流程

int main(int argc, char **argv)
{
...
is = stream_open(input_filename, file_iformat);//打开文件 ，同时开启解码线程
    if (!is) {
        av_log(NULL, AV_LOG_FATAL, "Failed to initialize VideoState!\n");
        do_exit(NULL);
    }
    event_loop(is);//主线程，用的于播放显示视频和SDL GUI处理。
...
}

ffplay 使用了 SDL 库做为播放框架，解码使用 ffmpeg 库。

SDL （Simple DirectMedia Layer）是一套开源代码的跨平台多媒体开发库，使用C语言写成。SDL提供了数种控制图像、声音、输出入的函数，让开发者只要用相同或是相似的代码就可以开发出跨多个平台（Linux、Windows、Mac OS X等）的应用软件。目前SDL多用于开发游戏、模拟器、媒体播放器等多媒体应用领域。
SDL使用GNU宽通用公共许可证为授权方式，意指动态链接（dynamic link）其库并不需要开放本身的源代码。因此诸如《雷神之锤4》等商业游戏也使用SDL来开发。

stream_open:

static VideoState *stream_open(const char *filename, AVInputFormat *iformat)
{
    VideoState *is;

    is = av_mallocz(sizeof(VideoState));
    if (!is)
        return NULL;
    is->filename = av_strdup(filename);
    if (!is->filename)
        goto fail;
    is->iformat = iformat;
    is->ytop    = 0;
    is->xleft   = 0;

    /* start video display */
    //为音、视频和字幕创建单独的帧队列
    if (frame_queue_init(&is->pictq, &is->videoq, VIDEO_PICTURE_QUEUE_SIZE, 1) < 0)
        goto fail;
    if (frame_queue_init(&is->subpq, &is->subtitleq, SUBPICTURE_QUEUE_SIZE, 0) < 0)
        goto fail;
    if (frame_queue_init(&is->sampq, &is->audioq, SAMPLE_QUEUE_SIZE, 1) < 0)
        goto fail;

    if (packet_queue_init(&is->videoq) < 0 ||
        packet_queue_init(&is->audioq) < 0 ||
        packet_queue_init(&is->subtitleq) < 0)
        goto fail;

    if (!(is->continue_read_thread = SDL_CreateCond())) {
        av_log(NULL, AV_LOG_FATAL, "SDL_CreateCond(): %s\n", SDL_GetError());
        goto fail;
    }
    //初始化时钟，serial从0开始
    init_clock(&is->vidclk, &is->videoq.serial);
    init_clock(&is->audclk, &is->audioq.serial);
    init_clock(&is->extclk, &is->extclk.serial);
    is->audio_clock_serial = -1;
    if (startup_volume < 0)
        av_log(NULL, AV_LOG_WARNING, "-volume=%d < 0, setting to 0\n", startup_volume);
    if (startup_volume > 100)
        av_log(NULL, AV_LOG_WARNING, "-volume=%d > 100, setting to 100\n", startup_volume);
    startup_volume = av_clip(startup_volume, 0, 100);
    startup_volume = av_clip(SDL_MIX_MAXVOLUME * startup_volume / 100, 0, SDL_MIX_MAXVOLUME);
    is->audio_volume = startup_volume;
    is->muted = 0;
    is->av_sync_type = av_sync_type;
    is->read_tid     = SDL_CreateThread(read_thread, "read_thread", is);//解码读取线程
    if (!is->read_tid) {
        av_log(NULL, AV_LOG_FATAL, "SDL_CreateThread(): %s\n", SDL_GetError());
fail:
        stream_close(is);
        return NULL;
    }
    return is;
}

stream_open为音、视频和字幕使用frame_queue_init初始化了单独的解码帧队列，初始化了时钟clock，并且开启了read_thread线程。

init_clock调用了set_clock然后av_gettime_relative方法。

av_gettime_relative方法，返回的是微秒为单位，代码有两种实现，具体看下面注释：

/**
 * Get the current time in microseconds since some unspecified starting point.
 * On platforms that support it, the time comes from a monotonic clock
 * This property makes this time source ideal for measuring relative time.
 * The returned values may not be monotonic on platforms where a monotonic
 * clock is not available.
 */
int64_t av_gettime_relative(void);

