transcode_step()在转码过程中对pts、dts、duration的处理

简介

对pts、dts、duration的处理主要集中在两大函数里面
1、process_input()读入数据并处理，放到滤镜里面
2、reap_filters()从滤镜读出数据，处理后写入文件

process_input()中对时间的处理

获取数据包

ret = get_input_packet(ifile, &pkt);

通过这条语句实际里面调用

return av_read_frame(f->ctx, pkt);

将读取到的帧放到pkt中。

处理获取的pkt中的时间

//如果pkt.dts或pkt.pts有值，而不是AV_NOPTS_VALUE时，做处理。
/* ifile->ts_offset在open_input_file函数中有处理，涉及到参数"itsoffset"(o->input_ts_offset)、 "copyts"(copy_ts)、"start_at_zero"(start_at_zero )、"ss"(ic->start_time) 语句如下。 f->ts_offset = o->input_ts_offset - (copy_ts ? (start_at_zero && ic->start_time != AV_NOPTS_VALUE ? ic->start_time : 0) : timestamp); 如果没有这些设置。f->ts_offset=0 */
if (pkt.dts != AV_NOPTS_VALUE)
    pkt.dts += av_rescale_q(ifile->ts_offset, AV_TIME_BASE_Q, ist->st->time_base);
if (pkt.pts != AV_NOPTS_VALUE)
    pkt.pts += av_rescale_q(ifile->ts_offset, AV_TIME_BASE_Q, ist->st->time_base);

//ist->ts_scale在add_input_streams函数中设置
/* ist->ts_scale = 1.0;//默认是1 MATCH_PER_STREAM_OPT(ts_scale, dbl, ist->ts_scale, ic, st); 涉及到的参数是"itsscale"：解释为"set the input ts scale" 貌似是单位 */
if (pkt.pts != AV_NOPTS_VALUE)
    pkt.pts *= ist->ts_scale;
if (pkt.dts != AV_NOPTS_VALUE)
    pkt.dts *= ist->ts_scale;

//做时间基的转换
pkt_dts = av_rescale_q_rnd(pkt.dts, ist->st->time_base, AV_TIME_BASE_Q, (enum AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
if ((ist->dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO ||
     ist->dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) &&
    pkt_dts != AV_NOPTS_VALUE && ist->next_dts == AV_NOPTS_VALUE && !copy_ts
    && (is->iformat->flags & AVFMT_TS_DISCONT) && ifile->last_ts != AV_NOPTS_VALUE) {
    int64_t delta   = pkt_dts - ifile->last_ts;
    if (delta < -1LL*dts_delta_threshold*AV_TIME_BASE ||
        delta >  1LL*dts_delta_threshold*AV_TIME_BASE){
        ifile->ts_offset -= delta;
        av_log(NULL, AV_LOG_DEBUG,
               "Inter stream timestamp discontinuity %"PRId64", new offset= %"PRId64"\n",
               delta, ifile->ts_offset);
        pkt.dts -= av_rescale_q(delta, AV_TIME_BASE_Q, ist->st->time_base);
        if (pkt.pts != AV_NOPTS_VALUE)
            pkt.pts -= av_rescale_q(delta, AV_TIME_BASE_Q, ist->st->time_base);
    }
}

duration = av_rescale_q(ifile->duration, ifile->time_base, ist->st->time_base);
if (pkt.pts != AV_NOPTS_VALUE) {
    pkt.pts += duration;
    ist->max_pts = FFMAX(pkt.pts, ist->max_pts);
    ist->min_pts = FFMIN(pkt.pts, ist->min_pts);
}

if (pkt.dts != AV_NOPTS_VALUE)
    pkt.dts += duration;

pkt_dts = av_rescale_q_rnd(pkt.dts, ist->st->time_base, AV_TIME_BASE_Q, (enum AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
if ((ist->dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO ||
     ist->dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) &&
     pkt_dts != AV_NOPTS_VALUE && ist->next_dts != AV_NOPTS_VALUE &&
    !copy_ts) {
    int64_t delta   = pkt_dts - ist->next_dts;
    if (is->iformat->flags & AVFMT_TS_DISCONT) {
        if (delta < -1LL*dts_delta_threshold*AV_TIME_BASE ||
            delta >  1LL*dts_delta_threshold*AV_TIME_BASE ||
            pkt_dts + AV_TIME_BASE/10 < FFMAX(ist->pts, ist->dts)) {
            ifile->ts_offset -= delta;
            av_log(NULL, AV_LOG_DEBUG,
                   "timestamp discontinuity %"PRId64", new offset= %"PRId64"\n",
                   delta, ifile->ts_offset);
            pkt.dts -= av_rescale_q(delta, AV_TIME_BASE_Q, ist->st->time_base);
            if (pkt.pts != AV_NOPTS_VALUE)
                pkt.pts -= av_rescale_q(delta, AV_TIME_BASE_Q, ist->st->time_base);
        }
    } else {
        if ( delta < -1LL*dts_error_threshold*AV_TIME_BASE ||
             delta >  1LL*dts_error_threshold*AV_TIME_BASE) {
            av_log(NULL, AV_LOG_WARNING, "DTS %"PRId64", next:%"PRId64" st:%d invalid dropping\n", pkt.dts, ist->next_dts, pkt.stream_index);
            pkt.dts = AV_NOPTS_VALUE;
        }
        if (pkt.pts != AV_NOPTS_VALUE){
            int64_t pkt_pts = av_rescale_q(pkt.pts, ist->st->time_base, AV_TIME_BASE_Q);
            delta   = pkt_pts - ist->next_dts;
            if ( delta < -1LL*dts_error_threshold*AV_TIME_BASE ||
                 delta >  1LL*dts_error_threshold*AV_TIME_BASE) {
                av_log(NULL, AV_LOG_WARNING, "PTS %"PRId64", next:%"PRId64" invalid dropping st:%d\n", pkt.pts, ist->next_dts, pkt.stream_index);
                pkt.pts = AV_NOPTS_VALUE;
            }
        }
    }
}

if (pkt.dts != AV_NOPTS_VALUE)
    ifile->last_ts = av_rescale_q(pkt.dts, ist->st->time_base, AV_TIME_BASE_Q);

