ffmpeg 中av_rescale_rnd 的含义

http://blog.csdn.net/fireroll/article/details/8485482

一、函数声明: 

int64_t av_rescale_rnd(int64_t a, int64_t b, int64_t c, enum AVRounding rnd); 

直接看代码, 它的作用是计算 "a * b / c" 的值并分五种方式来取整.

用在FFmpeg中,

则是将以 "时钟基c" 表示的 数值a 转换成以 "时钟基b" 来表示。

一共有5种方式:

 

enum AVRounding {
    AV_ROUND_ZERO     = 0, ///< Round toward zero.          趋近于0  
    AV_ROUND_INF      = 1, ///< Round away from zero.       趋远于0  
    AV_ROUND_DOWN     = 2, ///< Round toward -infinity.     趋于更小的整数  
    AV_ROUND_UP       = 3, ///< Round toward +infinity.     趋于更大的整数  
    AV_ROUND_NEAR_INF = 5, ///< Round to nearest and halfway cases away from zero.     四舍五入,小于0.5取值趋向0,大于0.5取值趋远于0  
    AV_ROUND_PASS_MINMAX = 8192, ///< Flag to pass INT64_MIN/MAX through instead of rescaling, this avoids special cases for AV_NOPTS_VALUE
} 

二、函数定义(见于libavutil/mathematics.c):

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--){
//            int o= a1 & 0x8000000000000000ULL;
            a1+= a1 + ((a0>>i)&1);
            t1+=t1;
            if(/*o || */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
}

三、实例分析
将以"1MHz时钟基" 表示的 "PTS/DTS值a" 转换成以 "90kHz时钟基" 表示。

av_rescale_q(a=-10949117256, 
             bq={num=1, den=1000000}, 
             cq={num=1, den=90000))
{
  int64_t b= bq.num * (int64_t)cq.den; // = 1 * 90000   = 90000;
  int64_t c= cq.num * (int64_t)bq.den; // = 1 * 1000000 = 1000000
  return av_rescale_rnd(a, b, c, 5);                   
}


av_rescale_rnd(a=10949117256, b=90000, c=1000000, rnd=5)
{
  if (rnd==5) 
    r = c / 2;                          // r =500000;
  
  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;    // =  10949117256 / 1000000 * 90000 + 
                                       //   (10949117256 % 1000000 * 90000 + 500000) / 1000000
                                       // =  985420553
  }
  else
  {
    ...
  }  
}