estimate_timings方法

estimate_timings方法在ffmepg中不是一个api方法， 只是在avformat_find_stream_info中调用，但这个方法的作用却是很大的， 它解出了三个参数， 分别如下

• start time：第一帧数据的PTS， 当前时间的计算是， 当前显示帧的PTS - start time。
• duration：视频的总长度。
• bitrate：视频的码率

计算duration的方法分为2种， estimate_timings_from_pts, estimate_timings_from_bit_rate， 这两种是在demux时没有解出来duration的情况下去做的。

static void estimate_timings(AVFormatContext *ic, int64_t old_offset)
{
    int64_t file_size;

    /* get the file size, if possible */
    if (ic->iformat->flags & AVFMT_NOFILE) {
        file_size = 0;
    } else {
        file_size = avio_size(ic->pb);
        file_size = FFMAX(0, file_size);
    }
    /*当demux是mpegts时，就使用 estimate_timings_from_pts方法去解出duration*/
    if ((!strcmp(ic->iformat->name, "mpeg") ||
         !strcmp(ic->iformat->name, "mpegts")) &&
        file_size && ic->pb->seekable) {
        /* get accurate estimate from the PTSes */
        estimate_timings_from_pts(ic, old_offset);
        ic->duration_estimation_method = AVFMT_DURATION_FROM_PTS;
    } else if (has_duration(ic)) {
        /* 在demux的read header的时候已经解出来了duration, 譬如mp4, hls rtsp */
        /* at least one component has timings - we use them for all
         * the components */
        fill_all_stream_timings(ic);
        ic->duration_estimation_method = AVFMT_DURATION_FROM_STREAM;
    } else {
        /* 通过码率和file size去计算duration */
        /* less precise: use bitrate info */
        estimate_timings_from_bit_rate(ic);
        ic->duration_estimation_method = AVFMT_DURATION_FROM_BITRATE;
    }
    update_stream_timings(ic);

    {
        int i;
        AVStream av_unused *st;
        for (i = 0; i < ic->nb_streams; i++) {
            st = ic->streams[i];
            av_log(ic, AV_LOG_TRACE, "stream %d: start_time: %0.3f duration: %0.3f\n", i,
                   (double) st->start_time * av_q2d(st->time_base),
                   (double) st->duration   * av_q2d(st->time_base));
        }
        av_log(ic, AV_LOG_TRACE,
                "format: start_time: %0.3f duration: %0.3f bitrate=%"PRId64" kb/s\n",
                (double) ic->start_time / AV_TIME_BASE,
                (double) ic->duration   / AV_TIME_BASE,
                (int64_t)ic->bit_rate / 1000);
    }
}

estimate_timings_from_pts方法

其实estimate_timings_from_pts方法说简单点， 就是通过最后一帧的pts-第一帧的pts来计算duration。

static void estimate_timings_from_pts(AVFormatContext *ic, int64_t old_offset)
{
    AVPacket pkt1, *pkt = &pkt1;
    AVStream *st;
    int num, den, read_size, i, ret;
    int found_duration = 0;
    int is_end;
    int64_t filesize, offset, duration;
    int retry = 0;

    /* flush packet queue */
    flush_packet_queue(ic);

    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];
        if (st->start_time == AV_NOPTS_VALUE &&
            st->first_dts == AV_NOPTS_VALUE &&
            st->codecpar->codec_type != AVMEDIA_TYPE_UNKNOWN)
            av_log(ic, AV_LOG_WARNING,
                   "start time for stream %d is not set in estimate_timings_from_pts\n", i);

        if (st->parser) {
            av_parser_close(st->parser);
            st->parser = NULL;
        }
    }

    av_opt_set(ic, "skip_changes", "1", AV_OPT_SEARCH_CHILDREN);
    /* estimate the end time (duration) */
    /* XXX: may need to support wrapping */
    filesize = ic->pb ? avio_size(ic->pb) : 0;
    do {
        is_end = found_duration;
        // 找到一个接近文件结尾的值
        offset = filesize - (DURATION_MAX_READ_SIZE << retry);
        if (offset < 0)
            offset = 0;

        avio_seek(ic->pb, offset, SEEK_SET);
        read_size = 0;
        // 循环， 尽可能的接近文件结尾， 获得最后一帧数据的Pts， 来计算druation
        for (;;) {
            if (read_size >= DURATION_MAX_READ_SIZE << (FFMAX(retry - 1, 0)))
                break;