如果pkt.dts == AV_NOPTS_VALUE 并且pkt.dts == AV_NOPTS_VALUE，以上这些语句都没有起作用。

之后调用如下语句

process_input_packet(ist, &pkt, 0);

process_input_packet

/* pkt = NULL means EOF (needed to flush decoder buffers) */
static int process_input_packet(InputStream *ist, const AVPacket *pkt, int no_eof)
{
    int ret = 0, i;
    int got_output = 0;

    AVPacket avpkt;
    //如果是文件的第一次读取，就处理
    if (!ist->saw_first_ts) {
        //有B帧的情况下dts和pts是不同的。
        ist->dts = ist->st->avg_frame_rate.num ? - ist->dec_ctx->has_b_frames * AV_TIME_BASE / av_q2d(ist->st->avg_frame_rate) : 0;
        ist->pts = 0;
        if (pkt && pkt->pts != AV_NOPTS_VALUE && !ist->decoding_needed) {
            ist->dts += av_rescale_q(pkt->pts, ist->st->time_base, AV_TIME_BASE_Q);
            ist->pts = ist->dts; //unused but better to set it to a value thats not totally wrong
        }
        ist->saw_first_ts = 1;
    }
    //此时在ist中dts和pts都是有值的，第一次进来是初始值，之后进来的值
    //初始化next_dts和next_pts
    if (ist->next_dts == AV_NOPTS_VALUE)
        ist->next_dts = ist->dts;
    if (ist->next_pts == AV_NOPTS_VALUE)
        ist->next_pts = ist->pts;
    /*如果pkt为NULL，表示流结束了。给avpkt赋值为此后在调用decode_video()->avcodec_decode_video2()时会将剩余的帧都返回。 在process_input()函数的出错处理里面有调用process_input_packet(ist, NULL, 0);就在此处处理 */
    if (!pkt) {
        /* EOF handling */
        av_init_packet(&avpkt);
        avpkt.data = NULL;
        avpkt.size = 0;
        goto handle_eof;
    } else {
        avpkt = *pkt;
    }

    if (pkt->dts != AV_NOPTS_VALUE) {
        ist->next_dts = ist->dts = av_rescale_q(pkt->dts, ist->st->time_base, AV_TIME_BASE_Q);
        if (ist->dec_ctx->codec_type != AVMEDIA_TYPE_VIDEO || !ist->decoding_needed)
            ist->next_pts = ist->pts = ist->dts;
    }

    // while we have more to decode or while the decoder did output something on EOF
    /*pkt里为视频是为一帧，为音频是可能为多帧。所以要一个循环*/
    while (ist->decoding_needed && (avpkt.size > 0 || (!pkt && got_output))) {
        int duration;
    handle_eof:
        //
        ist->pts = ist->next_pts;
        ist->dts = ist->next_dts;

        //没什么用。
        if (avpkt.size && avpkt.size != pkt->size &&
            !(ist->dec->capabilities & AV_CODEC_CAP_SUBFRAMES)) {
            av_log(NULL, ist->showed_multi_packet_warning ? AV_LOG_VERBOSE : AV_LOG_WARNING,
                   "Multiple frames in a packet from stream %d\n", pkt->stream_index);
            ist->showed_multi_packet_warning = 1;
        }
        //解码一个pkt
        switch (ist->dec_ctx->codec_type) {
        case AVMEDIA_TYPE_AUDIO:
            ret = decode_audio    (ist, &avpkt, &got_output);
            break;
        case AVMEDIA_TYPE_VIDEO:
            ret = decode_video    (ist, &avpkt, &got_output);
            if (avpkt.duration) {
                //返回的pkt中已经计算出了时长
                duration = av_rescale_q(avpkt.duration, ist->st->time_base, AV_TIME_BASE_Q);
            } else if(ist->dec_ctx->framerate.num != 0 && ist->dec_ctx->framerate.den != 0) {
                //根据帧率计算时长
                int ticks= av_stream_get_parser(ist->st) ? av_stream_get_parser(ist->st)->repeat_pict+1 : ist->dec_ctx->ticks_per_frame;
                duration = ((int64_t)AV_TIME_BASE *
                                ist->dec_ctx->framerate.den * ticks) /
                                ist->dec_ctx->framerate.num / ist->dec_ctx->ticks_per_frame;
            } else
                duration = 0;
            //dts和pts加时长，就是下一个帧的dts和pts，注意pkt中的都是临时的。只是在解码的时候用，过程当中的时间都保持在ist中。
            if(ist->dts != AV_NOPTS_VALUE && duration) {
                ist->next_dts += duration;
            }else
                ist->next_dts = AV_NOPTS_VALUE;

            if (got_output)
                ist->next_pts += duration; //FIXME the duration is not correct in some cases
            break;
        case AVMEDIA_TYPE_SUBTITLE:
            ret = transcode_subtitles(ist, &avpkt, &got_output);
            break;
        default:
            return -1;
        }

        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Error while decoding stream #%d:%d: %s\n",
                   ist->file_index, ist->st->index, av_err2str(ret));
            if (exit_on_error)
                exit_program(1);
            break;
        }

        avpkt.dts=
        avpkt.pts= AV_NOPTS_VALUE;

