## ffmpeg / libavcodec / jfdctfst.c @ 5917d17c

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1 | de6d9b64 | Fabrice Bellard | ```
/*
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2 | ```
* jfdctfst.c
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3 | ```
*
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4 | ```
* Copyright (C) 1994-1996, Thomas G. Lane.
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5 | ```
* This file is part of the Independent JPEG Group's software.
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6 | ```
* For conditions of distribution and use, see the accompanying README file.
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7 | ```
*
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8 | ```
* This file contains a fast, not so accurate integer implementation of the
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9 | ```
* forward DCT (Discrete Cosine Transform).
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10 | ```
*
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11 | ```
* A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
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12 | ```
* on each column. Direct algorithms are also available, but they are
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13 | ```
* much more complex and seem not to be any faster when reduced to code.
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14 | ```
*
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15 | ```
* This implementation is based on Arai, Agui, and Nakajima's algorithm for
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16 | ```
* scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
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17 | ```
* Japanese, but the algorithm is described in the Pennebaker & Mitchell
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18 | ```
* JPEG textbook (see REFERENCES section in file README). The following code
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19 | ```
* is based directly on figure 4-8 in P&M.
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20 | ```
* While an 8-point DCT cannot be done in less than 11 multiplies, it is
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21 | ```
* possible to arrange the computation so that many of the multiplies are
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22 | ```
* simple scalings of the final outputs. These multiplies can then be
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23 | ```
* folded into the multiplications or divisions by the JPEG quantization
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24 | ```
* table entries. The AA&N method leaves only 5 multiplies and 29 adds
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25 | ```
* to be done in the DCT itself.
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26 | ```
* The primary disadvantage of this method is that with fixed-point math,
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27 | ```
* accuracy is lost due to imprecise representation of the scaled
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28 | ```
* quantization values. The smaller the quantization table entry, the less
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29 | ```
* precise the scaled value, so this implementation does worse with high-
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30 | ```
* quality-setting files than with low-quality ones.
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31 | ```
*/
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32 | |||

33 | #include <stdlib.h> |
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34 | #include <stdio.h> |
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35 | #include "common.h" |
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36 | #include "dsputil.h" |
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37 | |||

38 | #define DCTSIZE 8 |
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39 | ```
#define GLOBAL(x) x
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40 | ```
#define RIGHT_SHIFT(x, n) ((x) >> (n))
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41 | ```
#define SHIFT_TEMPS
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42 | |||

43 | ```
/*
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44 | ```
* This module is specialized to the case DCTSIZE = 8.
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45 | ```
*/
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46 | |||

47 | #if DCTSIZE != 8 |
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48 | Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */ |
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49 | ```
#endif
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50 | |||

51 | |||

52 | ```
/* Scaling decisions are generally the same as in the LL&M algorithm;
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53 | ```
* see jfdctint.c for more details. However, we choose to descale
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54 | ```
* (right shift) multiplication products as soon as they are formed,
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55 | ```
* rather than carrying additional fractional bits into subsequent additions.
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56 | ```
* This compromises accuracy slightly, but it lets us save a few shifts.
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57 | ```
* More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
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58 | ```
* everywhere except in the multiplications proper; this saves a good deal
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59 | ```
* of work on 16-bit-int machines.
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60 | ```
*
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61 | ```
* Again to save a few shifts, the intermediate results between pass 1 and
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62 | ```
* pass 2 are not upscaled, but are represented only to integral precision.
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63 | ```
*
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64 | ```
* A final compromise is to represent the multiplicative constants to only
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65 | ```
* 8 fractional bits, rather than 13. This saves some shifting work on some
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66 | ```
* machines, and may also reduce the cost of multiplication (since there
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67 | ```
* are fewer one-bits in the constants).
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68 | ```
*/
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69 | |||

70 | #define CONST_BITS 8 |
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71 | |||

72 | |||

73 | ```
/* Some C compilers fail to reduce "FIX(constant)" at compile time, thus
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74 | ```
* causing a lot of useless floating-point operations at run time.
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75 | ```
* To get around this we use the following pre-calculated constants.
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76 | ```
* If you change CONST_BITS you may want to add appropriate values.
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77 | ```
* (With a reasonable C compiler, you can just rely on the FIX() macro...)
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78 | ```
*/
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79 | |||

