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:

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=;
     av_assert2(c > );
     av_assert2(b >=);
     av_assert2((unsigned)(rnd&~AV_ROUND_PASS_MINMAX)<= && (rnd&~AV_ROUND_PASS_MINMAX)!=);

     if (c <=  || b <  || !((unsigned)(rnd&~AV_ROUND_PASS_MINMAX)<= && (rnd&~AV_ROUND_PASS_MINMAX)!=))
         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< && a != INT64_MIN) return -av_rescale_rnd(-a, b, c, rnd ^ ((rnd>>)&));

     if(rnd==AV_ROUND_NEAR_INF) r= c/;
     else if(rnd&)             r= c-;

     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>>;
         uint64_t b0= b&0xFFFFFFFF;
         uint64_t b1= b>>;
         uint64_t t1= a0*b1 + a1*b0;
         uint64_t t1a= t1<<;
         int i;

         a0 = a0*b0 + t1a;
         a1 = a1*b1 + (t1>>) + (a0<t1a);
         a0 += r;
         a1 += a0<r;

         for(i=; i>=; i--){
 //            int o= a1 & 0x8000000000000000ULL;
             a1+= a1 + ((a0>>i)&);
             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=-,
              bq={num=, den=},
              cq={num=, den=))
 {
   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, );
 }

 av_rescale_rnd(a=, b=, c=, rnd=)
 {
   if (rnd==)
     r = c / ;                          // 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
   {
     ...
   }
 }