        // touch data and size only if not EOF
        if (pkt) {
            /*如果是视频帧，avpkt.size==0，ret==0，循环结束， 如果是音频帧，avpkt可能包含多帧，avpkt.size处理完可能不为0， 可能需要循环几次*/
            if(ist->dec_ctx->codec_type != AVMEDIA_TYPE_AUDIO)
                ret = avpkt.size;
            avpkt.data += ret;
            avpkt.size -= ret;
        }
        if (!got_output) {
            continue;
        }
        if (got_output && !pkt)
            break;
    }
    //pkt 传入是NULL时，给滤镜传入EOF信息
    /* after flushing, send an EOF on all the filter inputs attached to the stream */
    /* except when looping we need to flush but not to send an EOF */
    if (!pkt && ist->decoding_needed && !got_output && !no_eof) {
        int ret = send_filter_eof(ist);
        if (ret < 0) {
            av_log(NULL, AV_LOG_FATAL, "Error marking filters as finished\n");
            exit_program(1);
        }
    }
    /*如果流不需要重新编解码的情况，这里处理dts和next_dts*/
    /* handle stream copy */
    if (!ist->decoding_needed) {
        ist->dts = ist->next_dts;
        switch (ist->dec_ctx->codec_type) {
        case AVMEDIA_TYPE_AUDIO:
            ist->next_dts += ((int64_t)AV_TIME_BASE * ist->dec_ctx->frame_size) /
                             ist->dec_ctx->sample_rate;
            break;
        case AVMEDIA_TYPE_VIDEO:
            //优先按帧率计算
            if (ist->framerate.num) {
                // TODO: Remove work-around for c99-to-c89 issue 7
                AVRational time_base_q = AV_TIME_BASE_Q;
                //ist中的时间基是ffmpeg的内部时间基time_base_q，将next_dts转换为流本身的时间基
                int64_t next_dts = av_rescale_q(ist->next_dts, time_base_q, av_inv_q(ist->framerate));
                //next_dts为以帧率为时间基，+1就为下一帧的时间
                ist->next_dts = av_rescale_q(next_dts + 1, av_inv_q(ist->framerate), time_base_q);
            } else if (pkt->duration) {
                //pkt->duration的时间基为流本身的ist->st->time_base，将它进行转换，并加到ist->next_dts，
                //含义是现在的dts加上此帧经过的时长，表示下一帧的dts
                ist->next_dts += av_rescale_q(pkt->duration, ist->st->time_base, AV_TIME_BASE_Q);
            } else if(ist->dec_ctx->framerate.num != 0) {
                /* AVCodecParserContext中repeat_pict的注释如下： * This field is used for proper frame duration computation in lavf. * It signals, how much longer the frame duration of the current frame * is compared to normal frame duration. * * frame_duration = (1 + repeat_pict) * time_base * * It is used by codecs like H.264 to display telecined material. * ------------------------------------------- * AVCodecContext中ticks_per_frame的注释如下： * For some codecs, the time base is closer to the field rate than the frame rate. * Most notably, H.264 and MPEG-2 specify time_base as half of frame duration * if no telecine is used ... * * Set to time_base ticks per frame. Default 1, e.g., H.264/MPEG-2 set it to 2. */
                /*这里之后再分析吧。大概也是根据ist->dec_ctx->framerate算出一帧的时间加到next_dts上就是下一帧开始时间*/
                int ticks= av_stream_get_parser(ist->st) ? av_stream_get_parser(ist->st)->repeat_pict + 1 : ist->dec_ctx->ticks_per_frame;
                ist->next_dts += ((int64_t)AV_TIME_BASE *
                                  ist->dec_ctx->framerate.den * ticks) /
                                  ist->dec_ctx->framerate.num / ist->dec_ctx->ticks_per_frame;
            }
            break;
        }
        ist->pts = ist->dts;
        ist->next_pts = ist->next_dts;
    }
    for (i = 0; pkt && i < nb_output_streams; i++) {
        OutputStream *ost = output_streams[i];

        if (!check_output_constraints(ist, ost) || ost->encoding_needed)
            continue;
        //如果不需要重新编解码，就走到这里。ost->encoding_needed判断。
        do_streamcopy(ist, ost, pkt);
    }

    return got_output;
}

decode_video

针对时间的处理只有一句

pkt->dts  = av_rescale_q(ist->dts, AV_TIME_BASE_Q, ist->st->time_base);

将在ist中保存的dts给pkt中

static int decode_video(InputStream *ist, AVPacket *pkt, int *got_output)
{
    AVFrame *decoded_frame, *f;
    int i, ret = 0, err = 0, resample_changed;
    int64_t best_effort_timestamp;
    AVRational *frame_sample_aspect;

    if (!ist->decoded_frame && !(ist->decoded_frame = av_frame_alloc()))
        return AVERROR(ENOMEM);
    if (!ist->filter_frame && !(ist->filter_frame = av_frame_alloc()))
        return AVERROR(ENOMEM);
    //解码后的帧存放处
    decoded_frame = ist->decoded_frame;
    //开始给pkt中的时间赋值。只对dts赋值。
    pkt->dts  = av_rescale_q(ist->dts, AV_TIME_BASE_Q, ist->st->time_base);

    update_benchmark(NULL);
    //实际的解码函数，got_output不为0表示解码出了帧，解码出的帧放到decoded_frame中。
    ret = avcodec_decode_video2(ist->dec_ctx,
                                decoded_frame, got_output, pkt);
    update_benchmark("decode_video %d.%d", ist->file_index, ist->st->index);

    // The following line may be required in some cases where there is no parser
    // or the parser does not has_b_frames correctly
    if (ist->st->codec->has_b_frames < ist->dec_ctx->has_b_frames) {
        if (ist->dec_ctx->codec_id == AV_CODEC_ID_H264) {
            ist->st->codec->has_b_frames = ist->dec_ctx->has_b_frames;
        } else
            av_log(ist->dec_ctx, AV_LOG_WARNING,
                   "has_b_frames is larger in decoder than demuxer %d > %d.\n"
                   "If you want to help, upload a sample "
                   "of this file to ftp://upload.ffmpeg.org/incoming/ "
                   "and contact the ffmpeg-devel mailing list. ([email protected])",
                   ist->dec_ctx->has_b_frames,
                   ist->st->codec->has_b_frames);
    }
    //判断输出结果，涉及到参数"xerror"。
    check_decode_result(ist, got_output, ret);
    //如果解码返回成功，做个判断，看是否解码后的长宽、像素格式和最初分析的一样，不一样就输出。没啥用
    if (*got_output && ret >= 0) {
        if (ist->dec_ctx->width  != decoded_frame->width ||
            ist->dec_ctx->height != decoded_frame->height ||
            ist->dec_ctx->pix_fmt != decoded_frame->format) {
            av_log(NULL, AV_LOG_DEBUG, "Frame parameters mismatch context %d,%d,%d != %d,%d,%d\n",
                decoded_frame->width,
                decoded_frame->height,
                decoded_frame->format,
                ist->dec_ctx->width,
                ist->dec_ctx->height,
                ist->dec_ctx->pix_fmt);
        }
    }

