FFMPEG + SDL音频播放分析

曾经终败给现在 2022-08-24 14:29 225阅读 0赞

目录 \[[hide][]\]

*  [1 抽象流程：][1]
 *  [2 关键实现：][2]
    
     *  [2.1 main()函数][2.1_main]
     *  [2.2 decode\_thread()读取文件信息和音频包][2.2_decode_thread]
     *  [2.3 stream\_component\_open()：设置音频参数和打开设备][2.3_stream_component_open]
     *  [2.4 audio\_callback(): 回调函数，向SDL缓冲区填充数据][2.4_audio_callback_ _SDL]
     *  [2.5 audio\_decode\_frame()：解码音频][2.5_audio_decode_frame]
 *  [3 FFMPEG结构体][3_FFMPEG]
    
     *  [3.1 channel\_layout\_map][3.1_channel_layout_map]
 *  [4 FFMPEG宏定义][4_FFMPEG]
    
     *  [4.1 Audio channel convenience macros][4.1_Audio channel convenience macros]
 *  [5 SDL宏定义][5_SDL]
    
     *  [5.1 SDL\_AudioSpec format][5.1_SDL_AudioSpec format]

# 抽象流程： #

设置SDL的音频参数 —-> 打开声音设备，播放静音 —-> ffmpeg读取音频流中数据放入队列 —-> SDL调用用户设置的函数来获取音频数据 —-> 播放音频

SDL内部维护了一个buffer来存放解码后的数据，这个buffer中的数据来源是我们注册的回调函数（audio\_callback），audio\_callback调用audio\_decode\_frame来做具体的音频解码工作，需要引起注意的是：从流中读取出的一个音频包(avpacket)可能含有多个音频桢(avframe)，所以需要多次调用avcodec\_decode\_audio4来完成整个包的解码，解码出来的数据存放在我们自己的缓冲中（audio\_buf2）。SDL每一次回调都会引起数据从audio\_buf2拷贝到SDL内部缓冲区，当audio\_buf2中的数据大于SDL的缓冲区大小时，需要分多次拷贝。

# 关键实现： #

## main()函数 ##

int main(int argc, char **argv){
        SDL_Event event; //SDL事件变量
        VideoState    *is; // 纪录视频及解码器等信息的大结构体
        is = (VideoState*) av_mallocz(sizeof(VideoState));
        if(argc < 2){
            fprintf(stderr, "Usage: play <file>\n");
            exit(1);
        }
        av_register_all(); //注册所有ffmpeg的解码器
        /* 初始化SDL，这里只实用了AUDIO，如果有视频，好需要SDL_INIT_VIDEO等等 */
        if(SDL_Init(SDL_INIT_AUDIO)){
            fprintf(stderr, "Count not initialize SDL - %s\n", SDL_GetError());
            exit(1);
        }
        is_strlcpy(is->filename, argv[1], sizeof(is->filename));
        /* 创建一个SDL线程来做视频解码工作，主线程进入SDL事件循环 */
        is->parse_tid = SDL_CreateThread(decode_thread, is);
        if(!is->parse_tid){
            SDL_WaitEvent(&event);
            switch(event.type){
                case FF_QUIT_EVENT:
                case SDL_QUIT:
                     is->quit = 1;
                    SDL_Quit();
                    exit(0);
                    break;
                default:
                     break;
            }
        }
        return 0;
    }

## decode\_thread()读取文件信息和音频包 ##

static int decode_thread(void *arg){
        VideoState *is = (VideoState*)arg;
        AVFormatContext *ic = NULL;
        AVPacket pkt1, *packet = &pkt1;
        int ret, i, audio_index = -1;
    
