ffmpeg 中av_rescale_rnd 的含义

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:

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

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;
  assert(c > 0);
  assert(b >=0);
  assert((unsigned)rnd<=5 && rnd!=4);


  /* 将小于0的整数,转换成大于0的整来计算 */
  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)
  {
    /* 处理小于32位整数的情况 */
    if (a<=INT_MAX)
      return (a * b + r)/c;
    else
      return a/c*b + (a%c*b + r)/c;
  }
  else
  {
    /* 64位整数的算法 */
    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;
} 

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
  {
    ...
  }  
}