod ecpar);
 av_bsf_init(bsf_ctx);

#include <stdio.h>
#include <libavutil/log.h>
#include <libavformat/avio.h>
#include <libavformat/avformat.h>

#define ADTS_HEADER_LEN  7;

const int sampling_frequencies[] = {
    96000,  // 0x0
    88200,  // 0x1
    64000,  // 0x2
    48000,  // 0x3
    44100,  // 0x4
    32000,  // 0x5
    24000,  // 0x6
    22050,  // 0x7
    16000,  // 0x8
    12000,  // 0x9
    11025,  // 0xa
    8000   // 0xb
    // 0xc d e f是保留的
};

int adts_header(char * const p_adts_header, const int data_length,
                const int profile, const int samplerate,
                const int channels)
{

    int sampling_frequency_index = 3; // 默认使用48000hz
    int adtsLen = data_length + 7;

    int frequencies_size = sizeof(sampling_frequencies) / sizeof(sampling_frequencies[0]);
    int i = 0;
    for(i = 0; i < frequencies_size; i++)
    {
        if(sampling_frequencies[i] == samplerate)
        {
            sampling_frequency_index = i;
            break;
        }
    }
    if(i >= frequencies_size)
    {
        printf("unsupport samplerate:%d\n", samplerate);
        return -1;
    }

    p_adts_header[0] = 0xff;         //syncword:0xfff                          高8bits
    p_adts_header[1] = 0xf0;         //syncword:0xfff                          低4bits
    p_adts_header[1] |= (0 << 3);    //MPEG Version:0 for MPEG-4,1 for MPEG-2  1bit
    p_adts_header[1] |= (0 << 1);    //Layer:0                                 2bits
    p_adts_header[1] |= 1;           //protection absent:1                     1bit

    p_adts_header[2] = (profile)<<6;            //profile:profile               2bits
    p_adts_header[2] |= (sampling_frequency_index & 0x0f)<<2; //sampling frequency index:sampling_frequency_index  4bits
    p_adts_header[2] |= (0 << 1);             //private bit:0                   1bit
    p_adts_header[2] |= (channels & 0x04)>>2; //channel configuration:channels  高1bit

    p_adts_header[3] = (channels & 0x03)<<6; //channel configuration:channels 低2bits
    p_adts_header[3] |= (0 << 5);               //original：0                1bit
    p_adts_header[3] |= (0 << 4);               //home：0                    1bit
    p_adts_header[3] |= (0 << 3);               //copyright id bit：0        1bit
    p_adts_header[3] |= (0 << 2);               //copyright id start：0      1bit
    p_adts_header[3] |= ((adtsLen & 0x1800) >> 11);           //frame length：value   高2bits

    p_adts_header[4] = (uint8_t)((adtsLen & 0x7f8) >> 3);     //frame length:value    中间8bits
    p_adts_header[5] = (uint8_t)((adtsLen & 0x7) << 5);       //frame length:value    低3bits
    p_adts_header[5] |= 0x1f;                                 //buffer fullness:0x7ff 高5bits
    p_adts_header[6] = 0xfc;      //‭11111100‬       //buffer fullness:0x7ff 低6bits
    // number_of_raw_data_blocks_in_frame：
    //    表示ADTS帧中有number_of_raw_data_blocks_in_frame + 1个AAC原始帧。

    return 0;
}

int main(int argc, char *argv[])
{
    int ret = -1;
    char errors[1024];

    char *in_filename = NULL;
    char *aac_filename = NULL;

    FILE *aac_fd = NULL;

    int audio_index = -1;
    int len = 0;


    AVFormatContext *ifmt_ctx = NULL;
    AVPacket pkt;

    // 设置打印级别
    av_log_set_level(AV_LOG_DEBUG);

    if(argc < 3)
    {
        av_log(NULL, AV_LOG_DEBUG, "the count of parameters should be more than three!\n");
        return -1;
    }

    in_filename = argv[1];      // 输入文件
    aac_filename = argv[2];     // 输出文件

    if(in_filename == NULL || aac_filename == NULL)
    {
        av_log(NULL, AV_LOG_DEBUG, "src or dts file is null, plz check them!\n");
        return -1;
    }

    aac_fd = fopen(aac_filename, "wb");
    if (!aac_fd)
    {
        av_log(NULL, AV_LOG_DEBUG, "Could not open destination file %s\n", aac_filename);
        return -1;
    }

    // 打开输入文件
    if((ret = avformat_open_input(&ifmt_ctx, in_filename, NULL, NULL)) < 0)
    {
        av_strerror(ret, errors, 1024);
        av_log(NULL, AV_LOG_DEBUG, "Could not open source file: %s, %d(%s)\n",
               in_filename,
               ret,
               errors);
        return -1;
    }

    // 获取解码器信息
    if((ret = avformat_find_stream_info(ifmt_ctx, NULL)) < 0)
    {
        av_strerror(ret, errors, 1024);
        av_log(NULL, AV_LOG_DEBUG, "failed to find stream information: %s, %d(%s)\n",
               in_filename,
               ret,
               errors);
        return -1;
    }

    // dump媒体信息
    av_dump_format(ifmt_ctx, 0, in_filename, 0);

    // 初始化packet
    av_init_packet(&pkt);

    // 查找audio对应的steam index
    audio_index = av_find_best_stream(ifmt_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
    if(audio_index < 0)
    {
        av_log(NULL, AV_LOG_DEBUG, "Could not find %s stream in input file %s\n",
               av_get_media_type_string(AVMEDIA_TYPE_AUDIO),
               in_filename);
        return AVERROR(EINVAL);
    }

    // 打印AAC级别
    printf("audio profile:%d, FF_PROFILE_AAC_LOW:%d\n",
           ifmt_ctx->streams[audio_index]->codecpar->profile,
           FF_PROFILE_AAC_LOW);

    if(ifmt_ctx->streams[audio_index]->codecpar->codec_id != AV_CODEC_ID_AAC)
    {
        printf("the media file no contain AAC stream, it's codec_id is %d\n",
               ifmt_ctx->streams[audio_index]->codecpar->codec_id);
        goto failed;
    }
    // 读取媒体文件，并把aac数据帧写入到本地文件
    while(av_read_frame(ifmt_ctx, &pkt) >=0 )
    {
        if(pkt.stream_index == audio_index)
        {
            char adts_header_buf[7] = {0};
            adts_header(adts_header_buf, pkt.size,
                        ifmt_ctx->streams[audio_index]->codecpar->profile,
                        ifmt_ctx->streams[audio_index]->codecpar->sample_rate,
                        ifmt_ctx->streams[audio_index]->codecpar->channels);
            fwrite(adts_header_buf, 1, 7, aac_fd);  // 写adts header , ts流不适用，ts流分离出来的packet带了adts header
            len = fwrite( pkt.data, 1, pkt.size, aac_fd);   // 写adts data
            if(len != pkt.size)
            {
                av_log(NULL, AV_LOG_DEBUG, "warning, length of writed data isn't equal pkt.size(%d, %d)\n",
                       len,
                       pkt.size);
            }
        }
        av_packet_unref(&pkt);
    }

failed:
    // 关闭输入文件
    if(ifmt_ctx)
    {
        avformat_close_input(&ifmt_ctx);
    }
    if(aac_fd)
    {
        fclose(aac_fd);
    }

    return 0;
}