        is->audioStream = -1;
        global_video_state = is; 
        /*  使用ffmpeg打开视频，解码器等 常规工作 */
        if(avFormat_open_input(&ic, is->filename, NULL,  NULL) != 0)  {
            fprintf(stderr, "open file error: %s\n", is->filename);
            return -1;
        }
        is->ic = ic;
        if(avformat_find_stream_info(ic, NULL) < 0){
            fprintf(stderr, "find stream info error\n");
            return -1;
        }
        av_dump_format(ic, 0, is->filename, 0);
        for(i  = 0; i < ic->nb_streams; i++){
             if(ic->streams[i])->codec->codec_type == AVMEDIA_TYPE_AUDIO && audio_index == -1){
                audio_index = i;
                break;
            }
        }
        if(audio_index >= 0) {
            /* 所有设置SDL音频流信息的步骤都在这个函数里完成 */
            stream_component_open(is, audio_index);
        }
        if(is->audioStream < 0){
            fprintf(stderr, "could not open codecs for file: %s\n", is->filename);
            goto fail;
        }
        /* 读包的主循环， av_read_frame不停的从文件中读取数据包（这里只取音频包）*/
        for(;;){
            if(is->quit) break;
            /* 这里audioq.size是指队列中的所有数据包带的音频数据的总量，并不是包的数量 */
            if(is->audioq.size > MAX_AUDIO_SIZE){
                SDL_Delay(10); // 毫秒
                continue;
            }
             ret = av_read_frame(is->ic, packet);
             if(ret < 0){
                    if(ret == AVERROR_EOF || url_feof(is->ic->pb))    break;
                    if(is->ic->pb && is->ic->pb->error)    break;
                    contiue;                  
              }  
              if(packet->stream_index == is->audioStream){
                        packet_queue_put(&is->audioq, packet);
               } else{
                         av_free_packet(packet);
                }
        }
         while(!is->quit)    SDL_Delay(100);
    fail: {
                   SDL_Event event;
                   event.type = FF_QUIT_EVENT;
                   event.user.data1 = is;
                   SDL_PushEvent(&event);
            }
            return 0;
    }

## stream\_component\_open()：设置音频参数和打开设备 ##

int stream_component_open(videoState *is, int stream_index){
        AVFormatContext *ic = is->ic;
        AVCodecContext *codecCtx;
        AVCodec *codec;
        /* 在用SDL_OpenAudio()打开音频设备的时候需要这两个参数*/
        /* wanted_spec是我们期望设置的属性，spec是系统最终接受的参数 */
        /* 我们需要检查系统接受的参数是否正确 */
        SDL_AudioSpec wanted_spec, spec;
        int64_t wanted_channel_layout = 0; // 声道布局(SDL中的具体定义见“FFMPEG结构体”部分) 
        int wanted_nb_channels; // 声道数
        /*  SDL支持的声道数为 1, 2, 4, 6 */
        /*  后面我们会使用这个数组来纠正不支持的声道数目 */
        const int next_nb_channels[] = { 0, 0, 1, 6,  2, 6, 4, 6 }; 
    
        if(stream_index < 0 || stream_index >= ic->nb_streams)    return -1;
        codecCtx = ic->streams[stream_index]->codec;
        wanted_nb_channels = codecCtx->channels;
        if(!wanted_channel_layout || wanted_nb_channels != av_get_channel_layout_nb_channels(wanted_channel_layout)) {
            wanted_channel_layout = av_get_default_channel_lauout(wanted_channel_nb_channels);
            wanted_channel_layout &= ~AV_CH_LAYOUT_STEREO_DOWNMIX;
        }
        wanted_spec.channels = av_get_channels_layout_nb_channels(wanted_channel_layout);
        wanted_spec.freq = codecCtx->sample_rate;
        if(wanted_spec.freq <= 0 || wanted_spec.channels <=0){
               fprintf(stderr, "Invaild sample rate or channel count!\n");
                return -1;
        }
        wanted_spec.format = AUDIO_S16SYS; // 具体含义请查看“SDL宏定义”部分
        wanted_spec.silence = 0; // 0指示静音
        wanted_spec.samples = SDL_AUDIO_BUFFER_SIZE; // 自定义SDL缓冲区大小
        wanted_spec.callback = audio_callback; // 音频解码的关键回调函数
        wanted_spec.userdata = is; // 传给上面回调函数的外带数据
    