    if (!*got_output || ret < 0)
        return ret;
    //对应参数"top"，和场有关
    if(ist->top_field_first>=0)
        decoded_frame->top_field_first = ist->top_field_first;
    //记录解码的帧数
    ist->frames_decoded++;
    //和硬件相关
    if (ist->hwaccel_retrieve_data && decoded_frame->format == ist->hwaccel_pix_fmt) {
        err = ist->hwaccel_retrieve_data(ist->dec_ctx, decoded_frame);
        if (err < 0)
            goto fail;
    }
    ist->hwaccel_retrieved_pix_fmt = decoded_frame->format;
    //此句很重要，是解码计算出的帧时间戳
    best_effort_timestamp= av_frame_get_best_effort_timestamp(decoded_frame);
    if(best_effort_timestamp != AV_NOPTS_VALUE) {
        //ist中的时间单位是AV_TIME_BASE_Q，decoded_frame中的时间单位是ist->st->time_base
        int64_t ts = av_rescale_q(decoded_frame->pts = best_effort_timestamp, ist->st->time_base, AV_TIME_BASE_Q);

        if (ts != AV_NOPTS_VALUE)
            ist->next_pts = ist->pts = ts;
    }

    if (debug_ts) {
        av_log(NULL, AV_LOG_INFO, "decoder -> ist_index:%d type:video "
               "frame_pts:%s frame_pts_time:%s best_effort_ts:%"PRId64" best_effort_ts_time:%s keyframe:%d frame_type:%d time_base:%d/%d\n",
               ist->st->index, av_ts2str(decoded_frame->pts),
               av_ts2timestr(decoded_frame->pts, &ist->st->time_base),
               best_effort_timestamp,
               av_ts2timestr(best_effort_timestamp, &ist->st->time_base),
               decoded_frame->key_frame, decoded_frame->pict_type,
               ist->st->time_base.num, ist->st->time_base.den);
    }
    /*表示视频就一帧。解完这次就不解了。处理音频时就没有这一句。在process_input_packet中会对此值做处理。 如果处理完还有值就需要在解码*/
    pkt->size = 0;

    if (ist->st->sample_aspect_ratio.num)
        decoded_frame->sample_aspect_ratio = ist->st->sample_aspect_ratio;
    //判断判断为真，需要重新设置滤镜链表。
    resample_changed = ist->resample_width   != decoded_frame->width  ||
                       ist->resample_height  != decoded_frame->height ||
                       ist->resample_pix_fmt != decoded_frame->format;
    if (resample_changed) {
        av_log(NULL, AV_LOG_INFO,
               "Input stream #%d:%d frame changed from size:%dx%d fmt:%s to size:%dx%d fmt:%s\n",
               ist->file_index, ist->st->index,
               ist->resample_width,  ist->resample_height,  av_get_pix_fmt_name(ist->resample_pix_fmt),
               decoded_frame->width, decoded_frame->height, av_get_pix_fmt_name(decoded_frame->format));

        ist->resample_width   = decoded_frame->width;
        ist->resample_height  = decoded_frame->height;
        ist->resample_pix_fmt = decoded_frame->format;

        for (i = 0; i < nb_filtergraphs; i++) {
            if (ist_in_filtergraph(filtergraphs[i], ist) && ist->reinit_filters &&
                configure_filtergraph(filtergraphs[i]) < 0) {
                av_log(NULL, AV_LOG_FATAL, "Error reinitializing filters!\n");
                exit_program(1);
            }
        }
    }

    frame_sample_aspect= av_opt_ptr(avcodec_get_frame_class(), decoded_frame, "sample_aspect_ratio");
    for (i = 0; i < ist->nb_filters; i++) {
        if (!frame_sample_aspect->num)
            *frame_sample_aspect = ist->st->sample_aspect_ratio;

        if (i < ist->nb_filters - 1) {
            f = ist->filter_frame;
            err = av_frame_ref(f, decoded_frame);
            if (err < 0)
                break;
        } else
            f = decoded_frame;
        //放到滤镜里面
        ret = av_buffersrc_add_frame_flags(ist->filters[i]->filter, f, AV_BUFFERSRC_FLAG_PUSH);
        if (ret == AVERROR_EOF) {
            ret = 0; /* ignore */
        } else if (ret < 0) {
            av_log(NULL, AV_LOG_FATAL,
                   "Failed to inject frame into filter network: %s\n", av_err2str(ret));
            exit_program(1);
        }
    }

fail:
    av_frame_unref(ist->filter_frame);
    av_frame_unref(decoded_frame);
    return err < 0 ? err : ret;
}

reap_filters

/** * Get and encode new output from any of the filtergraphs, without causing * activity. * * @return 0 for success, <0 for severe errors */
static int reap_filters(int flush)
{
    AVFrame *filtered_frame = NULL;
    int i;

