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libjpeg库分析----4.rgb转yuv

分类： LINUX

2016-07-04 18:58:01

1. 原理
a.RGB转YUV的公式
  1. Y = 0.299*R + 0.587*G + 0.114*B
  2. U = -0.1687*R - 0.3313*G + 0.5*B + 128
  3. V = 0.5*R - 0.418*G - 0.0813*B + 128
b.说明
对于Y分量，每一个像素代入上面的公式处理即可
对于UV分量，四个像素共用一组UV分量，所以UV分量的计算需要取这四个像素的平均值
  1. (R0,G0,B0) (R1,G1,B1)    -->第1行
  2. (R2,G2,B2) (R3,G3,B3)    -->第2行

  4. Y0 Y1         -->Y0 Y1
  5. Y2 Y3         -->Y2 Y3
  6. U             -->(U0+U1+U2+U3)/4
  7. V             -->(V0+V1+V2+V3)/4
所以 YUV的大小=RGB/2
2.bmp转yuv的代码
  1. #include <stdio.h>
  2. #include <stdlib.h>
  3. #include <string.h>
  4. #include <math.h>
  5. #define dbmsg(fmt, args ...) printf("%s:%s[%d]: "fmt"\n", __FILE__,__FUNCTION__, __LINE__,##args)
  6. #pragma pack(1)
  7. typedef struct __BITMAPFILEHEADER__
  8. {
  9.     u_int16_t bfType;
  10.     u_int32_t bfSize;
  11.     u_int16_t bfReserved1;
  12.     u_int16_t bfReserved2;
  13.     u_int32_t bfOffBits;
  14. }BITMAPFILEHEADER;

  16. typedef struct __BITMAPINFOHEADER
  17. {
  18.     u_int32_t biSize;
  19.     u_int32_t biWidth;
  20.     u_int32_t biHeight;
  21.     u_int16_t biPlanes;
  22.     u_int16_t biBitCount;
  23.     u_int32_t biCompression;
  24.     u_int32_t biSizeImage;
  25.     u_int32_t biXPelsPerMeter;
  26.     u_int32_t biYPelsPerMeter;
  27.     u_int32_t biClrUsed;
  28.     u_int32_t biClrImportant;
  29. }BITMAPINFOHEADER;

  31. int32_t rgb_seq_change(unsigned char* rgb, int w, int h, int bpp)
  32. {
  33.     int i;
  34.     unsigned char t;
  35.     unsigned char* pt = NULL;
  36.     pt = (unsigned char*)malloc(w*bpp);
  37.     
  38.     //change pixel form <N--1> to <1--N>
  39.     for(i=0; i<h/2; i++)
  40.     {
  41.         memcpy(pt, &rgb[i*w*bpp], w*bpp);
  42.         memcpy(&rgb[i*w*bpp], &rgb[(h-1-i)*w*bpp], w*bpp);
  43.         memcpy(&rgb[(h-1-i)*w*bpp], pt, w*bpp);
  44.     }
  45.     free(pt);
  46.     //gbr2rgb
  47.     for(i=0; i<h*w*3; i+=3)
  48.     {
  49.         t = rgb[i];
  50.         rgb[i] = rgb[i+2];
  51.         rgb[i+2] = t;
  52.     }
  53.     return 0;
  54. }

  56. #define TY(r,g,b) (( r* 0.2989 + g* 0.5866 + b* 0.1145))
  57. #define TU(r,g,b) (( r*(-0.1688) + g*(-0.3312) + b*0.5000 + 128))
  58. #define TV(r,g,b) (( r* 0.5000 + g*(-0.4184) + b*(-0.0816) + 128))

  60. #define Y(r,g,b) (TY(r,g,b) > 255 ? 255 : (TY(r,g,b) < 0 ? 0 : TY(r,g,b)))
  61. #define U(r,g,b) (TU(r,g,b) > 255 ? 255 : (TU(r,g,b) < 0 ? 0 : TU(r,g,b)))
  62. #define V(r,g,b) (TV(r,g,b) > 255 ? 255 : (TV(r,g,b) < 0 ? 0 : TV(r,g,b)))
  63. int32_t rgb_yuv_convert (unsigned char* yuv, unsigned char* rgb, int w, int h, int bpp)
  64. {
  65.     int i, j, k=0;
  66.     int r, g, b;
  67.     int r0, g0, b0;
  68.     int r1, g1, b1;
  69.     int r2, g2, b2;
  70.     int r3, g3, b3;
  71.     unsigned char *pY = NULL;
  72.     unsigned char *pU = NULL;
  73.     unsigned char *pV = NULL;
  74.     
  75.     pY = yuv;
  76.     pU = yuv + w*h;
  77.     pV = pU + ((w*h)>>2);
  78.     for(i=0; i<h; i++)
  79.     for(j=0; j<w; j++)
  80.     {
  81.         r = rgb[(i*w+j)*bpp];
  82.         g = rgb[(i*w+j)*bpp+1];
  83.         b = rgb[(i*w+j)*bpp+2];