80 | #if CONST_BITS == 8 |
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81 | #define FIX_0_382683433 ((INT32) 98) /* FIX(0.382683433) */ |
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82 | #define FIX_0_541196100 ((INT32) 139) /* FIX(0.541196100) */ |
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83 | #define FIX_0_707106781 ((INT32) 181) /* FIX(0.707106781) */ |
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84 | #define FIX_1_306562965 ((INT32) 334) /* FIX(1.306562965) */ |
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85 | ```
#else
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86 | #define FIX_0_382683433 FIX(0.382683433) |
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87 | #define FIX_0_541196100 FIX(0.541196100) |
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88 | #define FIX_0_707106781 FIX(0.707106781) |
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89 | #define FIX_1_306562965 FIX(1.306562965) |
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90 | ```
#endif
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91 | |||

92 | |||

93 | ```
/* We can gain a little more speed, with a further compromise in accuracy,
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94 | ```
* by omitting the addition in a descaling shift. This yields an incorrectly
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95 | ```
* rounded result half the time...
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96 | ```
*/
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97 | |||

98 | ```
#ifndef USE_ACCURATE_ROUNDING
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99 | ```
#undef DESCALE
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100 | ```
#define DESCALE(x,n) RIGHT_SHIFT(x, n)
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101 | ```
#endif
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102 | |||

103 | |||

104 | ```
/* Multiply a DCTELEM variable by an INT32 constant, and immediately
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105 | ```
* descale to yield a DCTELEM result.
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106 | ```
*/
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107 | |||

108 | #define MULTIPLY(var,const) ((DCTELEM) DESCALE((var) * (const), CONST_BITS)) |
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109 | |||

110 | |||

111 | ```
/*
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112 | ```
* Perform the forward DCT on one block of samples.
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113 | ```
*/
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114 | |||

115 | ```
GLOBAL(void)
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116 | 03c94ede | Fabrice Bellard | fdct_ifast (DCTELEM * data) |

117 | de6d9b64 | Fabrice Bellard | { |

118 | DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; |
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119 | DCTELEM tmp10, tmp11, tmp12, tmp13; |
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120 | DCTELEM z1, z2, z3, z4, z5, z11, z13; |
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121 | DCTELEM *dataptr; |
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122 | ```
int ctr;
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123 | SHIFT_TEMPS |
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124 | |||

125 | ```
/* Pass 1: process rows. */
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126 | |||

127 | dataptr = data; |
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128 | for (ctr = DCTSIZE-1; ctr >= 0; ctr--) { |
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129 | tmp0 = dataptr[0] + dataptr[7]; |
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130 | tmp7 = dataptr[0] - dataptr[7]; |
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131 | tmp1 = dataptr[1] + dataptr[6]; |
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132 | tmp6 = dataptr[1] - dataptr[6]; |
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133 | tmp2 = dataptr[2] + dataptr[5]; |
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134 | tmp5 = dataptr[2] - dataptr[5]; |
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135 | tmp3 = dataptr[3] + dataptr[4]; |
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136 | tmp4 = dataptr[3] - dataptr[4]; |
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137 | |||

138 | ```
/* Even part */
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139 | |||

140 | ```
tmp10 = tmp0 + tmp3; /* phase 2 */
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141 | tmp13 = tmp0 - tmp3; |
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142 | tmp11 = tmp1 + tmp2; |
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143 | tmp12 = tmp1 - tmp2; |
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144 | |||

145 | dataptr[0] = tmp10 + tmp11; /* phase 3 */ |
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146 | ```
dataptr[4] = tmp10 - tmp11;
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147 | |||

148 | ```
z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
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149 | dataptr[2] = tmp13 + z1; /* phase 5 */ |
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150 | ```
dataptr[6] = tmp13 - z1;
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151 | |||

152 | ```
/* Odd part */
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153 | |||

154 | ```
tmp10 = tmp4 + tmp5; /* phase 2 */
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155 | tmp11 = tmp5 + tmp6; |
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156 | tmp12 = tmp6 + tmp7; |
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157 | |||