        /*  打开音频设备，这里使用一个while来循环尝试打开不同的声道数(由上面 */
        /*  next_nb_channels数组指定）直到成功打开，或者全部失败 */
        while(SDL_OpenAudio(&wanted_spec, &spec) < 0){
            fprintf(stderr, "SDL_OpenAudio(%d channels): %s\n", wanted_spec.channels, SDL_GetError());
            wanted_spec.channels = next_nb_channels[FFMIN(7, wanted_spec.channels)]; // FFMIN()由ffmpeg定义的宏，返回较小的数
            if(!wanted_spec.channels){
                  fprintf(stderr, "No more channel to try\n");
                  return -1;
            }
            wanted_channel_layout = av_get_default_channel_layout(wanted_spec.channels);
        }
        /* 检查实际使用的配置（保存在spec,由SDL_OpenAudio()填充） */
        if(spec.format != AUDIO_S16SYS){
            fprintf(stderr, "SDL advised audio format %d is not supported\n", spec.format);
            return -1;
        }
        if(spec.channels != wanted_spec.channels) {
            wanted_channel_layout = av_get_default_channel_layout(spec.channels);
            if(!wanted_channel_layout){
                    fprintf(stderr, "SDL advised channel count %d is not support\n", spec.channels);
                    return -1;
            }
        }
        /* 把设置好的参数保存到大结构中 */
        is->audio_src_fmt = is->audio_tgt_fmt = AV_SAMPLE_FMT_S16;
        is->audio_src_freq = is->audio_tgt_freq = spec.freq;
        is->audio_src_channel_layout = is->audio_tgt_layout = wanted_channel_layout;
        is->audio_src_channels = is->audio_tat_channels = spec.channels;
    
        codec = avcodec_find_decoder(codecCtx>codec_id);
        if(!codec || (avcodec_open2(codecCtx, codec, NULL) < 0)){
            fprintf(stderr, "Unsupported codec!\n");
            return -1;
        }
        ic->streams[stream_index]->discard = AVDISCARD_DEFAULT; //具体含义请查看“FFMPEG宏定义”部分
        is->audioStream = stream_index;
        is->audio_st = ic->streams[stream_index];
        is->audio_buf_size = 0;
        is->audio_buf_index = 0;
        memset(&is->audio_pkt, 0, sizeof(is->audio_pkt));
        packet_queue_init(&is->audioq);
        SDL_PauseAudio(0); // 开始播放静音
    }

## audio\_callback(): 回调函数，向SDL缓冲区填充数据 ##

void audio_callback(void *userdata, Uint8 *stream, int len){
        VideoState *is = (VideoState*)userdata;
        int len1, audio_data_size;
    
        /*   len是由SDL传入的SDL缓冲区的大小，如果这个缓冲未满，我们就一直往里填充数据 */
        while(len > 0){
            /*  audio_buf_index 和 audio_buf_size 标示我们自己用来放置解码出来的数据的缓冲区，*/
            /*   这些数据待copy到SDL缓冲区， 当audio_buf_index >= audio_buf_size的时候意味着我*/
            /*   们的缓冲为空，没有数据可供copy，这时候需要调用audio_decode_frame来解码出更
            /*   多的桢数据 */
            if(is->audio_buf_index >= is->audio_buf_size){
                    audio_data_size = audio_decode_frame(is);
                    /* audio_data_size < 0 标示没能解码出数据，我们默认播放静音 */
                    is(audio_data_size < 0){
                             is->audio_buf_size = 1024;
                             /* 清零，静音 */
    			 memset(is->audio_buf, 0, is->audio_buf_size);
                    } else{
                              is->audio_buf_size = audio_data_size;
                     }
                     is->audio_buf_index = 0;
            }
            /*  查看stream可用空间，决定一次copy多少数据，剩下的下次继续copy */
            len1 = is->audio_buf_size - is->audio_buf_index;
            if(len1 > len)    len1 = len;
    
            memcpy(stream, (uint8_t*)is->audio_buf + is->audio_buf_index, len1);
            len -= len1;
            stream += len1;
            is->audio_buf_index += len1;
        }
    }