  85.         pY[i*w+j] = Y(r, g, b);
  86.        // dbmsg("i=%d,j=%d", i,j);
  87.         if(i%2==1 && j%2==1)
  88.         {
  89.             //left up
  90.             r0 = rgb[((i-1)*w+j-1)*bpp];
  91.             g0 = rgb[((i-1)*w+j-1)*bpp+1];
  92.             b0 = rgb[((i-1)*w+j-1)*bpp+2];
  93.             //up
  94.             r1 = rgb[((i-1)*w+j)*bpp];
  95.             g1 = rgb[((i-1)*w+j)*bpp+1];
  96.             b1 = rgb[((i-1)*w+j)*bpp+2];

  98.             //right
  99.             r2 = rgb[(i*w+j-1)*bpp];
  100.             g2 = rgb[(i*w+j-1)*bpp+1];
  101.             b2 = rgb[(i*w+j-1)*bpp+2];

  103.             //now
  104.             r3 = r;
  105.             g3 = g;
  106.             b3 = b;

  108.             pU[k] = (U(r0,g0,b0)+ U(r1,g1,b1) + U(r2,g2,b2) + U(r3,g3,b3)) /4;
  109.             pV[k] = (V(r0,g0,b0)+ V(r1,g1,b1) + V(r2,g2,b2) + V(r3,g3,b3)) /4;
  110.             k++;
  111.             //printf("k=%d\n",k );
  112.         }
  113.     }
  114.     return 0;
  115. }

  117. int translate_bmp2yuv(FILE* out_yuv_fp, FILE* bmp_fp)
  118. {
  119.     int i, len;
  120.     int w, h, bpp; //bpp-->byte_per_pix
  121.     char* p_temp = NULL;
  122.     unsigned char* rgb = NULL;
  123.     unsigned char* yuv = NULL;

  125.     BITMAPFILEHEADER bmphead;
  126.     BITMAPINFOHEADER infohead;

  128.     if( (NULL==out_yuv_fp) || (NULL==bmp_fp))
  129.         return -1;
  130.     len = sizeof(bmphead)+sizeof(infohead);
  131.     printf("len=%d\n",len);
  132.     p_temp = (char*)malloc(len*sizeof(char));
  133.     if(p_temp == NULL)
  134.     {
  135.         printf("malloc error\n");
  136.         return -1;
  137.     }
  138.     fread(p_temp, len, 1, bmp_fp);
  139.     memcpy(&bmphead, p_temp, sizeof(bmphead));
  140.     memcpy(&infohead, p_temp+sizeof(bmphead), sizeof(infohead));
  141.     if(bmphead.bfType != 0x4D42) //must be 0x4D42='BM'
  142.     {
  143.         printf("not a bmp file\n");
  144.         return -1;
  145.     }
  146.     if(infohead.biBitCount < 24)
  147.     {
  148.         printf("error: now bitCount=%d < 24bit\n", infohead.biBitCount);
  149.         return -1;
  150.     }
  151.     free(p_temp);
  152.     w = infohead.biWidth;
  153.     h = infohead.biHeight;
  154.     bpp = infohead.biBitCount/8;
  155.     printf("w=%d,h=%d,bitcount=%d\n", w, h, bpp);

  157.     yuv = (char*) malloc(w*h*bpp/2);
  158.     if(yuv== NULL)
  159.     {
  160.         printf("malloc yuv buffer error\n");
  161.         return -1;
  162.     }

