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):

 1 int64_t av_rescale_rnd(int64_t a, int64_t b, int64_t c, enum AVRounding rnd){

 2     int64_t r=0;

 3     av_assert2(c > 0);

 4     av_assert2(b >=0);

 5     av_assert2((unsigned)(rnd&~AV_ROUND_PASS_MINMAX)<=5 && (rnd&~AV_ROUND_PASS_MINMAX)!=4);

 6 

 7     if (c <= 0 || b < 0 || !((unsigned)(rnd&~AV_ROUND_PASS_MINMAX)<=5 && (rnd&~AV_ROUND_PASS_MINMAX)!=4))

 8         return INT64_MIN;

 9 

10     if (rnd & AV_ROUND_PASS_MINMAX) {

11         if (a == INT64_MIN || a == INT64_MAX)

12             return a;

13         rnd -= AV_ROUND_PASS_MINMAX;

14     }

15 

16     if(a<0 && a != INT64_MIN) return -av_rescale_rnd(-a, b, c, rnd ^ ((rnd>>1)&1));

17 

18     if(rnd==AV_ROUND_NEAR_INF) r= c/2;

19     else if(rnd&1)             r= c-1;

20 

21     if(b<=INT_MAX && c<=INT_MAX){

22         if(a<=INT_MAX)

23             return (a * b + r)/c;

24         else

25             return a/c*b + (a%c*b + r)/c;

26     }else{

27 #if 1

28         uint64_t a0= a&0xFFFFFFFF;

29         uint64_t a1= a>>32;

30         uint64_t b0= b&0xFFFFFFFF;

31         uint64_t b1= b>>32;

32         uint64_t t1= a0*b1 + a1*b0;

33         uint64_t t1a= t1<<32;

34         int i;

35 

36         a0 = a0*b0 + t1a;

37         a1 = a1*b1 + (t1>>32) + (a0<t1a);

38         a0 += r;

39         a1 += a0<r;

40 

41         for(i=63; i>=0; i--){

42 //            int o= a1 & 0x8000000000000000ULL;

43             a1+= a1 + ((a0>>i)&1);

44             t1+=t1;

45             if(/*o || */c <= a1){

46                 a1 -= c;

47                 t1++;

48             }

49         }

50         return t1;

51     }

52 #else

53         AVInteger ai;

54         ai= av_mul_i(av_int2i(a), av_int2i(b));

55         ai= av_add_i(ai, av_int2i(r));

56 

57         return av_i2int(av_div_i(ai, av_int2i(c)));

58     }

59 #endif

60 }

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。

 1 av_rescale_q(a=-10949117256, 

 2              bq={num=1, den=1000000}, 

 3              cq={num=1, den=90000))

 4 {

 5   int64_t b= bq.num * (int64_t)cq.den; // = 1 * 90000   = 90000;

 6   int64_t c= cq.num * (int64_t)bq.den; // = 1 * 1000000 = 1000000

 7   return av_rescale_rnd(a, b, c, 5);                   

 8 }

 9 

10 

11 av_rescale_rnd(a=10949117256, b=90000, c=1000000, rnd=5)

12 {

13   if (rnd==5) 

14     r = c / 2;                          // r =500000;

15   

16   if (b<=INT_MAX && c<=INT_MAX)

17   {

18     if (a<=INT_MAX)

19       return (a * b + r)/c;

20     else

21       return a/c*b + (a%c*b + r)/c;    // =  10949117256 / 1000000 * 90000 + 

22                                        //   (10949117256 % 1000000 * 90000 + 500000) / 1000000

23                                        // =  985420553

24   }

25   else

26   {

27     ...

28   }  

29 }