//gettimeofday获得1970年1月1日到现在的时间
int64_t av_gettime(void)
{
#if HAVE_GETTIMEOFDAY //linux方法，tv.tv_usec为微秒数，tv_sec为秒数，所以返回的是微秒为单位
    struct timeval tv;
    gettimeofday(&tv, NULL);
    return (int64_t)tv.tv_sec * 1000000 + tv.tv_usec;
#elif HAVE_GETSYSTEMTIMEASFILETIME//windows方法
    FILETIME ft;
    int64_t t;
    GetSystemTimeAsFileTime(&ft);
    t = (int64_t)ft.dwHighDateTime << 32 | ft.dwLowDateTime;
    return t / 10 - 11644473600000000; /* Jan 1, 1601 */
#else
    return -1;
#endif
}

int64_t av_gettime_relative(void)//返回的是微秒为单位
{
//如果定义了CLOCK_MONOTONIC而且有clock_gettime方法，则使用clock_gettime
//clock_gettime(CLOCK_MONOTONIC,..):从系统启动这一刻起开始计时,不受系统时间被用户改变的影响
#if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
#ifdef __APPLE__
    if (clock_gettime)
#endif
    {
        struct timespec ts;
        clock_gettime(CLOCK_MONOTONIC, &ts);
        return (int64_t)ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
    }
#endif
//故意加上42小时的秒数
    return av_gettime() + 42 * 60 * 60 * INT64_C(1000000);
}

再来看看read_thread线程：

/* this thread gets the stream from the disk or the network */
static int read_thread(void *arg)
{
//前面打开一堆代码打开stream，获取stream信息等。
 ...

    is->realtime = is_realtime(ic);//判断是否实时网络流rtp,rtsp,sdp,udp

  ...
    //打开各种流，进行解码
    /* open the streams */
    if (st_index[AVMEDIA_TYPE_AUDIO] >= 0) {
        stream_component_open(is, st_index[AVMEDIA_TYPE_AUDIO]);
    }

    ret = -1;
    if (st_index[AVMEDIA_TYPE_VIDEO] >= 0) {
        ret = stream_component_open(is, st_index[AVMEDIA_TYPE_VIDEO]);
    }
    if (is->show_mode == SHOW_MODE_NONE)
        is->show_mode = ret >= 0 ? SHOW_MODE_VIDEO : SHOW_MODE_RDFT;

    if (st_index[AVMEDIA_TYPE_SUBTITLE] >= 0) {
        stream_component_open(is, st_index[AVMEDIA_TYPE_SUBTITLE]);
    }
 ...    
    if (infinite_buffer < 0 && is->realtime)//实时流
        infinite_buffer = 1;

    for (;;) {
        if (is->abort_request)
            break;
        if (is->paused != is->last_paused) {//暂停与继续播放处理
...
        }
...
        //seek处理
        if (is->seek_req) {
...
        }

        //非实时流最多读取15M或者音视频和字幕队列都满了则等待10毫秒
        /* if the queue are full, no need to read more */
        if (infinite_buffer<1 &&
              (is->audioq.size + is->videoq.size + is->subtitleq.size > MAX_QUEUE_SIZE
            || (stream_has_enough_packets(is->audio_st, is->audio_stream, &is->audioq) &&
                stream_has_enough_packets(is->video_st, is->video_stream, &is->videoq) &&
                stream_has_enough_packets(is->subtitle_st, is->subtitle_stream, &is->subtitleq)))) {
            /* wait 10 ms */
            SDL_LockMutex(wait_mutex);
            SDL_CondWaitTimeout(is->continue_read_thread, wait_mutex, 10);
            SDL_UnlockMutex(wait_mutex);
            continue;
        }
        //结束了，从头播放或者退出
        if (!is->paused &&
            (!is->audio_st || (is->auddec.finished == is->audioq.serial && frame_queue_nb_remaining(&is->sampq) == 0)) &&
            (!is->video_st || (is->viddec.finished == is->videoq.serial && frame_queue_nb_remaining(&is->pictq) == 0))) {
            if (loop != 1 && (!loop || --loop)) {
                stream_seek(is, start_time != AV_NOPTS_VALUE ? start_time : 0, 0, 0);
            } else if (autoexit) {
                ret = AVERROR_EOF;
                goto fail;
            }
        }
        //主要流程就是不停的读取帧到队列中，如果队列满了则等待。
        //同时外部播放线程不断从队列中取帧进行播放。
        ret = av_read_frame(ic, pkt);//关键代码：读取帧 
        if (ret < 0) {//错误或者结束，队列放入一个空包
            if ((ret == AVERROR_EOF || avio_feof(ic->pb)) && !is->eof) {
                if (is->video_stream >= 0)
                    packet_queue_put_nullpacket(&is->videoq, is->video_stream);
                if (is->audio_stream >= 0)
                    packet_queue_put_nullpacket(&is->audioq, is->audio_stream);
                if (is->subtitle_stream >= 0)
                    packet_queue_put_nullpacket(&is->subtitleq, is->subtitle_stream);
                is->eof = 1;
            }
            if (ic->pb && ic->pb->error)
                break;
            SDL_LockMutex(wait_mutex);
            SDL_CondWaitTimeout(is->continue_read_thread, wait_mutex, 10);
            SDL_UnlockMutex(wait_mutex);
            continue;
        } else {
            is->eof = 0;
        }