  164.     rgb = (char*)malloc(w*h*bpp);
  165.     if(rgb== NULL)
  166.     {
  167.         printf("malloc rgb buffer error\n");
  168.         return -1;
  169.     }
  170.     fread(rgb, w*h*bpp, 1, bmp_fp);
  171.     rgb_seq_change(rgb, w, h, bpp);
  172.     for(i=0; i<100; i++)                        //这儿主要是为了测试，真正用时千万别把这个搞上去了
  173.        rgb_yuv_convert(yuv, rgb, w, h, bpp);
  174.     fwrite(yuv, w*h*bpp/2, 1, out_yuv_fp);
  175.     free(rgb);
  176.     free(yuv);
  177.     return 0;
  178. }

  180. int main ( int argc, char *argv[] )
  181. {
  182.     char bmp_file[] = "./640_480.bmp";
  183.     char raw_file[] = "./640_480.yuv";
  184.     FILE * out_yuv_fp = NULL;
  185.     FILE * in_bmp_fp = NULL;

  187.     if (NULL == (in_bmp_fp = fopen(bmp_file,"rb")))
  188.     {
  189.         printf("error: %s not found\n",bmp_file);
  190.         return -1;
  191.     }

  193.     if (NULL == (out_yuv_fp = fopen(raw_file,"wb")))
  194.     {
  195.         printf("error: %s not found\n",raw_file);
  196.         return -1;
  197.     }
  198.     
  199.     translate_bmp2yuv(out_yuv_fp, in_bmp_fp);

  201.     fclose(out_yuv_fp);
  202.     fclose(in_bmp_fp);
  203.     printf("over\n");
  204.     return EXIT_SUCCESS;
  205. }


2.查表法
  1. #include <stdio.h>
  2. #include <stdlib.h>
  3. #include <string.h>
  4. #include <math.h>
  5. #define dbmsg(fmt, args ...) printf("%s:%s[%d]: "fmt"\n", __FILE__,__FUNCTION__, __LINE__,##args)
  6. #pragma pack(1)
  7. typedef struct __BITMAPFILEHEADER__
  8. {
  9.     u_int16_t bfType;
  10.     u_int32_t bfSize;
  11.     u_int16_t bfReserved1;
  12.     u_int16_t bfReserved2;
  13.     u_int32_t bfOffBits;
  14. }BITMAPFILEHEADER;

  16. typedef struct __BITMAPINFOHEADER
  17. {
  18.     u_int32_t biSize;
  19.     u_int32_t biWidth;
  20.     u_int32_t biHeight;
  21.     u_int16_t biPlanes;
  22.     u_int16_t biBitCount;
  23.     u_int32_t biCompression;
  24.     u_int32_t biSizeImage;
  25.     u_int32_t biXPelsPerMeter;
  26.     u_int32_t biYPelsPerMeter;
  27.     u_int32_t biClrUsed;
  28.     u_int32_t biClrImportant;
  29. }BITMAPINFOHEADER;

  31. int32_t rgb_seq_change(unsigned char* rgb, int w, int h, int bpp)
  32. {
  33.     int i;
  34.     unsigned char t;
  35.     unsigned char* pt = NULL;
  36.     pt = (unsigned char*)malloc(w*bpp);
  37.     
  38.     //change pixel form <N--1> to <1--N>
  39.     for(i=0; i<h/2; i++)
  40.     {
  41.         memcpy(pt, &rgb[i*w*bpp], w*bpp);
  42.         memcpy(&rgb[i*w*bpp], &rgb[(h-1-i)*w*bpp], w*bpp);
  43.         memcpy(&rgb[(h-1-i)*w*bpp], pt, w*bpp);
  44.     }
  45.     free(pt);
  46.     //gbr2rgb
  47.     for(i=0; i<h*w*3; i+=3)
  48.     {
  49.         t = rgb[i];
  50.         rgb[i] = rgb[i+2];
  51.         rgb[i+2] = t;
  52.     }
  53.     return 0;
  54. }
  55. #define Y(a) ((a)> 255 ? 255 : (a) < 0 ? 0 : (a))
  56. #define U(a) ((a) > 255 ? 255 : (a) < 0 ? 0 : (a))
  57. #define V(a) ((a) > 255 ? 255 : (a) < 0 ? 0 : (a))

