transcode_step()介绍

transcode_step()

transcode_step()

static int transcode_step(void)
{
    OutputStream *ost;
    InputStream  *ist;
    int ret;
    //选出一路输出流中dts（解码时间戳）最小的。
    ost = choose_output();//后面有详细介绍
    if (!ost) {
        if (got_eagain()) {
            reset_eagain();
            av_usleep(10000);
            return 0;
        }
        av_log(NULL, AV_LOG_VERBOSE, "No more inputs to read from, finishing.\n");
        return AVERROR_EOF;
    }
    //判断是否配置了滤镜
    if (ost->filter) {
        if ((ret = transcode_from_filter(ost->filter->graph, &ist)) < 0)
            return ret;
        if (!ist)
            return 0;
    } else {
        av_assert0(ost->source_index >= 0);
        ist = input_streams[ost->source_index];
    }
    //完成解码工作，并放入滤镜中
    ret = process_input(ist->file_index);
    if (ret == AVERROR(EAGAIN)) {
        if (input_files[ist->file_index]->eagain)
            ost->unavailable = 1;
        return 0;
    }

    if (ret < 0)
        return ret == AVERROR_EOF ? 0 : ret;
    //从滤镜中获取数据，并完成编码工作，写入文件中
    return reap_filters(0);
}

choose_output()

选出一路输出流中dts（解码时间戳）最小的。
其中av_rescale_q的解释参考ffmpeg中的时间
简单的说就是做一个不同时间基之间的转换

/** * Select the output stream to process. * * @return selected output stream, or NULL if none available */
static OutputStream *choose_output(void)
{
    int i;
    int64_t opts_min = INT64_MAX;
    OutputStream *ost_min = NULL;

    for (i = 0; i < nb_output_streams; i++) {
        OutputStream *ost = output_streams[i];
        int64_t opts = ost->st->cur_dts == AV_NOPTS_VALUE ? INT64_MIN :
                       av_rescale_q(ost->st->cur_dts, ost->st->time_base,
                                    AV_TIME_BASE_Q);
        if (ost->st->cur_dts == AV_NOPTS_VALUE)
            av_log(NULL, AV_LOG_DEBUG, "cur_dts is invalid (this is harmless if it occurs once at the start per stream)\n");
        /*find the stream that opts is Minimum */
        if (!ost->finished && opts < opts_min) {
            opts_min = opts;
            ost_min  = ost->unavailable ? NULL : ost;
        }
    }
    return ost_min;
}

transcode_from_filter

对指定的滤镜做一步转码。
这个函数需要再分析完滤镜整体结构后才能解释。先放到这里。

/** * Perform a step of transcoding for the specified filter graph. * * @param[in] graph filter graph to consider * @param[out] best_ist input stream where a frame would allow to continue * @return 0 for success, <0 for error */
static int transcode_from_filter(FilterGraph *graph, InputStream **best_ist)
{
    int i, ret;
    int nb_requests, nb_requests_max = 0;
    InputFilter *ifilter;
    InputStream *ist;

    *best_ist = NULL;
    ret = avfilter_graph_request_oldest(graph->graph);
    if (ret >= 0)
        return reap_filters(0);

    if (ret == AVERROR_EOF) {
        ret = reap_filters(1);
        for (i = 0; i < graph->nb_outputs; i++)
            close_output_stream(graph->outputs[i]->ost);
        return ret;
    }
    if (ret != AVERROR(EAGAIN))
        return ret;

    for (i = 0; i < graph->nb_inputs; i++) {
        ifilter = graph->inputs[i];
        ist = ifilter->ist;
        if (input_files[ist->file_index]->eagain ||
            input_files[ist->file_index]->eof_reached)
            continue;
        nb_requests = av_buffersrc_get_nb_failed_requests(ifilter->filter);
        if (nb_requests > nb_requests_max) {
            nb_requests_max = nb_requests;
            *best_ist = ist;
        }
    }

    if (!*best_ist)
        for (i = 0; i < graph->nb_outputs; i++)
            graph->outputs[i]->ost->unavailable = 1;

    return 0;
}

process_input()