158 | ```
/* The rotator is modified from fig 4-8 to avoid extra negations. */
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159 | ```
z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
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160 | ```
z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
``` |
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161 | ```
z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
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162 | ```
z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
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163 | |||

164 | ```
z11 = tmp7 + z3; /* phase 5 */
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165 | z13 = tmp7 - z3; |
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166 | |||

167 | dataptr[5] = z13 + z2; /* phase 6 */ |
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168 | ```
dataptr[3] = z13 - z2;
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169 | ```
dataptr[1] = z11 + z4;
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170 | ```
dataptr[7] = z11 - z4;
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171 | |||

172 | ```
dataptr += DCTSIZE; /* advance pointer to next row */
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173 | } |
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174 | |||

175 | ```
/* Pass 2: process columns. */
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176 | |||

177 | dataptr = data; |
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178 | for (ctr = DCTSIZE-1; ctr >= 0; ctr--) { |
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179 | tmp0 = dataptr[DCTSIZE*0] + dataptr[DCTSIZE*7]; |
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180 | tmp7 = dataptr[DCTSIZE*0] - dataptr[DCTSIZE*7]; |
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181 | tmp1 = dataptr[DCTSIZE*1] + dataptr[DCTSIZE*6]; |
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182 | tmp6 = dataptr[DCTSIZE*1] - dataptr[DCTSIZE*6]; |
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183 | tmp2 = dataptr[DCTSIZE*2] + dataptr[DCTSIZE*5]; |
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184 | tmp5 = dataptr[DCTSIZE*2] - dataptr[DCTSIZE*5]; |
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185 | tmp3 = dataptr[DCTSIZE*3] + dataptr[DCTSIZE*4]; |
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186 | tmp4 = dataptr[DCTSIZE*3] - dataptr[DCTSIZE*4]; |
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187 | |||

188 | ```
/* Even part */
``` |
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189 | |||

190 | ```
tmp10 = tmp0 + tmp3; /* phase 2 */
``` |
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191 | tmp13 = tmp0 - tmp3; |
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192 | tmp11 = tmp1 + tmp2; |
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193 | tmp12 = tmp1 - tmp2; |
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194 | |||

195 | dataptr[DCTSIZE*0] = tmp10 + tmp11; /* phase 3 */ |
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196 | ```
dataptr[DCTSIZE*4] = tmp10 - tmp11;
``` |
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197 | |||

198 | ```
z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
``` |
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199 | dataptr[DCTSIZE*2] = tmp13 + z1; /* phase 5 */ |
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200 | ```
dataptr[DCTSIZE*6] = tmp13 - z1;
``` |
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201 | |||

202 | ```
/* Odd part */
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203 | |||

204 | ```
tmp10 = tmp4 + tmp5; /* phase 2 */
``` |
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205 | tmp11 = tmp5 + tmp6; |
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206 | tmp12 = tmp6 + tmp7; |
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207 | |||

208 | ```
/* The rotator is modified from fig 4-8 to avoid extra negations. */
``` |
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209 | ```
z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
``` |
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210 | ```
z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
``` |
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211 | ```
z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
``` |
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212 | ```
z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
``` |
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213 | |||

214 | ```
z11 = tmp7 + z3; /* phase 5 */
``` |
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215 | z13 = tmp7 - z3; |
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216 | |||

217 | dataptr[DCTSIZE*5] = z13 + z2; /* phase 6 */ |
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218 | ```
dataptr[DCTSIZE*3] = z13 - z2;
``` |
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219 | ```
dataptr[DCTSIZE*1] = z11 + z4;
``` |
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220 | ```
dataptr[DCTSIZE*7] = z11 - z4;
``` |
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221 | |||

222 | ```
dataptr++; /* advance pointer to next column */
``` |
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223 | } |
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224 | } |
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225 | cd4af68a | Zdenek Kabelac | |

226 | |||

227 | ```
#undef GLOBAL
``` |
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228 | ```
#undef CONST_BITS
``` |
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229 | ```
#undef DESCALE
``` |
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230 | ```
#undef FIX_0_541196100
``` |
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231 | `#undef FIX_1_306562965` |