    /* Reap all buffers present in the buffer sinks */
    for (i = 0; i < nb_output_streams; i++) {
        OutputStream *ost = output_streams[i];
        OutputFile    *of = output_files[ost->file_index];
        AVFilterContext *filter;
        AVCodecContext *enc = ost->enc_ctx;
        int ret = 0;

        if (!ost->filter)
            continue;
        filter = ost->filter->filter;

        if (!ost->filtered_frame && !(ost->filtered_frame = av_frame_alloc())) {
            return AVERROR(ENOMEM);
        }
        //从滤镜处理后的帧存放在这里
        filtered_frame = ost->filtered_frame;

        while (1) {
            double float_pts = AV_NOPTS_VALUE; // this is identical to filtered_frame.pts but with higher precision
            //从滤镜获取帧
            ret = av_buffersink_get_frame_flags(filter, filtered_frame,
                                               AV_BUFFERSINK_FLAG_NO_REQUEST);
            if (ret < 0) {
                if (ret != AVERROR(EAGAIN) && ret != AVERROR_EOF) {
                    av_log(NULL, AV_LOG_WARNING,
                           "Error in av_buffersink_get_frame_flags(): %s\n", av_err2str(ret));
                } else if (flush && ret == AVERROR_EOF) {
                    //没有数据了。已经到了滤镜的结束。
                    if (filter->inputs[0]->type == AVMEDIA_TYPE_VIDEO)
                        do_video_out(of->ctx, ost, NULL, AV_NOPTS_VALUE);
                }
                break;
            }
            //如果设置了此值，获取到的帧丢弃
            if (ost->finished) {
                av_frame_unref(filtered_frame);
                continue;
            }
            //下面这段不知道啥意思
            if (filtered_frame->pts != AV_NOPTS_VALUE) {
                int64_t start_time = (of->start_time == AV_NOPTS_VALUE) ? 0 : of->start_time;
                AVRational tb = enc->time_base;
                int extra_bits = av_clip(29 - av_log2(tb.den), 0, 16);

                tb.den <<= extra_bits;
                float_pts =
                    av_rescale_q(filtered_frame->pts, filter->inputs[0]->time_base, tb) -
                    av_rescale_q(start_time, AV_TIME_BASE_Q, tb);
                float_pts /= 1 << extra_bits;
                // avoid exact midoints to reduce the chance of rounding differences, this can be removed in case the fps code is changed to work with integers
                float_pts += FFSIGN(float_pts) * 1.0 / (1<<17);

                filtered_frame->pts =
                    av_rescale_q(filtered_frame->pts, filter->inputs[0]->time_base, enc->time_base) -
                    av_rescale_q(start_time, AV_TIME_BASE_Q, enc->time_base);
            }
            //if (ost->source_index >= 0)
            // *filtered_frame= *input_streams[ost->source_index]->decoded_frame; //for me_threshold

            switch (filter->inputs[0]->type) {
            case AVMEDIA_TYPE_VIDEO:
                //设置宽高比
                if (!ost->frame_aspect_ratio.num)
                    enc->sample_aspect_ratio = filtered_frame->sample_aspect_ratio;

                if (debug_ts) {
                    av_log(NULL, AV_LOG_INFO, "filter -> pts:%s pts_time:%s exact:%f time_base:%d/%d\n",
                            av_ts2str(filtered_frame->pts), av_ts2timestr(filtered_frame->pts, &enc->time_base),
                            float_pts,
                            enc->time_base.num, enc->time_base.den);
                }
                //编码并输出到文件
                do_video_out(of->ctx, ost, filtered_frame, float_pts);
                break;
            case AVMEDIA_TYPE_AUDIO:
                if (!(enc->codec->capabilities & AV_CODEC_CAP_PARAM_CHANGE) &&
                    enc->channels != av_frame_get_channels(filtered_frame)) {
                    av_log(NULL, AV_LOG_ERROR,
                           "Audio filter graph output is not normalized and encoder does not support parameter changes\n");
                    break;
                }
                do_audio_out(of->ctx, ost, filtered_frame);
                break;
            default:
                // TODO support subtitle filters
                av_assert0(0);
            }

            av_frame_unref(filtered_frame);
        }
    }

    return 0;
}

do_video_out

static void do_video_out(AVFormatContext *s,
                         OutputStream *ost,
                         AVFrame *next_picture,
                         double sync_ipts)
{
    int ret, format_video_sync;
    AVPacket pkt;
    AVCodecContext *enc = ost->enc_ctx;
    AVCodecContext *mux_enc = ost->st->codec;
    int nb_frames, nb0_frames, i;
    double delta, delta0;
    double duration = 0;
    int frame_size = 0;
    InputStream *ist = NULL;
    AVFilterContext *filter = ost->filter->filter;

    if (ost->source_index >= 0)
        ist = input_streams[ost->source_index];
    /*求duration的值*/
    if (filter->inputs[0]->frame_rate.num > 0 &&
        filter->inputs[0]->frame_rate.den > 0)
        duration = 1/(av_q2d(filter->inputs[0]->frame_rate) * av_q2d(enc->time_base));

    if(ist && ist->st->start_time != AV_NOPTS_VALUE && ist->st->first_dts != AV_NOPTS_VALUE && ost->frame_rate.num)
        duration = FFMIN(duration, 1/(av_q2d(ost->frame_rate) * av_q2d(enc->time_base)));

    if (!ost->filters_script &&
        !ost->filters &&
        next_picture &&
        ist &&
        lrintf(av_frame_get_pkt_duration(next_picture) * av_q2d(ist->st->time_base) / av_q2d(enc->time_base)) > 0) {
        duration = lrintf(av_frame_get_pkt_duration(next_picture) * av_q2d(ist->st->time_base) / av_q2d(enc->time_base));
    }

    if (!next_picture) {
        //reap_filters函数中在从滤镜中取数据结束后调用do_video_out(of->ctx, ost, NULL, AV_NOPTS_VALUE);会走到这里
        //end, flushing
        nb0_frames = nb_frames = mid_pred(ost->last_nb0_frames[0],
                                          ost->last_nb0_frames[1],
                                          ost->last_nb0_frames[2]);
    } else {
        delta0 = sync_ipts - ost->sync_opts; // delta0 is the "drift" between the input frame (next_picture) and where it would fall in the output.
        delta  = delta0 + duration;