/* * Return * - 0 -- one packet was read and processed * - AVERROR(EAGAIN) -- no packets were available for selected file, * this function should be called again * - AVERROR_EOF -- this function should not be called again */
int Cffmpeg::process_input(int file_index)
{
    InputFile *ifile = input_files[file_index];
    AVFormatContext *is;
    InputStream *ist;
    AVPacket pkt;
    int ret, i, j;
    int64_t duration;
    int64_t pkt_dts;

    is  = ifile->ctx;
    //也就是调用av_read_frame()，其中对帧率仿真做了处理
    ret = get_input_packet(ifile, &pkt);

    if (ret == AVERROR(EAGAIN)) {
        ifile->eagain = 1;
        return ret;
    }
    /*ifile->loop针对参数"stream_loop"，在open_input_file()中有赋值 *表示"set number of times input stream shall be looped" *get_input_packet函数出错，就seek到文件的开始，ifile->loop表示循环次数， *在seek_to_start中递减 */
    if (ret < 0 && ifile->loop) {
        if ((ret = seek_to_start(ifile, is)) < 0)
            return ret;
        ret = get_input_packet(ifile, &pkt);
    }
    if (ret < 0) {
        //出错
        if (ret != AVERROR_EOF) {
            print_error(is->filename, ret);
            if (exit_on_error)
                exit_program(1);
        }
        //文件结束
        for (i = 0; i < ifile->nb_streams; i++) {
            ist = input_streams[ifile->ist_index + i];
            if (ist->decoding_needed) {
                /* 调用流程如下： process_input_packet()->decode_video()->avcodec_decode_video2(); 在avcodec_decode_video2()函数的注释中有这样一段 * @note Codecs which have the AV_CODEC_CAP_DELAY capability set have a delay * between input and output, these need to be fed with avpkt->data=NULL, * avpkt->size=0 at the end to return the remaining frames. * * process_input_packet中将一个avpkt->data=NULL,avpkt->size=0的avpkt传给了avcodec_decode_video2 * 以返回剩余的帧。结束这路流的输入 */
                ret = process_input_packet(ist, NULL, 0);
                if (ret>0)
                    return 0;
            }

            /* mark all outputs that don't go through lavfi as finished */
            for (j = 0; j < nb_output_streams; j++) {
                OutputStream *ost = output_streams[j];

                if (ost->source_index == ifile->ist_index + i &&
                    (ost->stream_copy || ost->enc->type == AVMEDIA_TYPE_SUBTITLE))
                    finish_output_stream(ost);//设置结束标识。
            }
        }

        ifile->eof_reached = 1;
        return AVERROR(EAGAIN);
    }

    reset_eagain();
    //对应参数"dump"，"dump each input packet"
    if (do_pkt_dump) {
        av_pkt_dump_log2(NULL, AV_LOG_INFO, &pkt, do_hex_dump,
                         is->streams[pkt.stream_index]);
    }
    /* the following test is needed in case new streams appear dynamically in stream : we ignore them 如果获取的流在初始化的时候没有，就丢弃 */
    if (pkt.stream_index >= ifile->nb_streams) {
        report_new_stream(file_index, &pkt);
        goto discard_packet;
    }
    //找到读取的帧对应的流信息结构体。ifile->ist_index表示这个文件的流在input_streams中的开始小标
    ist = input_streams[ifile->ist_index + pkt.stream_index];

    ist->data_size += pkt.size;
    ist->nb_packets++;
    //配置中让丢弃的流就丢弃
    if (ist->discard)
        goto discard_packet;
    //读取的帧损坏退出
    if (exit_on_error && (pkt.flags & AV_PKT_FLAG_CORRUPT)) {
        av_log(NULL, AV_LOG_FATAL, "%s: corrupt input packet in stream %d\n", is->filename, pkt.stream_index);
        exit_program(1);
    }

    if (debug_ts) {
        av_log(NULL, AV_LOG_INFO, "demuxer -> ist_index:%d type:%s "
               "next_dts:%s next_dts_time:%s next_pts:%s next_pts_time:%s pkt_pts:%s pkt_pts_time:%s pkt_dts:%s pkt_dts_time:%s off:%s off_time:%s\n",
               ifile->ist_index + pkt.stream_index, av_get_media_type_string(ist->dec_ctx->codec_type),
               av_ts2str(ist->next_dts), av_ts2timestr(ist->next_dts, &AV_TIME_BASE_Q),
               av_ts2str(ist->next_pts), av_ts2timestr(ist->next_pts, &AV_TIME_BASE_Q),
               av_ts2str(pkt.pts), av_ts2timestr(pkt.pts, &ist->st->time_base),
               av_ts2str(pkt.dts), av_ts2timestr(pkt.dts, &ist->st->time_base),
               av_ts2str(input_files[ist->file_index]->ts_offset),
               av_ts2timestr(input_files[ist->file_index]->ts_offset, &AV_TIME_BASE_Q));
    }
    //在设置了is->start_time的情况下才执行这里。对应参数"ss"
    if(!ist->wrap_correction_done && is->start_time != AV_NOPTS_VALUE && ist->st->pts_wrap_bits < 64){
        int64_t stime, stime2;
        // Correcting starttime based on the enabled streams
        // FIXME this ideally should be done before the first use of starttime but we do not know which are the enabled streams at that point.
        // so we instead do it here as part of discontinuity handling
        if (   ist->next_dts == AV_NOPTS_VALUE
            && ifile->ts_offset == -is->start_time
            && (is->iformat->flags & AVFMT_TS_DISCONT)) {
            int64_t new_start_time = INT64_MAX;
            for (i=0; i<is->nb_streams; i++) {
                AVStream *st = is->streams[i];
                if(st->discard == AVDISCARD_ALL || st->start_time == AV_NOPTS_VALUE)
                    continue;
                new_start_time = FFMIN(new_start_time, av_rescale_q(st->start_time, st->time_base, AV_TIME_BASE_Q));
            }
            if (new_start_time > is->start_time) {
                av_log(is, AV_LOG_VERBOSE, "Correcting start time by %"PRId64"\n", new_start_time - is->start_time);
                ifile->ts_offset = -new_start_time;
            }
        }

