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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
标签：style blog http java color art
原文地址：http://www.cnblogs.com/youngt/p/3698886.html
