ffmpeg时间戳相关函数

1、概述

ffmpeg中与时间戳相关的函数主要有：av_rescale_q，av_rescale_q_rnd，av_compare_ts。

av_rescale_q

int64_t av_rescale_q(int64_t a, AVRational bq, AVRational cq)
{
    return av_rescale_q_rnd(a, bq, cq, AV_ROUND_NEAR_INF);
}

av_rescale_q_rnd：

int64_t av_rescale_q_rnd(int64_t a, AVRational bq, AVRational cq,
                         enum AVRounding rnd)
{
    int64_t b = bq.num * (int64_t)cq.den;
    int64_t c = cq.num * (int64_t)bq.den;
    return av_rescale_rnd(a, b, c, rnd);
}

av_rescale_rnd：

int64_t av_rescale_rnd(int64_t a, int64_t b, int64_t c, enum AVRounding rnd)
{
    int64_t r = 0;
    av_assert2(c > 0);
    av_assert2(b >=0);
    av_assert2((unsigned)(rnd&~AV_ROUND_PASS_MINMAX)<=5 && (rnd&~AV_ROUND_PASS_MINMAX)!=4);
 
    if (c <= 0 || b < 0 || !((unsigned)(rnd&~AV_ROUND_PASS_MINMAX)<=5 && (rnd&~AV_ROUND_PASS_MINMAX)!=4))
        return INT64_MIN;
 
    if (rnd & AV_ROUND_PASS_MINMAX) {
        if (a == INT64_MIN || a == INT64_MAX)
            return a;
        rnd -= AV_ROUND_PASS_MINMAX;
    }
 
    if (a < 0 && a != INT64_MIN)
        return -av_rescale_rnd(-a, b, c, rnd ^ ((rnd >> 1) & 1));
 
    if (rnd == AV_ROUND_NEAR_INF)
        r = c / 2;
    else if (rnd & 1)
        r = c - 1;
 
    if (b <= INT_MAX && c <= INT_MAX) {
        if (a <= INT_MAX)
            return (a * b + r) / c;
        else
            return a / c * b + (a % c * b + r) / c;
    } else {
#if 1
        uint64_t a0  = a & 0xFFFFFFFF;
        uint64_t a1  = a >> 32;
        uint64_t b0  = b & 0xFFFFFFFF;
        uint64_t b1  = b >> 32;
        uint64_t t1  = a0 * b1 + a1 * b0;
        uint64_t t1a = t1 << 32;
        int i;
 
        a0  = a0 * b0 + t1a;
        a1  = a1 * b1 + (t1 >> 32) + (a0 < t1a);
        a0 += r;
        a1 += a0 < r;
 
        for (i = 63; i >= 0; i--) {
            a1 += a1 + ((a0 >> i) & 1);
            t1 += t1;
            if (c <= a1) {
                a1 -= c;
                t1++;
            }
        }
        return t1;
    }
#else
        AVInteger ai;
        ai = av_mul_i(av_int2i(a), av_int2i(b));
        ai = av_add_i(ai, av_int2i(r));
 
        return av_i2int(av_div_i(ai, av_int2i(c)));
    }
#endif
}

函数av_rescale_rnd其实就是返回(a * b)/c，只是加了一些舍入方案。

那么整个结构就清晰了，当我们调用av_rescale_q(pts, timebase1, timebase2)或者av_rescale_q_rnd(pts, timebase1, timebase2, XX)时
其实就是按照下面的公式计算了一下， 公式如下：
x = pts * (timebase1.num / timebase1.den )* (timebase2.den / timebase2.num);
这个x就是转换后的时间戳。

明白了时间基转换对理解av_compare_ts很有帮助，下面来看看av_compare_ts的代码：

int av_compare_ts(int64_t ts_a, AVRational tb_a, int64_t ts_b, AVRational tb_b)
{
    int64_t a = tb_a.num * (int64_t)tb_b.den;
    int64_t b = tb_b.num * (int64_t)tb_a.den;
    if ((FFABS(ts_a)|a|FFABS(ts_b)|b) <= INT_MAX)
        return (ts_a*a > ts_b*b) - (ts_a*a < ts_b*b);
    if (av_rescale_rnd(ts_a, a, b, AV_ROUND_DOWN) < ts_b)
        return -1;
    if (av_rescale_rnd(ts_b, b, a, AV_ROUND_DOWN) < ts_a)
        return 1;
    return 0;
}

从代码中可以了总结出一句伪代码（假设2个时间基相等）：

return ts_a == ts_b ? 0 : ts_a < ts_b ? -1 : 1