## audio\_decode\_frame()：解码音频 ##

int audio_decode_frame(VideoState *is){
        int len1, len2, decoded_data_size;
        AVPacket *pkt = &is->audio_pkt;
        int got_frame = 0;
        int64_t dec_channel_layout;
        int wanted_nb_samples, resampled_data_size;
    
        for(;;){
          while(is->audio_pkt_size > 0){
            if(!is->audio_frame){
                if(!(is->audio_frame = avacodec_alloc_frame())){
                    return AVERROR(ENOMEM);
                }
            } else
              avcodec_get_frame_defaults(is->audio_frame);
    
            len1 = avcodec_decode_audio4(is->audio_st_codec, is->audio_frame, got_frame, pkt);
            /* 解码错误，跳过整个包 */
            if(len1 < 0){
               is->audio_pkt_size = 0;
               break;
            }
            is->audio_pkt_data += len1;
            is->audio_pkt_size -= len1;
            if(!got_frame)   continue;
            /* 计算解码出来的桢需要的缓冲大小 */
            decoded_data_size = av_samples_get_buffer_size(NULL,
                                is->audio_frame_channels,
                                is->audio_frame_nb_samples,
                                is->audio_frame_format, 1);
            dec_channel_layout = (is->audio_frame->channel_layout && is->audio_frame->channels
                       == av_get_channel_layout_nb_channels(is->audio_frame->channel_layout))
                       ? is->audio_frame->channel_layout : av_get_default_channel_layout(is->audio_frame->channels);                       
            wanted_nb_samples =  is->audio_frame->nb_samples;
            if (is->audio_frame->format != is->audio_src_fmt || 
                dec_channel_layout != is->audio_src_channel_layout ||
                is->audio_frame->sample_rate != is->audio_src_freq || 
                (wanted_nb_samples != is->audio_frame->nb_samples && !is->swr_ctx)) {
                    if (is->swr_ctx) swr_free(&is->swr_ctx);
                    is->swr_ctx = swr_alloc_set_opts(NULL,
                                                     is->audio_tgt_channel_layout,
                                                     is->audio_tgt_fmt,
                                                     is->audio_tgt_freq,
                                                     dec_channel_layout,
                                                     is->audio_frame->format,
                                                     is->audio_frame->sample_rate,
                                                     0, NULL);
                     if (!is->swr_ctx || swr_init(is->swr_ctx) < 0) {
                         fprintf(stderr, "swr_init() failed\n");
                         break;
                     }
                     is->audio_src_channel_layout = dec_channel_layout;
                     is->audio_src_channels = is->audio_st->codec->channels;
                     is->audio_src_freq = is->audio_st->codec->sample_rate;
                     is->audio_src_fmt = is->audio_st->codec->sample_fmt;
             }
             /* 这里我们可以对采样数进行调整，增加或者减少，一般可以用来做声画同步 */
             if (is->swr_ctx) {
                 const uint8_t **in = (const uint8_t **)is->audio_frame->extended_data;
                 uint8_t *out[] = { is->audio_buf2 };
                 if (wanted_nb_samples != is->audio_frame->nb_samples) {
                    if(swr_set_compensation(is->swr_ctx, 
                      (wanted_nb_samples - is->audio_frame->nb_samples)*is->audio_tgt_freq/is->audio_frame->sample_rate,
                       wanted_nb_samples * is->audio_tgt_freq/is->audio_frame->sample_rate) < 0) {
                            fprintf(stderr, "swr_set_compensation() failed\n");
                            break;
                       }
                 }
                 len2 = swr_convert(is->swr_ctx, out,  
                      sizeof(is->audio_buf2)/is->audio_tgt_channels/av_get_bytes_per_sample(is->audio_tgt_fmt),  
                      in, is->audio_frame->nb_samples);
                 if (len2 < 0) {
                      fprintf(stderr, "swr_convert() failed\n");
                      break;
                 }
                 if(len2 == sizeof(is->audio_buf2)/is->audio_tgt_channels/av_get_bytes_per_sample(is->audio_tgt_fmt)) {
                     fprintf(stderr, "warning: audio buffer is probably too small\n");
                     swr_init(is->swr_ctx);
                 }
                 is->audio_buf = is->audio_buf2;
                 resampled_data_size = len2*is->audio_tgt_channels*av_get_bytes_per_sample(is->audio_tgt_fmt);
               } else {
                 resampled_data_size = decoded_data_size;
                 is->audio_buf = is->audio_frame->data[0];
               }
               /*  返回得到的数据 */
               return resampled_data_size;
           }
           if (pkt->data) av_free_packet(pkt);
           memset(pkt, 0, sizeof(*pkt));
           if (is->quit) return -1;
           if (packet_queue_get(&is->audioq, pkt, 1) < 0) return -1;
           is->audio_pkt_data = pkt->data;
           is->audio_pkt_size = pkt->size;
    
