4、ffplay解碼線程源碼分析

ffplay解碼線程原理分析

一、視頻解碼模塊

1、從AVPacket 解碼出AVFrame

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) {//第一次解碼時Decoder的pkt_serial爲初始值-1，PacketQueue的值爲1，調好if塊。 Decoder初始化見函數decoder_init(...)
            do {
                if (d->queue->abort_request)
                    return -1;

                switch (d->avctx->codec_type) {
                    case AVMEDIA_TYPE_VIDEO:
                        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) {
                                //若命令行指定"-drp 0"參數，則要求重新設置pts，使用pkt_dts
                                frame->pts = frame->pkt_dts;
                            }
                        }
                        break;
                    case AVMEDIA_TYPE_AUDIO:
                        ret = avcodec_receive_frame(d->avctx, frame);
                        if (ret >= 0) {
                            //音頻播放時的時間基準
                            AVRational tb = (AVRational){1, frame->sample_rate};
                            if (frame->pts != AV_NOPTS_VALUE)
                            {
                                //將解碼時的pts轉換爲播放時的pts
                                frame->pts = av_rescale_q(frame->pts, d->avctx->pkt_timebase, tb);
                            }
                            else if (d->next_pts != AV_NOPTS_VALUE)
                            {
                                //如果當前frame沒有pts，上一幀有pts採用上一幀的pts
                                frame->pts = av_rescale_q(d->next_pts, d->next_pts_tb, tb);
                            }
                            if (frame->pts != AV_NOPTS_VALUE) {
                                //設置下一幀的pts,time_base
                                d->next_pts = frame->pts + frame->nb_samples;
                                d->next_pts_tb = tb;
                            }
                        }
                        break;
                }
                if (ret == AVERROR_EOF) {//解碼結束
                    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);//AVPacket隊列爲空，通知該cond的目的是???
            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)//重AVPacket隊列中取出一條記錄，並將VAPackt的序列號賦值給d->pkt_serial。若隊列爲空就阻塞在PacketQueue的字段的cond上
                    return -1;
            }
        } while (d->queue->serial != d->pkt_serial);//若從PacketQueue中取出的AVPacket的序列號不等於當前PacketQueue的序列號，則不斷從PacketQueue取出AVPackt直到相等。該段代碼到達到去除過時AVPackt的功能（例如使用快進功能時，PacketQueue 會進行flush序列號會增加）

        if (pkt.data == flush_pkt.data) {
            //取出的pkt爲標識數據時，說明重新進行讀取AVPacket數據,Reset the internal decoder state / flush internal buffers. Should be called e.g. when seeking or when switching to a different stream.

            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) {
                //該if塊解碼字幕
                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) {
                        //若此次未解碼完成，將數據設置給decoder暫存，下次繼續解碼
                       d->packet_pending = 1;
                       av_packet_move_ref(&d->pkt, &pkt);
                    }
                    ret = got_frame ? 0 : (pkt.data ? AVERROR(EAGAIN) : AVERROR_EOF);
                }
            } else {
                //解碼AVPacket
                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);
        }
    }
}


static void decoder_init(Decoder *d, AVCodecContext *avctx, PacketQueue *queue, SDL_cond *empty_queue_cond) {
    memset(d, 0, sizeof(Decoder));
    d->avctx = avctx;
    d->queue = queue;
    d->empty_queue_cond = empty_queue_cond;
    d->start_pts = AV_NOPTS_VALUE;
    d->pkt_serial = -1;
}

get_video_frame(…)獲取一幀數據,當解碼出的數據幀的渲染時間與同步時鐘的差值大於閾值時，進行丟幀處理。

static int get_video_frame(VideoState *is, AVFrame *frame)
{
    int got_picture;

    if ((got_picture = decoder_decode_frame(&is->viddec, frame, NULL)) < 0)
        return -1;

    if (got_picture) {
        double dpts = NAN;

        if (frame->pts != AV_NOPTS_VALUE)
          { 
            //pts轉換爲second 
            dpts = av_q2d(is->video_st->time_base) * frame->pts;
          }

        frame->sample_aspect_ratio = av_guess_sample_aspect_ratio(is->ic, is->video_st, frame);

        if (framedrop>0 || (framedrop && get_master_sync_type(is) != AV_SYNC_VIDEO_MASTER)) {
            if (frame->pts != AV_NOPTS_VALUE) {
                double diff = dpts - get_master_clock(is);
                //滿足一下添加進行丟幀處理
                //1.diff誤差不是太大
                //2.diff - is->frame_last_filter_delay < 0，diff小於avfilter輸出數據的耗時，也就是輸出數據轉換速度不夠快
                //3.is->viddec.pkt_serial == is->vidclk.serial
                //4.videoq PacketQueue隊列不爲空
                if (!isnan(diff) && fabs(diff) < AV_NOSYNC_THRESHOLD &&
                    diff - is->frame_last_filter_delay < 0 &&
                    is->viddec.pkt_serial == is->vidclk.serial &&
                    is->videoq.nb_packets) {
                    is->frame_drops_early++;
                    av_frame_unref(frame);
                    got_picture = 0;
                }
            }
        }
    }

