六.FFmpeg对mp4文件操作（ubuntu）

一.抽取aac数据

1.代码抽取aac没有声音，使用ffmpeg命令行也无法转换为wav

问题解决：ADTS头的采样率没有设置正确，av_dump_format 获取视频信息的时候可以看到aac的详细信息。

void Widget::adts_header(char *szAdtsHeader, int dataLen)
{

    int audio_object_type = 2; //音频对象类型 2 aac_lc
    int sampling_frequency_index = 4;//采样率
    int channel_config = 2;//声道数

    int adtsLen = dataLen + 7;

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

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

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

    szAdtsHeader[4] = (uint8_t)((adtsLen & 0x7f8) >> 3);     //frame length:value    中间8bits
    szAdtsHeader[5] = (uint8_t)((adtsLen & 0x7) << 5);       //frame length:value    低3bits
    szAdtsHeader[5] |= 0x1f;                                 //buffer fullness:0x7ff 高5bits
    szAdtsHeader[6] = 0xfc;
}
void Widget::loadMp4Info()
{
    AVFormatContext *fmt_ctx=NULL;
    AVPacket avpkt;
    av_log_set_level(AV_LOG_INFO);

    aac_file=fopen("./2159.aac","wb");
    if(!aac_file)
    {
        qDebug()<<"failed open aac file";
    }

    int ret=0;
    ret=avformat_open_input(&fmt_ctx,"./1.mp4",NULL,NULL);
    if(ret!=0)
    {
        qDebug()<<"failed open avformat";
        return;
    }
    av_dump_format(fmt_ctx,0,"/2.mp4",0);

    //get aac data
    int audio_index=av_find_best_stream(fmt_ctx,AVMEDIA_TYPE_AUDIO,-1,-1,NULL,0);
    if(audio_index<0)
    {
        av_log(NULL,AV_LOG_ERROR,"can't find the best stream\n");
        return;
    }
    //write data to file
    av_init_packet(&avpkt);

    int wirteLen=0;

    while(av_read_frame(fmt_ctx,&avpkt)>=0)
    {
        if(avpkt.stream_index==audio_index)
        {            //add aac header
            char adts_header_buff[7];
            adts_header(adts_header_buff,avpkt.size);
            fwrite(adts_header_buff,1,7,aac_file);

            wirteLen=fwrite(avpkt.data,1,avpkt.size,aac_file);
            if(wirteLen!=avpkt.size)
            {
                av_log(NULL,AV_LOG_WARNING,"warning wirteLen != pkt.size %d , %d",wirteLen,avpkt.size);
            }
        }
        av_packet_unref(&avpkt);
    }


    avformat_close_input(&fmt_ctx);

}

二.抽取h264数据