            do {
                ret = ff_read_packet(ic, pkt);
            } while (ret == AVERROR(EAGAIN));
            if (ret != 0)
                break;
            read_size += pkt->size;
            st         = ic->streams[pkt->stream_index];
            if (pkt->pts != AV_NOPTS_VALUE &&
                (st->start_time != AV_NOPTS_VALUE ||
                 st->first_dts  != AV_NOPTS_VALUE)) {
                if (pkt->duration == 0) {
                    ff_compute_frame_duration(ic, &num, &den, st, st->parser, pkt);
                    if (den && num) {
                        pkt->duration = av_rescale_rnd(1,
                                           num * (int64_t) st->time_base.den,
                                           den * (int64_t) st->time_base.num,
                                           AV_ROUND_DOWN);
                    }
                }
                duration = pkt->pts + pkt->duration;
                found_duration = 1;
                if (st->start_time != AV_NOPTS_VALUE)
                    duration -= st->start_time;
                else
                    duration -= st->first_dts;
                if (duration > 0) {
                    if (st->duration == AV_NOPTS_VALUE || st->info->last_duration<= 0 ||
                        (st->duration < duration && FFABS(duration - st->info->last_duration) < 60LL*st->time_base.den / st->time_base.num))
                        st->duration = duration;
                    st->info->last_duration = duration;
                }
            }
            av_packet_unref(pkt);
        }

        /* check if all audio/video streams have valid duration */
        if (!is_end) {
            is_end = 1;
            for (i = 0; i < ic->nb_streams; i++) {
                st = ic->streams[i];
                switch (st->codecpar->codec_type) {
                    case AVMEDIA_TYPE_VIDEO:
                    case AVMEDIA_TYPE_AUDIO:
                        if (st->duration == AV_NOPTS_VALUE)
                            is_end = 0;
                }
            }
        }
    } while (!is_end &&
             offset &&
             ++retry <= DURATION_MAX_RETRY);

    av_opt_set(ic, "skip_changes", "0", AV_OPT_SEARCH_CHILDREN);

    /* warn about audio/video streams which duration could not be estimated */
    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];
        if (st->duration == AV_NOPTS_VALUE) {
            switch (st->codecpar->codec_type) {
            case AVMEDIA_TYPE_VIDEO:
            case AVMEDIA_TYPE_AUDIO:
                if (st->start_time != AV_NOPTS_VALUE || st->first_dts  != AV_NOPTS_VALUE) {
                    av_log(ic, AV_LOG_DEBUG, "stream %d : no PTS found at end of file, duration not set\n", i);
                } else
                    av_log(ic, AV_LOG_DEBUG, "stream %d : no TS found at start of file, duration not set\n", i);
            }
        }
    }
    fill_all_stream_timings(ic);
    /* 重新将文件的position置到old_offset位置上 */
    avio_seek(ic->pb, old_offset, SEEK_SET);
    for (i = 0; i < ic->nb_streams; i++) {
        int j;

        st              = ic->streams[i];
        st->cur_dts     = st->first_dts;
        st->last_IP_pts = AV_NOPTS_VALUE;
        st->last_dts_for_order_check = AV_NOPTS_VALUE;
        for (j = 0; j < MAX_REORDER_DELAY + 1; j++)
            st->pts_buffer[j] = AV_NOPTS_VALUE;
    }
}

estimate_timings_from_bit_rate

这个方法是当码率和file size已知的情况下去计算duration
计算公式如下：
duration = file size(字节为单位) * 8 / bit rate(bps为单位)

static void estimate_timings_from_bit_rate(AVFormatContext *ic)
{
    int64_t filesize, duration;
    int i, show_warning = 0;
    AVStream *st;

    /* if bit_rate is already set, we believe it */
    if (ic->bit_rate <= 0) {
        int64_t bit_rate = 0;
        for (i = 0; i < ic->nb_streams; i++) {
            st = ic->streams[i];
            if (st->codecpar->bit_rate <= 0 && st->internal->avctx->bit_rate > 0)
                st->codecpar->bit_rate = st->internal->avctx->bit_rate;
            if (st->codecpar->bit_rate > 0) {
                if (INT64_MAX - st->codecpar->bit_rate < bit_rate) {
                    bit_rate = 0;
                    break;
                }
                bit_rate += st->codecpar->bit_rate;
            } else if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO && st->codec_info_nb_frames > 1) {
                // If we have a videostream with packets but without a bitrate
                // then consider the sum not known
                bit_rate = 0;
                break;
            }
        }
        ic->bit_rate = bit_rate;
    }

    /* if duration is already set, we believe it */
    if (ic->duration == AV_NOPTS_VALUE &&
        ic->bit_rate != 0) {
        filesize = ic->pb ? avio_size(ic->pb) : 0;
        if (filesize > ic->internal->data_offset) {
            filesize -= ic->internal->data_offset;
            for (i = 0; i < ic->nb_streams; i++) {
                st      = ic->streams[i];
                if (   st->time_base.num <= INT64_MAX / ic->bit_rate
                    && st->duration == AV_NOPTS_VALUE) {
                    duration = av_rescale(8 * filesize, st->time_base.den,
                                          ic->bit_rate *
                                          (int64_t) st->time_base.num);
                    st->duration = duration;
                    show_warning = 1;
                }
            }
        }
    }
    if (show_warning)
        av_log(ic, AV_LOG_WARNING,
               "Estimating duration from bitrate, this may be inaccurate\n");
}

update_stream_timings

update_stream_timings方法用来计算统一的的start time、duration.

