1 /* fdctref.c, forward discrete cosine transform, double precision */
3 /* Copyright (C) 1996, MPEG Software Simulation Group. All Rights Reserved. */
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38 # define PI 3.14159265358979323846
42 /* global declarations */
43 void init_fdct _ANSI_ARGS_((void));
44 void fdct _ANSI_ARGS_((short *block));
47 static double c[8][8]; /* transform coefficients */
56 s = (i==0) ? sqrt(0.125) : 0.5;
59 c[i][j] = s * cos((PI/8.0)*i*(j+0.5));
70 for(i = 0; i < 8; i++)
71 for(j = 0; j < 8; j++)
76 * for(k = 0; k < 8; k++)
77 * s += c[j][k] * block[8 * i + k];
79 s += c[j][0] * block[8 * i + 0];
80 s += c[j][1] * block[8 * i + 1];
81 s += c[j][2] * block[8 * i + 2];
82 s += c[j][3] * block[8 * i + 3];
83 s += c[j][4] * block[8 * i + 4];
84 s += c[j][5] * block[8 * i + 5];
85 s += c[j][6] * block[8 * i + 6];
86 s += c[j][7] * block[8 * i + 7];
91 for(j = 0; j < 8; j++)
92 for(i = 0; i < 8; i++)
97 * for(k = 0; k < 8; k++)
98 * s += c[i][k] * tmp[8 * k + j];
100 s += c[i][0] * tmp[8 * 0 + j];
101 s += c[i][1] * tmp[8 * 1 + j];
102 s += c[i][2] * tmp[8 * 2 + j];
103 s += c[i][3] * tmp[8 * 3 + j];
104 s += c[i][4] * tmp[8 * 4 + j];
105 s += c[i][5] * tmp[8 * 5 + j];
106 s += c[i][6] * tmp[8 * 6 + j];
107 s += c[i][7] * tmp[8 * 7 + j];
109 block[8 * i + j] = (int)floor(s + 0.499999);
111 * reason for adding 0.499999 instead of 0.5:
112 * s is quite often x.5 (at least for i and/or j = 0 or 4)
113 * and setting the rounding threshold exactly to 0.5 leads to an
114 * extremely high arithmetic implementation dependency of the result;
115 * s being between x.5 and x.500001 (which is now incorrectly rounded
116 * downwards instead of upwards) is assumed to occur less often