         }
    }

# FFMPEG结构体 #

## channel\_layout\_map ##

static const struct {
    const char *name;
    int nb_channels;
    uint64_t layout;
    } channel_layout_map[] = {
    { "mono", 1, AV_CH_LAYOUT_MONO },
    { "stereo", 2, AV_CH_LAYOUT_STEREO },
    { "2.1", 3, AV_CH_LAYOUT_2POINT1 },
    { "3.0", 3, AV_CH_LAYOUT_SURROUND },
    { "3.0(back)", 3, AV_CH_LAYOUT_2_1 },
    { "4.0", 4, AV_CH_LAYOUT_4POINT0 },
    { "quad", 4, AV_CH_LAYOUT_QUAD },
    { "quad(side)", 4, AV_CH_LAYOUT_2_2 },
    { "3.1", 4, AV_CH_LAYOUT_3POINT1 },
    { "5.0", 5, AV_CH_LAYOUT_5POINT0_BACK },
    { "5.0(side)", 5, AV_CH_LAYOUT_5POINT0 },
    { "4.1", 5, AV_CH_LAYOUT_4POINT1 },
    { "5.1", 6, AV_CH_LAYOUT_5POINT1_BACK },
    { "5.1(side)", 6, AV_CH_LAYOUT_5POINT1 },
    { "6.0", 6, AV_CH_LAYOUT_6POINT0 },
    { "6.0(front)", 6, AV_CH_LAYOUT_6POINT0_FRONT },
    { "hexagonal", 6, AV_CH_LAYOUT_HEXAGONAL },
    { "6.1", 7, AV_CH_LAYOUT_6POINT1 },
    { "6.1", 7, AV_CH_LAYOUT_6POINT1_BACK },
    { "6.1(front)", 7, AV_CH_LAYOUT_6POINT1_FRONT },
    { "7.0", 7, AV_CH_LAYOUT_7POINT0 },
    { "7.0(front)", 7, AV_CH_LAYOUT_7POINT0_FRONT },
    { "7.1", 8, AV_CH_LAYOUT_7POINT1 },
    { "7.1(wide)", 8, AV_CH_LAYOUT_7POINT1_WIDE },
    { "octagonal", 8, AV_CH_LAYOUT_OCTAGONAL },
    { "downmix", 2, AV_CH_LAYOUT_STEREO_DOWNMIX, },
    };