        //检测包如果在播放的范围就放入队列，否则丢弃
        /* check if packet is in play range specified by user, then queue, otherwise discard */
        stream_start_time = ic->streams[pkt->stream_index]->start_time;
        pkt_ts = pkt->pts == AV_NOPTS_VALUE ? pkt->dts : pkt->pts;//包的时间戳，先取pts播放时间戳，取不到则取dts解码时间戳
        //duration == AV_NOPTS_VALUE表示命令行未设置此参数，如果设置了则duration表示播放总时长。
        //后面一段是 包的时间戳与开始播放的时间戳之差即当前包的时间位置，也就是这个包应该在第几秒播放。
        //跟start_time真正播放开始的时间之差如果小于等于duration总时长则表示包有效然后加入队列，无效则丢弃。
        pkt_in_play_range = duration == AV_NOPTS_VALUE ||
                (pkt_ts - (stream_start_time != AV_NOPTS_VALUE ? stream_start_time : 0)) *
                av_q2d(ic->streams[pkt->stream_index]->time_base) -
                (double)(start_time != AV_NOPTS_VALUE ? start_time : 0) / 1000000
                <= ((double)duration / 1000000);
        if (pkt->stream_index == is->audio_stream && pkt_in_play_range) {
            packet_queue_put(&is->audioq, pkt);
        } else if (pkt->stream_index == is->video_stream && pkt_in_play_range
                   && !(is->video_st->disposition & AV_DISPOSITION_ATTACHED_PIC)) {
            packet_queue_put(&is->videoq, pkt);
        } else if (pkt->stream_index == is->subtitle_stream && pkt_in_play_range) {
            packet_queue_put(&is->subtitleq, pkt);
        } else {
            av_packet_unref(pkt);
        }
    }

    ret = 0;
 fail:
...
}

先打开各种stream，stream_component_open开启独立的解码线程，本线程不断循环av_read_frame读取帧数据packet_queue_put放到流的各自队列中。

比较关键的av_read_frame从文件或者网络流中读取未解码的帧，具体代码属于ffmpeg库这儿就不展开分析了，这儿只讨ffplay。

stream_component_open：

/* open a given stream. Return 0 if OK */
static int stream_component_open(VideoState *is, int stream_index)
{
...
    codec = avcodec_find_decoder(avctx->codec_id);//查找解码器
...
//每个stream解码都有自己独立的线程，
    switch (avctx->codec_type) {
    case AVMEDIA_TYPE_AUDIO:
...
        //打开声音播放设备，同时设置了sdl音频播放回调方法 sdl_audio_callback
        /* prepare audio output */
        if ((ret = audio_open(is, channel_layout, nb_channels, sample_rate, &is->audio_tgt)) < 0)
            goto fail;
        is->audio_hw_buf_size = ret;
        is->audio_src = is->audio_tgt;
        is->audio_buf_size  = 0;
        is->audio_buf_index = 0;