  59. #define RGB_RED 0 /* Offset of Red in an RGB scanline element */
  60. #define RGB_GREEN 1 /* Offset of Green */
  61. #define RGB_BLUE 2 /* Offset of Blue */
  62. #define GETJSAMPLE(value) ((int) (value) & 0xFF)
  63. #define MAXJSAMPLE 255
  64. #define CENTERJSAMPLE 128
  65. #define SCALEBITS    16    /* speediest right-shift on some machines */
  66. #define FIX(x) ((int32_t) ((x) * (1L<<SCALEBITS) + 0.5))
  67. #define CBCR_OFFSET    ((int32_t) CENTERJSAMPLE << SCALEBITS)
  68. #define ONE_HALF    ((int32_t) 1 << (SCALEBITS-1))
  69. #define FIX(x)        ((int32_t) ((x) * (1L<<SCALEBITS) + 0.5))

  71. /* We allocate one big table and divide it up into eight parts, instead of
  72.  * doing eight alloc_small requests. This lets us use a single table base
  73.  * address, which can be held in a register in the inner loops on many
  74.  * machines (more than can hold all eight addresses, anyway).
  75.  */

  77. #define R_Y_OFF        0            /* offset to R => Y section */
  78. #define G_Y_OFF        (1*(MAXJSAMPLE+1))    /* offset to G => Y section */
  79. #define B_Y_OFF        (2*(MAXJSAMPLE+1))    /* etc. */
  80. #define R_CB_OFF    (3*(MAXJSAMPLE+1))
  81. #define G_CB_OFF    (4*(MAXJSAMPLE+1))
  82. #define B_CB_OFF    (5*(MAXJSAMPLE+1))
  83. #define R_CR_OFF    B_CB_OFF        /* B=>Cb, R=>Cr are the same */
  84. #define G_CR_OFF    (6*(MAXJSAMPLE+1))
  85. #define B_CR_OFF    (7*(MAXJSAMPLE+1))
  86. #define TABLE_SIZE    (8*(MAXJSAMPLE+1))


  89. /* * Initialize for RGB->YCC colorspace conversion. */
  90. int32_t* rgb_yuv_start()
  91. {
  92.     int32_t * rgb_yuv_tab;
  93.     int32_t i;
  94.     dbmsg("rgb_yuv_start");
  95.     rgb_yuv_tab = (int32_t*) malloc(TABLE_SIZE * sizeof(int32_t));
  96.     if(NULL==rgb_yuv_tab)
  97.         return NULL;

  99.     for (i = 0; i <= MAXJSAMPLE; i++) {
  100.         rgb_yuv_tab[i+R_Y_OFF] = FIX(0.29900) * i;
  101.         rgb_yuv_tab[i+G_Y_OFF] = FIX(0.58700) * i;
  102.         rgb_yuv_tab[i+B_Y_OFF] = FIX(0.11400) * i + ONE_HALF;
  103.         rgb_yuv_tab[i+R_CB_OFF] = (-FIX(0.16874)) * i;
  104.         rgb_yuv_tab[i+G_CB_OFF] = (-FIX(0.33126)) * i;
  105.         rgb_yuv_tab[i+B_CB_OFF] = FIX(0.50000) * i + CBCR_OFFSET + ONE_HALF-1;
  106.         rgb_yuv_tab[i+G_CR_OFF] = (-FIX(0.41869)) * i;
  107.         rgb_yuv_tab[i+B_CR_OFF] = (-FIX(0.08131)) * i;
  108.     }
  109.     return rgb_yuv_tab;
  110. }

  112. void rgb_yuv_convert(char* yuv, char* rgb, int w, int h,int bpp, int32_t* ctab)
  113. {
  114.     int r, g, b;
  115.     int r0, g0, b0;
  116.     int r1, g1, b1;
  117.     int r2, g2, b2;
  118.     int r3, g3, b3;
  119.     unsigned char *pY = NULL;
  120.     unsigned char *pU = NULL;
  121.     unsigned char *pV = NULL;
  122.     unsigned int i, j, k=0;
  123.     pY = yuv;
  124.     pU = yuv + w*h;
  125.     pV = pU + ((w*h)>>2);