# FFMPEG宏定义 #

## Audio channel convenience macros ##

#define AV_CH_LAYOUT_MONO              (AV_CH_FRONT_CENTER)
     #define AV_CH_LAYOUT_STEREO            (AV_CH_FRONT_LEFT|AV_CH_FRONT_RIGHT)
     #define AV_CH_LAYOUT_2POINT1           (AV_CH_LAYOUT_STEREO|AV_CH_LOW_FREQUENCY)
     #define AV_CH_LAYOUT_2_1               (AV_CH_LAYOUT_STEREO|AV_CH_BACK_CENTER)
     #define AV_CH_LAYOUT_SURROUND          (AV_CH_LAYOUT_STEREO|AV_CH_FRONT_CENTER)
     #define AV_CH_LAYOUT_3POINT1           (AV_CH_LAYOUT_SURROUND|AV_CH_LOW_FREQUENCY)
     #define AV_CH_LAYOUT_4POINT0           (AV_CH_LAYOUT_SURROUND|AV_CH_BACK_CENTER)
     #define AV_CH_LAYOUT_4POINT1           (AV_CH_LAYOUT_4POINT0|AV_CH_LOW_FREQUENCY)
     #define AV_CH_LAYOUT_2_2               (AV_CH_LAYOUT_STEREO|AV_CH_SIDE_LEFT|AV_CH_SIDE_RIGHT)
     #define AV_CH_LAYOUT_QUAD              (AV_CH_LAYOUT_STEREO|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
     #define AV_CH_LAYOUT_5POINT0           (AV_CH_LAYOUT_SURROUND|AV_CH_SIDE_LEFT|AV_CH_SIDE_RIGHT)
     #define AV_CH_LAYOUT_5POINT1           (AV_CH_LAYOUT_5POINT0|AV_CH_LOW_FREQUENCY)
     #define AV_CH_LAYOUT_5POINT0_BACK      (AV_CH_LAYOUT_SURROUND|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
     #define AV_CH_LAYOUT_5POINT1_BACK      (AV_CH_LAYOUT_5POINT0_BACK|AV_CH_LOW_FREQUENCY)
     #define AV_CH_LAYOUT_6POINT0           (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_CENTER)
     #define AV_CH_LAYOUT_6POINT0_FRONT     (AV_CH_LAYOUT_2_2|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
     #define AV_CH_LAYOUT_HEXAGONAL         (AV_CH_LAYOUT_5POINT0_BACK|AV_CH_BACK_CENTER)
     #define AV_CH_LAYOUT_6POINT1           (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_CENTER)
     #define AV_CH_LAYOUT_6POINT1_BACK      (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_BACK_CENTER)
     #define AV_CH_LAYOUT_6POINT1_FRONT     (AV_CH_LAYOUT_6POINT0_FRONT|AV_CH_LOW_FREQUENCY)
     #define AV_CH_LAYOUT_7POINT0           (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
     #define AV_CH_LAYOUT_7POINT0_FRONT     (AV_CH_LAYOUT_5POINT0|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
     #define AV_CH_LAYOUT_7POINT1           (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)
    #define AV_CH_LAYOUT_7POINT1_WIDE      (AV_CH_LAYOUT_5POINT1|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
    #define AV_CH_LAYOUT_7POINT1_WIDE_BACK (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)
    #define AV_CH_LAYOUT_OCTAGONAL         (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_LEFT|AV_CH_BACK_CENTER|AV_CH_BACK_RIGHT)
    #define AV_CH_LAYOUT_STEREO_DOWNMIX    (AV_CH_STEREO_LEFT|AV_CH_STEREO_RIGHT)

# SDL宏定义 #

## SDL\_AudioSpec format ##

AUDIO_U8           Unsigned 8-bit samples
    AUDIO_S8            Signed 8-bit samples
    AUDIO_U16LSB    Unsigned 16-bit samples, in little-endian byte order
    AUDIO_S16LSB    Signed 16-bit samples, in little-endian byte order
    AUDIO_U16MSB    Unsigned 16-bit samples, in big-endian byte order
    AUDIO_S16MSB    Signed 16-bit samples, in big-endian byte order
    AUDIO_U16           same as AUDIO_U16LSB (for backwards compatability probably)
    AUDIO_S16           same as AUDIO_S16LSB (for backwards compatability probably)
    AUDIO_U16SYS    Unsigned 16-bit samples, in system byte order
    AUDIO_S16SYS     Signed 16-bit samples, in system byte order

git clone https://github.com/lnmcc/musicPlayer.git

转帖：http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/

[hide]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#
[1]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#i
[2]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#i-2
[2.1_main]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#main
[2.2_decode_thread]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#decode_thread
[2.3_stream_component_open]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#stream_component_open
[2.4_audio_callback_ _SDL]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#audio_callback_SDL
[2.5_audio_decode_frame]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#audio_decode_frame
[3_FFMPEG]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#FFMPEG
[3.1_channel_layout_map]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#channel_layout_map
[4_FFMPEG]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#FFMPEG-2
[4.1_Audio channel convenience macros]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#Audio_channel_convenience_macros
[5_SDL]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#SDL
[5.1_SDL_AudioSpec format]: http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/#SDL_AudioSpec_format