        stime = av_rescale_q(is->start_time, AV_TIME_BASE_Q, ist->st->time_base);
        stime2= stime + (1ULL<<ist->st->pts_wrap_bits);
        ist->wrap_correction_done = 1;

        if(stime2 > stime && pkt.dts != AV_NOPTS_VALUE && pkt.dts > stime + (1LL<<(ist->st->pts_wrap_bits-1))) {
            pkt.dts -= 1ULL<<ist->st->pts_wrap_bits;
            ist->wrap_correction_done = 0;
        }
        if(stime2 > stime && pkt.pts != AV_NOPTS_VALUE && pkt.pts > stime + (1LL<<(ist->st->pts_wrap_bits-1))) {
            pkt.pts -= 1ULL<<ist->st->pts_wrap_bits;
            ist->wrap_correction_done = 0;
        }
    }

    /* add the stream-global side data to the first packet */
    if (ist->nb_packets == 1) {
        if (ist->st->nb_side_data)
            av_packet_split_side_data(&pkt);
        for (i = 0; i < ist->st->nb_side_data; i++) {
            AVPacketSideData *src_sd = &ist->st->side_data[i];
            uint8_t *dst_data;

            if (av_packet_get_side_data(&pkt, src_sd->type, NULL))
                continue;
            if (ist->autorotate && src_sd->type == AV_PKT_DATA_DISPLAYMATRIX)
                continue;

            dst_data = av_packet_new_side_data(&pkt, src_sd->type, src_sd->size);
            if (!dst_data)
                exit_program(1);

            memcpy(dst_data, src_sd->data, src_sd->size);
        }
    }

    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);

    if (pkt.pts != AV_NOPTS_VALUE)
        pkt.pts *= ist->ts_scale;
    if (pkt.dts != AV_NOPTS_VALUE)
        pkt.dts *= ist->ts_scale;

    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);

    if (debug_ts) {
        av_log(NULL, AV_LOG_INFO, "demuxer+ffmpeg -> ist_index:%d type:%s pkt_pts:%s pkt_pts_time:%s pkt_dts:%s pkt_dts_time:%s off:%s off_time:%s\n",
               ifile->ist_index + pkt.stream_index, av_get_media_type_string(ist->dec_ctx->codec_type),
               av_ts2str(pkt.pts), av_ts2timestr(pkt.pts, &ist->st->time_base),
               av_ts2str(pkt.dts), av_ts2timestr(pkt.dts, &ist->st->time_base),
               av_ts2str(input_files[ist->file_index]->ts_offset),
               av_ts2timestr(input_files[ist->file_index]->ts_offset, &AV_TIME_BASE_Q));
    }

    sub2video_heartbeat(ist, pkt.pts);

    process_input_packet(ist, &pkt, 0);

discard_packet:
    av_packet_unref(&pkt);

    return 0;
}

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);
        }
        filtered_frame = ost->filtered_frame;

        while (1) {
            double float_pts = AV_NOPTS_VALUE; // this is identical to filtered_frame.pts but with higher precision
            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) {
                    if (filter->inputs[0]->type == AVMEDIA_TYPE_VIDEO)
                        do_video_out(of->ctx, ost, NULL, AV_NOPTS_VALUE);
                }
                break;
            }
            if (ost->finished) {
                av_frame_unref(filtered_frame);
                continue;
            }
            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:
                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);
                }

                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;
}

av_frame_get_best_effort_timestamp(const AVFrame *frame)

声明在libavutil\frame.h中
定义在libavutil\frame.c中

MAKE_ACCESSORS(AVFrame, frame, int64_t, best_effort_timestamp)

//libavutil\internal.h
#define MAKE_ACCESSORS(str, name, type, field) \
    type av_##name##_get_##field(const str *s) { return s->field; } \
    void av_##name##_set_##field(str *s, type v) { s->field = v; }

功能可见，就是获取和设置一个结构体中的一个成员的值。