  127.     for(i=0; i<h; i++)
  128.     for(j=0; j<w; j++)
  129.     {
  130.         r = GETJSAMPLE(rgb[(i*w+j)*bpp+RGB_RED]);
  131.         g = GETJSAMPLE(rgb[(i*w+j)*bpp+RGB_GREEN]);
  132.         b = GETJSAMPLE(rgb[(i*w+j)*bpp+RGB_BLUE]);
  133.         pY[i*w+j] = Y((ctab[r+R_Y_OFF] + ctab[g+G_Y_OFF] + ctab[b+B_Y_OFF]) >> SCALEBITS);
  134.         if(i%2==1 && j%2==1)
  135.         {
  136.             //left up
  137.             r0 = GETJSAMPLE(rgb[((i-1)*w+j-1)*bpp]);
  138.             g0 = GETJSAMPLE(rgb[((i-1)*w+j-1)*bpp+1]);
  139.             b0 = GETJSAMPLE(rgb[((i-1)*w+j-1)*bpp+2]);
  140.             //up
  141.             r1 = GETJSAMPLE(rgb[((i-1)*w+j)*bpp]);
  142.             g1 = GETJSAMPLE(rgb[((i-1)*w+j)*bpp+1]);
  143.             b1 = GETJSAMPLE(rgb[((i-1)*w+j)*bpp+2]);

  145.             //right
  146.             r2 = GETJSAMPLE(rgb[(i*w+j-1)*bpp]);
  147.             g2 = GETJSAMPLE(rgb[(i*w+j-1)*bpp+1]);
  148.             b2 = GETJSAMPLE(rgb[(i*w+j-1)*bpp+2]);

  150.             r3 = r;
  151.             g3 = g;
  152.             b3 = b;
  153.             pU[k] = (
  154.                     U((ctab[r0+R_CB_OFF] + ctab[g0+G_CB_OFF] + ctab[b0+B_CB_OFF]) >> SCALEBITS) +
  155.                     U((ctab[r1+R_CB_OFF] + ctab[g1+G_CB_OFF] + ctab[b1+B_CB_OFF]) >> SCALEBITS) +
  156.                     U((ctab[r2+R_CB_OFF] + ctab[g2+G_CB_OFF] + ctab[b2+B_CB_OFF]) >> SCALEBITS) +
  157.                     U((ctab[r3+R_CB_OFF] + ctab[g3+G_CB_OFF] + ctab[b3+B_CB_OFF]) >> SCALEBITS)
  158.                     )/4;
  159.                 
  160.             pV[k] = (
  161.                     V((ctab[r0+R_CR_OFF] + ctab[g0+G_CR_OFF] + ctab[b0+B_CR_OFF]) >> SCALEBITS) +
  162.                     V((ctab[r1+R_CR_OFF] + ctab[g1+G_CR_OFF] + ctab[b1+B_CR_OFF]) >> SCALEBITS) +
  163.                     V((ctab[r2+R_CR_OFF] + ctab[g2+G_CR_OFF] + ctab[b2+B_CR_OFF]) >> SCALEBITS) +
  164.                     V((ctab[r3+R_CR_OFF] + ctab[g3+G_CR_OFF] + ctab[b3+B_CR_OFF]) >> SCALEBITS)
  165.                     )/4;
  166. #if 0
  167.             dbmsg("(%d+%d+%d+%d)/4=%d",
  168.                     U((ctab[r0+R_CB_OFF] + ctab[g0+G_CB_OFF] + ctab[b0+B_CB_OFF]) >> SCALEBITS) ,
  169.                     U((ctab[r1+R_CB_OFF] + ctab[g1+G_CB_OFF] + ctab[b1+B_CB_OFF]) >> SCALEBITS) ,
  170.                     U((ctab[r2+R_CB_OFF] + ctab[g2+G_CB_OFF] + ctab[b2+B_CB_OFF]) >> SCALEBITS) ,
  171.                     U((ctab[r3+R_CB_OFF] + ctab[g3+G_CB_OFF] + ctab[b3+B_CB_OFF]) >> SCALEBITS), pU[k]
  172.                  );
  173.             dbmsg("(%d+%d+%d+%d)/4=%d",
  174.                     V((ctab[r0+R_CR_OFF] + ctab[g0+G_CR_OFF] + ctab[b0+B_CR_OFF]) >> SCALEBITS) ,
  175.                     V((ctab[r1+R_CR_OFF] + ctab[g1+G_CR_OFF] + ctab[b1+B_CR_OFF]) >> SCALEBITS) ,
  176.                     V((ctab[r2+R_CR_OFF] + ctab[g2+G_CR_OFF] + ctab[b2+B_CR_OFF]) >> SCALEBITS) ,
  177.                     V((ctab[r3+R_CR_OFF] + ctab[g3+G_CR_OFF] + ctab[b3+B_CR_OFF]) >> SCALEBITS),pV[k]
  178.                     );
  179. #endif
  180.             k++;
  181.         }
  182.    }
  183. }