        /* by default, we output a single frame */
        nb0_frames = 0; // tracks the number of times the PREVIOUS frame should be duplicated, mostly for variable framerate (VFR)
        nb_frames = 1;

        format_video_sync = video_sync_method;
        if (format_video_sync == VSYNC_AUTO) {
            if(!strcmp(s->oformat->name, "avi")) {
                format_video_sync = VSYNC_VFR;
            } else
                format_video_sync = (s->oformat->flags & AVFMT_VARIABLE_FPS) ? ((s->oformat->flags & AVFMT_NOTIMESTAMPS) ? VSYNC_PASSTHROUGH : VSYNC_VFR) : VSYNC_CFR;
            if (   ist
                && format_video_sync == VSYNC_CFR
                && input_files[ist->file_index]->ctx->nb_streams == 1
                && input_files[ist->file_index]->input_ts_offset == 0) {
                format_video_sync = VSYNC_VSCFR;
            }
            if (format_video_sync == VSYNC_CFR && copy_ts) {
                format_video_sync = VSYNC_VSCFR;
            }
        }
        ost->is_cfr = (format_video_sync == VSYNC_CFR || format_video_sync == VSYNC_VSCFR);

        if (delta0 < 0 &&
            delta > 0 &&
            format_video_sync != VSYNC_PASSTHROUGH &&
            format_video_sync != VSYNC_DROP) {
            if (delta0 < -0.6) {
                av_log(NULL, AV_LOG_WARNING, "Past duration %f too large\n", -delta0);
            } else
                av_log(NULL, AV_LOG_DEBUG, "Clipping frame in rate conversion by %f\n", -delta0);
            sync_ipts = ost->sync_opts;
            duration += delta0;
            delta0 = 0;
        }

        switch (format_video_sync) {
        case VSYNC_VSCFR:
            if (ost->frame_number == 0 && delta0 >= 0.5) {
                av_log(NULL, AV_LOG_DEBUG, "Not duplicating %d initial frames\n", (int)lrintf(delta0));
                delta = duration;
                delta0 = 0;
                ost->sync_opts = lrint(sync_ipts);
            }
        case VSYNC_CFR:
            // FIXME set to 0.5 after we fix some dts/pts bugs like in avidec.c
            if (frame_drop_threshold && delta < frame_drop_threshold && ost->frame_number) {
                nb_frames = 0;
            } else if (delta < -1.1)
                nb_frames = 0;
            else if (delta > 1.1) {
                nb_frames = lrintf(delta);
                if (delta0 > 1.1)
                    nb0_frames = lrintf(delta0 - 0.6);
            }
            break;
        case VSYNC_VFR:
            if (delta <= -0.6)
                nb_frames = 0;
            else if (delta > 0.6)
                ost->sync_opts = lrint(sync_ipts);
            break;
        case VSYNC_DROP:
        case VSYNC_PASSTHROUGH:
            ost->sync_opts = lrint(sync_ipts);
            break;
        default:
            av_assert0(0);
        }
    }
    //之前的都是在算包含几个帧
    nb_frames = FFMIN(nb_frames, ost->max_frames - ost->frame_number);
    nb0_frames = FFMIN(nb0_frames, nb_frames);

    //缓存有三个nb0_frames在last_nb0_frames中，每次[0]是最新的。写之前都向后移。
    memmove(ost->last_nb0_frames + 1,
            ost->last_nb0_frames,
            sizeof(ost->last_nb0_frames[0]) * (FF_ARRAY_ELEMS(ost->last_nb0_frames) - 1));
    ost->last_nb0_frames[0] = nb0_frames;

    if (nb0_frames == 0 && ost->last_dropped) {
        nb_frames_drop++;
        av_log(NULL, AV_LOG_VERBOSE,
               "*** dropping frame %d from stream %d at ts %"PRId64"\n",
               ost->frame_number, ost->st->index, ost->last_frame->pts);
    }
    if (nb_frames > (nb0_frames && ost->last_dropped) + (nb_frames > nb0_frames)) {
        if (nb_frames > dts_error_threshold * 30) {
            av_log(NULL, AV_LOG_ERROR, "%d frame duplication too large, skipping\n", nb_frames - 1);
            nb_frames_drop++;
            return;
        }
        nb_frames_dup += nb_frames - (nb0_frames && ost->last_dropped) - (nb_frames > nb0_frames);
        av_log(NULL, AV_LOG_VERBOSE, "*** %d dup!\n", nb_frames - 1);
    }
    ost->last_dropped = nb_frames == nb0_frames && next_picture;

  /* duplicates frame if needed */
  for (i = 0; i < nb_frames; i++) {
    AVFrame *in_picture;
    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;

    if (i < nb0_frames && ost->last_frame) {
        in_picture = ost->last_frame;
    } else
        in_picture = next_picture;

    if (!in_picture)
        return;

    in_picture->pts = ost->sync_opts;

#if 1
    if (!check_recording_time(ost))
#else
    if (ost->frame_number >= ost->max_frames)
#endif
        return;

#if FF_API_LAVF_FMT_RAWPICTURE
    if (s->oformat->flags & AVFMT_RAWPICTURE &&
        enc->codec->id == AV_CODEC_ID_RAWVIDEO) {
        /* raw pictures are written as AVPicture structure to avoid any copies. We support temporarily the older method. */
        if (in_picture->interlaced_frame)
            mux_enc->field_order = in_picture->top_field_first ? AV_FIELD_TB:AV_FIELD_BT;
        else
            mux_enc->field_order = AV_FIELD_PROGRESSIVE;
        pkt.data   = (uint8_t *)in_picture;
        pkt.size   =  sizeof(AVPicture);
        pkt.pts    = av_rescale_q(in_picture->pts, enc->time_base, ost->st->time_base);
        pkt.flags |= AV_PKT_FLAG_KEY;

        write_frame(s, &pkt, ost);
    } else
#endif
    {
        int got_packet, forced_keyframe = 0;
        double pts_time;

        if (enc->flags & (AV_CODEC_FLAG_INTERLACED_DCT | AV_CODEC_FLAG_INTERLACED_ME) &&
            ost->top_field_first >= 0)
            in_picture->top_field_first = !!ost->top_field_first;

        if (in_picture->interlaced_frame) {
            //对场帧的处理
            if (enc->codec->id == AV_CODEC_ID_MJPEG)
                mux_enc->field_order = in_picture->top_field_first ? AV_FIELD_TT:AV_FIELD_BB;
            else
                mux_enc->field_order = in_picture->top_field_first ? AV_FIELD_TB:AV_FIELD_BT;
        } else
            mux_enc->field_order = AV_FIELD_PROGRESSIVE;