• start time：比较每个流的start time, 获得最小的，赋值给ic->start_time
• duration：先获取每个节目的中最小的start time, 再获取每个节目的最大的end time, 然后相减，就得到了每个节目的duraion, 再对比每个节目中的duration, 将最大的赋值给ic->duration。
• bitrate：通过file size和 duration计算出

static void update_stream_timings(AVFormatContext *ic)
{
    int64_t start_time, start_time1, start_time_text, end_time, end_time1, end_time_text;
    int64_t duration, duration1, filesize;
    int i;
    AVStream *st;
    AVProgram *p;

    start_time = INT64_MAX;
    start_time_text = INT64_MAX;
    end_time   = INT64_MIN;
    end_time_text   = INT64_MIN;
    duration   = INT64_MIN;
    for (i = 0; i < ic->nb_streams; i++) {
        st = ic->streams[i];
        if (st->start_time != AV_NOPTS_VALUE && st->time_base.den) {
            start_time1 = av_rescale_q(st->start_time, st->time_base,
                                       AV_TIME_BASE_Q);
            if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE || st->codecpar->codec_type == AVMEDIA_TYPE_DATA) {
                if (start_time1 < start_time_text)
                    start_time_text = start_time1;
            } else
                start_time = FFMIN(start_time, start_time1);
            end_time1 = av_rescale_q_rnd(st->duration, st->time_base,
                                         AV_TIME_BASE_Q,
                                         AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
            if (end_time1 != AV_NOPTS_VALUE && (end_time1 > 0 ? start_time1 <= INT64_MAX - end_time1 : start_time1 >= INT64_MIN - end_time1)) {
                end_time1 += start_time1;
                if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE || st->codecpar->codec_type == AVMEDIA_TYPE_DATA)
                    end_time_text = FFMAX(end_time_text, end_time1);
                else
                    end_time = FFMAX(end_time, end_time1);
            }
            for (p = NULL; (p = av_find_program_from_stream(ic, p, i)); ) {
                if (p->start_time == AV_NOPTS_VALUE || p->start_time > start_time1)
                    p->start_time = start_time1;
                if (p->end_time < end_time1)
                    p->end_time = end_time1;
            }
        }
        if (st->duration != AV_NOPTS_VALUE) {
            duration1 = av_rescale_q(st->duration, st->time_base,
                                     AV_TIME_BASE_Q);
            duration  = FFMAX(duration, duration1);
        }
    }
    if (start_time == INT64_MAX || (start_time > start_time_text && start_time - start_time_text < AV_TIME_BASE))
        start_time = start_time_text;
    else if (start_time > start_time_text)
        av_log(ic, AV_LOG_VERBOSE, "Ignoring outlier non primary stream starttime %f\n", start_time_text / (float)AV_TIME_BASE);

    if (end_time == INT64_MIN || (end_time < end_time_text && end_time_text - end_time < AV_TIME_BASE)) {
        end_time = end_time_text;
    } else if (end_time < end_time_text) {
        av_log(ic, AV_LOG_VERBOSE, "Ignoring outlier non primary stream endtime %f\n", end_time_text / (float)AV_TIME_BASE);
    }

    if (start_time != INT64_MAX) {
        ic->start_time = start_time;
        if (end_time != INT64_MIN) {
            if (ic->nb_programs > 1) {
                for (i = 0; i < ic->nb_programs; i++) {
                    p = ic->programs[i];
                    if (p->start_time != AV_NOPTS_VALUE && p->end_time > p->start_time)
                        duration = FFMAX(duration, p->end_time - p->start_time);
                }
            } else
                duration = FFMAX(duration, end_time - start_time);
        }
    }
    if (duration != INT64_MIN && duration > 0 && ic->duration == AV_NOPTS_VALUE) {
        ic->duration = duration;
    }
    if (ic->pb && (filesize = avio_size(ic->pb)) > 0 && ic->duration > 0) {
        /* compute the bitrate */
        double bitrate = (double) filesize * 8.0 * AV_TIME_BASE /
                         (double) ic->duration;
        if (bitrate >= 0 && bitrate <= INT64_MAX)
            ic->bit_rate = bitrate;
    }
}