  185. int translate_bmp2yuv(FILE* out_yuv_fp, FILE* bmp_fp)
  186. {
  187.     int i, len;
  188.     int w, h, bpp; //bpp-->byte_per_pix
  189.     char* p_temp = NULL;
  190.     unsigned char* rgb = NULL;
  191.     unsigned char* yuv = NULL;
  192.     int32_t* p_color_tab = NULL;
  193.     BITMAPFILEHEADER bmphead;
  194.     BITMAPINFOHEADER infohead;

  196.     if( (NULL==out_yuv_fp) || (NULL==bmp_fp))
  197.         return -1;
  198.     len = sizeof(bmphead)+sizeof(infohead);
  199.     printf("len=%d\n",len);
  200.     p_temp = (char*)malloc(len*sizeof(char));
  201.     if(p_temp == NULL)
  202.     {
  203.         printf("malloc error\n");
  204.         return -1;
  205.     }
  206.     fread(p_temp, len, 1, bmp_fp);
  207.     memcpy(&bmphead, p_temp, sizeof(bmphead));
  208.     memcpy(&infohead, p_temp+sizeof(bmphead), sizeof(infohead));
  209.     if(bmphead.bfType != 0x4D42) //must be 0x4D42='BM'
  210.     {
  211.         printf("not a bmp file\n");
  212.         return -1;
  213.     }
  214.     if(infohead.biBitCount < 24)
  215.     {
  216.         printf("error: now bitCount=%d < 24bit\n", infohead.biBitCount);
  217.         return -1;
  218.     }
  219.     free(p_temp);
  220.     w = infohead.biWidth;
  221.     h = infohead.biHeight;
  222.     bpp = infohead.biBitCount/8;
  223.     printf("w=%d,h=%d,bitcount=%d\n", w, h, bpp);

  225.     yuv = (char*) malloc(w*h*bpp/2);
  226.     if(yuv== NULL)
  227.     {
  228.         printf("malloc yuv buffer error\n");
  229.         return -1;
  230.     }

  232.     rgb = (char*)malloc(w*h*bpp);
  233.     if(rgb== NULL)
  234.     {
  235.         printf("malloc rgb buffer error\n");
  236.         return -1;
  237.     }
  238.     fread(rgb, w*h*bpp, 1, bmp_fp);
  239.     rgb_seq_change(rgb, w, h, bpp);
  240.     p_color_tab = rgb_yuv_start();
  241.     for(i=0; i<100; i++)                        //这儿主要是为了测试，真正用时千万别把这个搞上去了
  242.         rgb_yuv_convert(yuv, rgb, w, h, bpp, p_color_tab);
  243.     fwrite(yuv, w*h*bpp/2, 1, out_yuv_fp);
  244.     free(rgb);
  245.     free(yuv);
  246.     return 0;
  247. }

  249. int main ( int argc, char *argv[] )
  250. {
  251.     char bmp_file[] = "./640_480.bmp";
  252.     char raw_file[] = "./640_480.yuv";
  253.     FILE * out_yuv_fp = NULL;
  254.     FILE * in_bmp_fp = NULL;

  256.     if (NULL == (in_bmp_fp = fopen(bmp_file,"rb")))
  257.     {
  258.         printf("error: %s not found\n",bmp_file);
  259.         return -1;
  260.     }

  262.     if (NULL == (out_yuv_fp = fopen(raw_file,"wb")))
  263.     {
  264.         printf("error: %s not found\n",raw_file);
  265.         return -1;
  266.     }
  267.     
  268.     translate_bmp2yuv(out_yuv_fp, in_bmp_fp);

  270.     fclose(out_yuv_fp);
  271.     fclose(in_bmp_fp);
  272.     printf("over\n");
  273.     return EXIT_SUCCESS;
  274. }
3. 测试
分别对两个rgb_yuv_convert进行测试，执行100次发现
 -->直接转的耗时
real 0m2.183s
user 0m2.179s
sys 0m0.000s    

-->查表法的耗时
real 0m0.679s
user 0m0.674s
sys 0m0.004s    
发现查表法是有一定的提高。



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