        in_picture->quality = enc->global_quality;
        in_picture->pict_type = 0;

        pts_time = in_picture->pts != AV_NOPTS_VALUE ?
            in_picture->pts * av_q2d(enc->time_base) : NAN;
        //对应参数"force_key_frames"，指定时间戳强制I帧
        if (ost->forced_kf_index < ost->forced_kf_count &&
            in_picture->pts >= ost->forced_kf_pts[ost->forced_kf_index]) {
            ost->forced_kf_index++;
            forced_keyframe = 1;
        } else if (ost->forced_keyframes_pexpr) {
            double res;
            ost->forced_keyframes_expr_const_values[FKF_T] = pts_time;
            res = av_expr_eval(ost->forced_keyframes_pexpr,
                               ost->forced_keyframes_expr_const_values, NULL);
            ff_dlog(NULL, "force_key_frame: n:%f n_forced:%f prev_forced_n:%f t:%f prev_forced_t:%f -> res:%f\n",
                    ost->forced_keyframes_expr_const_values[FKF_N],
                    ost->forced_keyframes_expr_const_values[FKF_N_FORCED],
                    ost->forced_keyframes_expr_const_values[FKF_PREV_FORCED_N],
                    ost->forced_keyframes_expr_const_values[FKF_T],
                    ost->forced_keyframes_expr_const_values[FKF_PREV_FORCED_T],
                    res);
            if (res) {
                forced_keyframe = 1;
                ost->forced_keyframes_expr_const_values[FKF_PREV_FORCED_N] =
                    ost->forced_keyframes_expr_const_values[FKF_N];
                ost->forced_keyframes_expr_const_values[FKF_PREV_FORCED_T] =
                    ost->forced_keyframes_expr_const_values[FKF_T];
                ost->forced_keyframes_expr_const_values[FKF_N_FORCED] += 1;
            }

            ost->forced_keyframes_expr_const_values[FKF_N] += 1;
        } else if (   ost->forced_keyframes
                   && !strncmp(ost->forced_keyframes, "source", 6)
                   && in_picture->key_frame==1) {
            forced_keyframe = 1;
        }

        if (forced_keyframe) {
            in_picture->pict_type = AV_PICTURE_TYPE_I;
            av_log(NULL, AV_LOG_DEBUG, "Forced keyframe at time %f\n", pts_time);
        }

        update_benchmark(NULL);
        if (debug_ts) {
            av_log(NULL, AV_LOG_INFO, "encoder <- type:video "
                   "frame_pts:%s frame_pts_time:%s time_base:%d/%d\n",
                   av_ts2str(in_picture->pts), av_ts2timestr(in_picture->pts, &enc->time_base),
                   enc->time_base.num, enc->time_base.den);
        }

        ost->frames_encoded++;

        ret = avcodec_encode_video2(enc, &pkt, in_picture, &got_packet);
        update_benchmark("encode_video %d.%d", ost->file_index, ost->index);
        if (ret < 0) {
            av_log(NULL, AV_LOG_FATAL, "Video encoding failed\n");
            exit_program(1);
        }
        //got_packet为1表示有数据编出。不是每次调用编码器都有帧输出
        if (got_packet) {
            if (debug_ts) {
                av_log(NULL, AV_LOG_INFO, "encoder -> type:video "
                       "pkt_pts:%s pkt_pts_time:%s pkt_dts:%s pkt_dts_time:%s\n",
                       av_ts2str(pkt.pts), av_ts2timestr(pkt.pts, &enc->time_base),
                       av_ts2str(pkt.dts), av_ts2timestr(pkt.dts, &enc->time_base));
            }

            if (pkt.pts == AV_NOPTS_VALUE && !(enc->codec->capabilities & AV_CODEC_CAP_DELAY))
                pkt.pts = ost->sync_opts;
            //转换时间基
            av_packet_rescale_ts(&pkt, enc->time_base, ost->st->time_base);

            if (debug_ts) {
                av_log(NULL, AV_LOG_INFO, "encoder -> type:video "
                    "pkt_pts:%s pkt_pts_time:%s pkt_dts:%s pkt_dts_time:%s\n",
                    av_ts2str(pkt.pts), av_ts2timestr(pkt.pts, &ost->st->time_base),
                    av_ts2str(pkt.dts), av_ts2timestr(pkt.dts, &ost->st->time_base));
            }

            frame_size = pkt.size;
            write_frame(s, &pkt, ost);

            /* if two pass, output log */
            if (ost->logfile && enc->stats_out) {
                fprintf(ost->logfile, "%s", enc->stats_out);
            }
        }
    }
    ost->sync_opts++;
    /* * For video, number of frames in == number of packets out. * But there may be reordering, so we can't throw away frames on encoder * flush, we need to limit them here, before they go into encoder. */
    ost->frame_number++;

    if (vstats_filename && frame_size)
        do_video_stats(ost, frame_size);
  }

    if (!ost->last_frame)
        ost->last_frame = av_frame_alloc();
    av_frame_unref(ost->last_frame);
    if (next_picture && ost->last_frame)
        av_frame_ref(ost->last_frame, next_picture);
    else
        av_frame_free(&ost->last_frame);
}

write_frame

void Cffmpeg::write_frame(AVFormatContext *s, AVPacket *pkt, OutputStream *ost)
{
    AVBitStreamFilterContext *bsfc = ost->bitstream_filters;
    AVCodecContext          *avctx = ost->encoding_needed ? ost->enc_ctx : ost->st->codec;
    int ret;

    if (!ost->st->codec->extradata_size && ost->enc_ctx->extradata_size) {
        ost->st->codec->extradata = (uint8_t *)av_mallocz(ost->enc_ctx->extradata_size + AV_INPUT_BUFFER_PADDING_SIZE);
        if (ost->st->codec->extradata) {
            memcpy(ost->st->codec->extradata, ost->enc_ctx->extradata, ost->enc_ctx->extradata_size);
            ost->st->codec->extradata_size = ost->enc_ctx->extradata_size;
        }
    }

    if ((avctx->codec_type == AVMEDIA_TYPE_VIDEO && video_sync_method == VSYNC_DROP) ||
        (avctx->codec_type == AVMEDIA_TYPE_AUDIO && audio_sync_method < 0))
        pkt->pts = pkt->dts = AV_NOPTS_VALUE;