        /* init averaging filter */
        is->audio_diff_avg_coef  = exp(log(0.01) / AUDIO_DIFF_AVG_NB);
        is->audio_diff_avg_count = 0;
        /* since we do not have a precise anough audio FIFO fullness,
           we correct audio sync only if larger than this threshold */
        is->audio_diff_threshold = (double)(is->audio_hw_buf_size) / is->audio_tgt.bytes_per_sec;

        is->audio_stream = stream_index;
        is->audio_st = ic->streams[stream_index];

        decoder_init(&is->auddec, avctx, &is->audioq, is->continue_read_thread);
        if ((is->ic->iformat->flags & (AVFMT_NOBINSEARCH | AVFMT_NOGENSEARCH | AVFMT_NO_BYTE_SEEK)) && !is->ic->iformat->read_seek) {
            is->auddec.start_pts = is->audio_st->start_time;
            is->auddec.start_pts_tb = is->audio_st->time_base;
        }
        if ((ret = decoder_start(&is->auddec, audio_thread, is)) < 0)//音频解码线程audio_thread
            goto out;
        SDL_PauseAudioDevice(audio_dev, 0);
        break;
    case AVMEDIA_TYPE_VIDEO:
        //视频解码线程video_thread
        is->video_stream = stream_index;
        is->video_st = ic->streams[stream_index];

        decoder_init(&is->viddec, avctx, &is->videoq, is->continue_read_thread);
        if ((ret = decoder_start(&is->viddec, video_thread, is)) < 0)
            goto out;
        is->queue_attachments_req = 1;
        break;
    case AVMEDIA_TYPE_SUBTITLE:
        is->subtitle_stream = stream_index;
        is->subtitle_st = ic->streams[stream_index];

        decoder_init(&is->subdec, avctx, &is->subtitleq, is->continue_read_thread);
        if ((ret = decoder_start(&is->subdec, subtitle_thread, is)) < 0)
            goto out;
        break;
    default:
        break;
    }
    goto out;

fail:
    avcodec_free_context(&avctx);
out:
    av_dict_free(&opts);

    return ret;
}

音、视、字幕各自拥有独立的解码线程。

音频：
audio_open打开了声音播放设备，设置了sdl音频播放回调方法sdl_audio_callback，在sdl_audio_callback中不断循环调用audio_decode_frame取出已经解码的音频帧，并且有需要的话会转换到要求的音频格式，然后交给SDL进行播放。

audio_thread循环调用decoder_decode_frame进行音频解码。

视频：
video_thread线程循环调用get_video_frame，get_video_frame又调用了decoder_decode_frame进行解码。

字幕同理。

三者都调用了decoder_decode_frame进行解码，根据codec_type区分处理。

static int decoder_decode_frame(Decoder *d, AVFrame *frame, AVSubtitle *sub) {
    int ret = AVERROR(EAGAIN);

    for (;;) {
        AVPacket pkt;

        if (d->queue->serial == d->pkt_serial) {
            do {
                if (d->queue->abort_request)
                    return -1;

                switch (d->avctx->codec_type) {
                    case AVMEDIA_TYPE_VIDEO://接收解码后的数据
                        ret = avcodec_receive_frame(d->avctx, frame);
                        if (ret >= 0) {
                            if (decoder_reorder_pts == -1) {
                                frame->pts = frame->best_effort_timestamp;
                            } else if (!decoder_reorder_pts) {
                                frame->pts = frame->pkt_dts;
                            }
                        }
                        break;
                    case AVMEDIA_TYPE_AUDIO:
                        ret = avcodec_receive_frame(d->avctx, frame);
                        if (ret >= 0) {
                            AVRational tb = (AVRational){1, frame->sample_rate};
                            if (frame->pts != AV_NOPTS_VALUE)
                                frame->pts = av_rescale_q(frame->pts, d->avctx->pkt_timebase, tb);
                            else if (d->next_pts != AV_NOPTS_VALUE)
                                frame->pts = av_rescale_q(d->next_pts, d->next_pts_tb, tb);
                            if (frame->pts != AV_NOPTS_VALUE) {
                                d->next_pts = frame->pts + frame->nb_samples;
                                d->next_pts_tb = tb;
                            }
                        }
                        break;
                }
                if (ret == AVERROR_EOF) {
                    d->finished = d->pkt_serial;
                    avcodec_flush_buffers(d->avctx);
                    return 0;
                }
                if (ret >= 0)
                    return 1;
            } while (ret != AVERROR(EAGAIN));
        }

        do {
            if (d->queue->nb_packets == 0)
                SDL_CondSignal(d->empty_queue_cond);
            if (d->packet_pending) {
                av_packet_move_ref(&pkt, &d->pkt);
                d->packet_pending = 0;
            } else {
                if (packet_queue_get(d->queue, &pkt, 1, &d->pkt_serial) < 0)//从队列中获取包
                    return -1;
            }
        } while (d->queue->serial != d->pkt_serial);

