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 }
ffmpeg 中av_rescale_rnd 的含义,码迷,mamicode.com
时间： 2024-10-07 09:20:48