ffmpeg 接收rtsp时的pts dts设置

发表于 | 更新于 | 阅读次数：
本文字数： 6.1k | 阅读时长 ≈ 11 分钟

设置pts

Pts的设置是根据timestamp和rtcp的ntp time设置的，在函数finalize_packet()内设置。跟ntp time挂钩是音视频同步的基础。
基本上可以认为timestamp的变化值就是pts的变化值，在没有B frame 时25fps 就是3600递增， 29.97fps 是3003 递增。存在B frame 时 timestamp 值 不是线性增加。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
/**
 * This was the second switch in rtp_parse packet.
 * Normalizes time, if required, sets stream_index, etc.
 */
 
static void finalize_packet(RTPDemuxContext *s, AVPacket *pkt, uint32_t timestamp)
{
    if (pkt->pts != AV_NOPTS_VALUE || pkt->dts != AV_NOPTS_VALUE)
        return; /* Timestamp already set by depacketizer */
    if (timestamp == RTP_NOTS_VALUE)
        return;
    if (s->last_rtcp_ntp_time != AV_NOPTS_VALUE && s->ic->nb_streams > 1) {
        int64_t addend;
        int delta_timestamp;
        /* compute pts from timestamp with received ntp_time */
        delta_timestamp = timestamp - s->last_rtcp_timestamp;
        /* convert to the PTS timebase */
        addend = av_rescale(s->last_rtcp_ntp_time - s->first_rtcp_ntp_time,
                            s->st->time_base.den,
                            (uint64_t) s->st->time_base.num << 32);
        pkt->pts = s->range_start_offset + s->rtcp_ts_offset + addend +
                   delta_timestamp;
        return;
    }

    if (!s->base_timestamp)
        s->base_timestamp = timestamp;
    /* assume that the difference is INT32_MIN < x < INT32_MAX,
     * but allow the first timestamp to exceed INT32_MAX */
    if (!s->timestamp)
        s->unwrapped_timestamp += timestamp;
    else
        s->unwrapped_timestamp += (int32_t)(timestamp - s->timestamp);
    s->timestamp = timestamp;
    pkt->pts     = s->unwrapped_timestamp + s->range_start_offset -
                   s->base_timestamp;
 
}

其中的s->st->time_base.den = 90000 / 44100 等， s->st->time_base.num = 1。

设置dts

Dts的值是从pts的值里面选择的，在没有B帧时设置成和pts一样；

有B frame存在时，根据B frame num ， 缓存相应B frame num个数加1的pts， 缓存的pts按从小到大排列。然后从数组0位置取出来当dts。

1
2
3
4
5
6
7
8
9
10
11
12
13
delay = st->internal->avctx->has_b_frames;

......

if (pkt->pts != AV_NOPTS_VALUE && delay <= MAX_REORDER_DELAY) {
        st->pts_buffer[0] = pkt->pts;

        for (i = 0; i<delay && st->pts_buffer[i] > st->pts_buffer[i + 1]; i++)
            FFSWAP(int64_t, st->pts_buffer[i], st->pts_buffer[i + 1]);

        if(has_decode_delay_been_guessed(st))
            pkt->dts = select_from_pts_buffer(st, st->pts_buffer, pkt->dts);
    }