    /* * Audio encoders may split the packets -- #frames in != #packets out. * But there is no reordering, so we can limit the number of output packets * by simply dropping them here. * Counting encoded video frames needs to be done separately because of * reordering, see do_video_out() */
    if (!(avctx->codec_type == AVMEDIA_TYPE_VIDEO && avctx->codec)) {
        if (ost->frame_number >= ost->max_frames) {
            av_packet_unref(pkt);
            return;
        }
        ost->frame_number++;
    }
    if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
        int i;
        uint8_t *sd = av_packet_get_side_data(pkt, AV_PKT_DATA_QUALITY_STATS,
                                              NULL);
        ost->quality = sd ? AV_RL32(sd) : -1;
        ost->pict_type = sd ? sd[4] : AV_PICTURE_TYPE_NONE;

        for (i = 0; i<FF_ARRAY_ELEMS(ost->error); i++) {
            if (sd && i < sd[5])
                ost->error[i] = AV_RL64(sd + 8 + 8*i);
            else
                ost->error[i] = -1;
        }

        if (ost->frame_rate.num && ost->is_cfr) {
            if (pkt->duration > 0)
                av_log(NULL, AV_LOG_WARNING, "Overriding packet duration by frame rate, this should not happen\n");
            pkt->duration = av_rescale_q(1, av_inv_q(ost->frame_rate),
                                         ost->st->time_base);
        }
    }
    //对应参数"bsf"
    if (bsfc)
        av_packet_split_side_data(pkt);
    //对应参数"bsf"
    if ((ret = av_apply_bitstream_filters(avctx, pkt, bsfc)) < 0) {
        print_error("", ret);
        if (exit_on_error)
            exit_program(1);
    }
    if (pkt->size == 0 && pkt->side_data_elems == 0)
        return;
    if (!ost->st->codecpar->extradata && avctx->extradata) {
        ost->st->codecpar->extradata = (uint8_t *)av_malloc(avctx->extradata_size + FF_INPUT_BUFFER_PADDING_SIZE);
        if (!ost->st->codecpar->extradata) {
            av_log(NULL, AV_LOG_ERROR, "Could not allocate extradata buffer to copy parser data.\n");
            exit_program(1);
        }
        ost->st->codecpar->extradata_size = avctx->extradata_size;
        memcpy(ost->st->codecpar->extradata, avctx->extradata, avctx->extradata_size);
    }

    if (!(s->oformat->flags & AVFMT_NOTIMESTAMPS)) {
        if (pkt->dts != AV_NOPTS_VALUE &&
            pkt->pts != AV_NOPTS_VALUE &&
            pkt->dts > pkt->pts) {
            av_log(s, AV_LOG_WARNING, "Invalid DTS: %"PRId64" PTS: %"PRId64" in output stream %d:%d, replacing by guess\n",
                   pkt->dts, pkt->pts,
                   ost->file_index, ost->st->index);
            pkt->pts =
            pkt->dts = pkt->pts + pkt->dts + ost->last_mux_dts + 1
                     - FFMIN3(pkt->pts, pkt->dts, ost->last_mux_dts + 1)
                     - FFMAX3(pkt->pts, pkt->dts, ost->last_mux_dts + 1);
        }
     if(
        (avctx->codec_type == AVMEDIA_TYPE_AUDIO || avctx->codec_type == AVMEDIA_TYPE_VIDEO) &&
        pkt->dts != AV_NOPTS_VALUE &&
        !(avctx->codec_id == AV_CODEC_ID_VP9 && ost->stream_copy) &&
        ost->last_mux_dts != AV_NOPTS_VALUE) {
      int64_t max = ost->last_mux_dts + !(s->oformat->flags & AVFMT_TS_NONSTRICT);
      if (pkt->dts < max) {
        int loglevel = max - pkt->dts > 2 || avctx->codec_type == AVMEDIA_TYPE_VIDEO ? AV_LOG_WARNING : AV_LOG_DEBUG;
        av_log(s, loglevel, "Non-monotonous DTS in output stream "
               "%d:%d; previous: %"PRId64", current: %"PRId64"; ",
               ost->file_index, ost->st->index, ost->last_mux_dts, pkt->dts);
        if (exit_on_error) {
            av_log(NULL, AV_LOG_FATAL, "aborting.\n");
            exit_program(1);
        }
        av_log(s, loglevel, "changing to %"PRId64". This may result "
               "in incorrect timestamps in the output file.\n",
               max);
        if(pkt->pts >= pkt->dts)
            pkt->pts = FFMAX(pkt->pts, max);
        pkt->dts = max;
      }
     }
    }
    ost->last_mux_dts = pkt->dts;

    ost->data_size += pkt->size;
    ost->packets_written++;

    pkt->stream_index = ost->index;

    if (debug_ts) {
        av_log(NULL, AV_LOG_INFO, "muxer <- type:%s "
                "pkt_pts:%s pkt_pts_time:%s pkt_dts:%s pkt_dts_time:%s size:%d\n",
                av_get_media_type_string(ost->enc_ctx->codec_type),
                av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, &ost->st->time_base),
                av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, &ost->st->time_base),
                pkt->size
              );
    }

    ret = av_interleaved_write_frame(s, pkt);
    if (ret < 0) {
        print_error("av_interleaved_write_frame()", ret);
        main_return_code = 1;
        close_all_output_streams(ost, (OSTFinished)(MUXER_FINISHED | ENCODER_FINISHED), ENCODER_FINISHED);
    }
    av_packet_unref(pkt);
}