    return got_picture;
}

解碼線程處理邏輯

• 對解碼出的AVFrame通過avfilter處理，如數據格式轉換，圖形旋轉、翻轉等處理
• avfilter處理後的AVFrame加入FrameQueue中

static int video_thread(void *arg)
{
    VideoState *is = arg;
    AVFrame *frame = av_frame_alloc();
    double pts;
    double duration;
    int ret;
    AVRational tb = is->video_st->time_base;
    AVRational frame_rate = av_guess_frame_rate(is->ic, is->video_st, NULL);

#if CONFIG_AVFILTER
    AVFilterGraph *graph = avfilter_graph_alloc();
    AVFilterContext *filt_out = NULL, *filt_in = NULL;
    int last_w = 0;
    int last_h = 0;
    enum AVPixelFormat last_format = -2;
    int last_serial = -1;
    int last_vfilter_idx = 0;
    if (!graph) {
        av_frame_free(&frame);
        return AVERROR(ENOMEM);
    }

#endif

    if (!frame) {
#if CONFIG_AVFILTER
        avfilter_graph_free(&graph);
#endif
        return AVERROR(ENOMEM);
    }

    for (;;) {
        ret = get_video_frame(is, frame);
        if (ret < 0)
            goto the_end;
        if (!ret)
            continue;

#if CONFIG_AVFILTER
        if (   last_w != frame->width
            || last_h != frame->height
            || last_format != frame->format
            || last_serial != is->viddec.pkt_serial
            || last_vfilter_idx != is->vfilter_idx) {
            av_log(NULL, AV_LOG_DEBUG,
                   "Video frame changed from size:%dx%d format:%s serial:%d to size:%dx%d format:%s serial:%d\n",
                   last_w, last_h,
                   (const char *)av_x_if_null(av_get_pix_fmt_name(last_format), "none"), last_serial,
                   frame->width, frame->height,
                   (const char *)av_x_if_null(av_get_pix_fmt_name(frame->format), "none"), is->viddec.pkt_serial);
            avfilter_graph_free(&graph);
            graph = avfilter_graph_alloc();
            if ((ret = configure_video_filters(graph, is, vfilters_list ? vfilters_list[is->vfilter_idx] : NULL, frame)) < 0) {
                SDL_Event event;
                event.type = FF_QUIT_EVENT;
                event.user.data1 = is;
                SDL_PushEvent(&event);
                goto the_end;
            }
            filt_in  = is->in_video_filter;
            filt_out = is->out_video_filter;
            last_w = frame->width;
            last_h = frame->height;
            last_format = frame->format;
            last_serial = is->viddec.pkt_serial;
            last_vfilter_idx = is->vfilter_idx;
            frame_rate = av_buffersink_get_frame_rate(filt_out);
        }

        ret = av_buffersrc_add_frame(filt_in, frame);
        if (ret < 0)
            goto the_end;

        while (ret >= 0) {
            is->frame_last_returned_time = av_gettime_relative() / 1000000.0;

            ret = av_buffersink_get_frame_flags(filt_out, frame, 0);
            if (ret < 0) {
                if (ret == AVERROR_EOF)
                    is->viddec.finished = is->viddec.pkt_serial;
                ret = 0;
                break;
            }

            is->frame_last_filter_delay = av_gettime_relative() / 1000000.0 - is->frame_last_returned_time;
            if (fabs(is->frame_last_filter_delay) > AV_NOSYNC_THRESHOLD / 10.0)
                is->frame_last_filter_delay = 0;
            tb = av_buffersink_get_time_base(filt_out);
#endif
            duration = (frame_rate.num && frame_rate.den ? av_q2d((AVRational){frame_rate.den, frame_rate.num}) : 0);
            pts = (frame->pts == AV_NOPTS_VALUE) ? NAN : frame->pts * av_q2d(tb);
            ret = queue_picture(is, frame, pts, duration, frame->pkt_pos, is->viddec.pkt_serial);//將avfilter處理後的AVFrame加入隊列
            av_frame_unref(frame);
#if CONFIG_AVFILTER
            if (is->videoq.serial != is->viddec.pkt_serial)
                break;
        }
#endif

        if (ret < 0)
            goto the_end;
    }
 the_end:
#if CONFIG_AVFILTER
    avfilter_graph_free(&graph);
#endif
    av_frame_free(&frame);
    return 0;
}