        if (pkt.data == flush_pkt.data) {
            avcodec_flush_buffers(d->avctx);
            d->finished = 0;
            d->next_pts = d->start_pts;
            d->next_pts_tb = d->start_pts_tb;
        } else {
            if (d->avctx->codec_type == AVMEDIA_TYPE_SUBTITLE) {
                int got_frame = 0;
                ret = avcodec_decode_subtitle2(d->avctx, sub, &got_frame, &pkt);
                if (ret < 0) {
                    ret = AVERROR(EAGAIN);
                } else {
                    if (got_frame && !pkt.data) {
                       d->packet_pending = 1;
                       av_packet_move_ref(&d->pkt, &pkt);
                    }
                    ret = got_frame ? 0 : (pkt.data ? AVERROR(EAGAIN) : AVERROR_EOF);
                }
            } else {
                //发送包用于解码
                if (avcodec_send_packet(d->avctx, &pkt) == AVERROR(EAGAIN)) {
                    av_log(d->avctx, AV_LOG_ERROR, "Receive_frame and send_packet both returned EAGAIN, which is an API violation.\n");
                    d->packet_pending = 1;
                    av_packet_move_ref(&d->pkt, &pkt);
                }
            }
            av_packet_unref(&pkt);
        }
    }
}

packet_queue_get取出从read_thread加入的未解码包。交给avcodec_send_packet。

avcodec_send_packet和avcodec_receive_frame是libavcodec中用于配对的解码方法。

avcodec_send_packet 给解码器发送未解码包，avcodec_receive_frame取出解码后的帧。

avcodec_send_packet 发送的未解码包可能需要多次调用 avcodec_receive_frame 才可以解码1帧。

解码后的数据存储在AVFrame *frame中，函数返回。

视频使用queue_picture把解码后的图像加入到队列中。解码后的音频使用frame_queue_push(&is->sampq);加入队列。

音频的播放：
音频在sdl_audio_callback中从解码音频队列不断取出解码音频进行播放。

视频的播放：
视频在主线程中播放显示。

event_loop中处理播放显示和GUI消息处理。

/* handle an event sent by the GUI */
static void event_loop(VideoState *cur_stream)//主线程，用的于播放显示视频和SDL GUI处理。
    SDL_Event event;
    double incr, pos, frac;

    for (;;) {
        double x;
        refresh_loop_wait_event(cur_stream, &event);//播放显示
        switch (event.type) {
        case SDL_KEYDOWN://各种键盘操作
            if (exit_on_keydown || event.key.keysym.sym == SDLK_ESCAPE || event.key.keysym.sym == SDLK_q) {
                do_exit(cur_stream);
                break;
            }
            // If we don't yet have a window, skip all key events, because read_thread might still be initializing...
            if (!cur_stream->width)
                continue;
            switch (event.key.keysym.sym) {
            case SDLK_f:
                toggle_full_screen(cur_stream);
                cur_stream->force_refresh = 1;
                break;
            case SDLK_p:
            case SDLK_SPACE:
                toggle_pause(cur_stream);
                break;
            case SDLK_m:
                toggle_mute(cur_stream);
                break;
            case SDLK_KP_MULTIPLY:
            case SDLK_0:
                update_volume(cur_stream, 1, SDL_VOLUME_STEP);
                break;
            case SDLK_KP_DIVIDE:
            case SDLK_9:
                update_volume(cur_stream, -1, SDL_VOLUME_STEP);
                break;
            case SDLK_s: // S: Step to next frame
                step_to_next_frame(cur_stream);
                break;
            case SDLK_a:
                stream_cycle_channel(cur_stream, AVMEDIA_TYPE_AUDIO);
                break;
            case SDLK_v:
                stream_cycle_channel(cur_stream, AVMEDIA_TYPE_VIDEO);
                break;
            case SDLK_c:
               ...
                break;
            case SDLK_t:
                stream_cycle_channel(cur_stream, AVMEDIA_TYPE_SUBTITLE);
                break;
            case SDLK_w:
              ...
                break;
            case SDLK_PAGEUP:
            ...
                break;
            case SDLK_PAGEDOWN:
             ...
                break;
            case SDLK_LEFT:
                incr = seek_interval ? -seek_interval : -10.0;
                goto do_seek;
            case SDLK_RIGHT:
                incr = seek_interval ? seek_interval : 10.0;
                goto do_seek;
            case SDLK_UP:
                incr = 60.0;
                goto do_seek;
            case SDLK_DOWN:
                incr = -60.0;
            do_seek:
                    ...
                break;
            default:
                break;
            }
            break;
        case SDL_MOUSEBUTTONDOWN://鼠标单击
            if (exit_on_mousedown) {
                do_exit(cur_stream);
                break;
            }
            if (event.button.button == SDL_BUTTON_LEFT) {
               ...
            }
        case SDL_MOUSEMOTION://鼠标移动
           ...
            break;
        case SDL_WINDOWEVENT://窗口消息
            switch (event.window.event) {
                case SDL_WINDOWEVENT_RESIZED:
                  ...
                case SDL_WINDOWEVENT_EXPOSED:
                  ...
            }
            break;
        case SDL_QUIT:
        case FF_QUIT_EVENT:
            do_exit(cur_stream);
            break;
        default:
            break;
        }
    }
}

refresh_loop_wait_event 进行播放显示图像:

static void refresh_loop_wait_event(VideoState *is, SDL_Event *event) {
    ...
            video_refresh(is, &remaining_time);//播放显示
   ...
}

/* called to display each frame */
static void video_refresh(void *opaque, double *remaining_time)
{
    VideoState *is = opaque;
    double time;

    Frame *sp, *sp2;

    if (!is->paused && get_master_sync_type(is) == AV_SYNC_EXTERNAL_CLOCK && is->realtime)//实时播放
        check_external_clock_speed(is);

    if (!display_disable && is->show_mode != SHOW_MODE_VIDEO && is->audio_st) {
        time = av_gettime_relative() / 1000000.0;
        if (is->force_refresh || is->last_vis_time + rdftspeed < time) {
            video_display(is);
            is->last_vis_time = time;
        }
        *remaining_time = FFMIN(*remaining_time, is->last_vis_time + rdftspeed - time);
    }

    if (is->video_st) {
retry:
        if (frame_queue_nb_remaining(&is->pictq) == 0) {
            // nothing to do, no picture to display in the queue
        } else {
            double last_duration, duration, delay;
            Frame *vp, *lastvp;

            /* dequeue the picture */
            lastvp = frame_queue_peek_last(&is->pictq);
            vp = frame_queue_peek(&is->pictq);

            if (vp->serial != is->videoq.serial) {
                frame_queue_next(&is->pictq);
                goto retry;
            }

            if (lastvp->serial != vp->serial)
                is->frame_timer = av_gettime_relative() / 1000000.0;

            if (is->paused)
                goto display;

            /* compute nominal last_duration */
            last_duration = vp_duration(is, lastvp, vp);//名义上的帧持续时间，
            delay = compute_target_delay(last_duration, is);//猜测计算延迟
            /*frame_timer实际上就是上一帧的播放时间，该时间是一个系统时间，
            而 frame_timer + delay 实际上就是当前这一帧的播放时间*/
            time= av_gettime_relative()/1000000.0;
            if (time < is->frame_timer + delay) {//播放太慢了马上播放
                *remaining_time = FFMIN(is->frame_timer + delay - time, *remaining_time);
                goto display;
            }

            is->frame_timer += delay;
            if (delay > 0 && time - is->frame_timer > AV_SYNC_THRESHOLD_MAX)
                is->frame_timer = time;

            SDL_LockMutex(is->pictq.mutex);
            if (!isnan(vp->pts))
                update_video_pts(is, vp->pts, vp->pos, vp->serial);
            SDL_UnlockMutex(is->pictq.mutex);

            if (frame_queue_nb_remaining(&is->pictq) > 1) {
                Frame *nextvp = frame_queue_peek_next(&is->pictq);
                duration = vp_duration(is, vp, nextvp);
                if(!is->step && (framedrop>0 || (framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) && time > is->frame_timer + duration){
                    is->frame_drops_late++;//丢弃帧
                    frame_queue_next(&is->pictq);
                    goto retry;
                }
            }

            if (is->subtitle_st) {
                    ...
            }

            frame_queue_next(&is->pictq);
            is->force_refresh = 1;

            if (is->step && !is->paused)
                stream_toggle_pause(is);
        }
display:
        /* display picture */
        if (!display_disable && is->force_refresh && is->show_mode == SHOW_MODE_VIDEO && is->pictq.rindex_shown)
            video_display(is); //绘制图像
    }
    is->force_refresh = 0;
    ...
}