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1 /* |
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2 * Copyright 2000-2002 Sun Microsystems, Inc. All Rights Reserved. |
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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4 * |
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5 * This code is free software; you can redistribute it and/or modify it |
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6 * under the terms of the GNU General Public License version 2 only, as |
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7 * published by the Free Software Foundation. Sun designates this |
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8 * particular file as subject to the "Classpath" exception as provided |
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9 * by Sun in the LICENSE file that accompanied this code. |
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10 * |
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11 * This code is distributed in the hope that it will be useful, but WITHOUT |
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12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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14 * version 2 for more details (a copy is included in the LICENSE file that |
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15 * accompanied this code). |
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16 * |
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17 * You should have received a copy of the GNU General Public License version |
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18 * 2 along with this work; if not, write to the Free Software Foundation, |
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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20 * |
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21 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
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22 * CA 95054 USA or visit www.sun.com if you need additional information or |
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23 * have any questions. |
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24 */ |
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25 |
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26 #include "AlphaMacros.h" |
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27 |
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28 /* |
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29 * The following equation is used to blend each pixel in a compositing |
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30 * operation between two images (a and b). If we have Ca (Component of a) |
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31 * and Cb (Component of b) representing the alpha and color components |
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32 * of a given pair of corresponding pixels in the two source images, |
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33 * then Porter & Duff have defined blending factors Fa (Factor for a) |
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34 * and Fb (Factor for b) to represent the contribution of the pixel |
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35 * from the corresponding image to the pixel in the result. |
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36 * |
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37 * Cresult = Fa * Ca + Fb * Cb |
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38 * |
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39 * The blending factors Fa and Fb are computed from the alpha value of |
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40 * the pixel from the "other" source image. Thus, Fa is computed from |
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41 * the alpha of Cb and vice versa on a per-pixel basis. |
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42 * |
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43 * A given factor (Fa or Fb) is computed from the other alpha using |
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44 * one of the following blending factor equations depending on the |
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45 * blending rule and depending on whether we are computing Fa or Fb: |
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46 * |
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47 * Fblend = 0 |
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48 * Fblend = ONE |
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49 * Fblend = alpha |
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50 * Fblend = (ONE - alpha) |
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51 * |
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52 * The value ONE in these equations represents the same numeric value |
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53 * as is used to represent "full coverage" in the alpha component. For |
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54 * example it is the value 0xff for 8-bit alpha channels and the value |
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55 * 0xffff for 16-bit alpha channels. |
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56 * |
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57 * Each Porter-Duff blending rule thus defines a pair of the above Fblend |
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58 * equations to define Fa and Fb independently and thus to control |
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59 * the contributions of the two source pixels to the destination pixel. |
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60 * |
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61 * Rather than use conditional tests per pixel in the inner loop, |
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62 * we note that the following 3 logical and mathematical operations |
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63 * can be applied to any alpha value to produce the result of one |
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64 * of the 4 Fblend equations: |
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65 * |
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66 * Fcomp = ((alpha AND Fk1) XOR Fk2) PLUS Fk3 |
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67 * |
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68 * Through appropriate choices for the 3 Fk values we can cause |
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69 * the result of this Fcomp equation to always match one of the |
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70 * defined Fblend equations. More importantly, the Fcomp equation |
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71 * involves no conditional tests which can stall pipelined processor |
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72 * execution and typically compiles very tightly into 3 machine |
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73 * instructions. |
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74 * |
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75 * For each of the 4 Fblend equations the desired Fk values are |
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76 * as follows: |
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77 * |
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78 * Fblend Fk1 Fk2 Fk3 |
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79 * ------ --- --- --- |
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80 * 0 0 0 0 |
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81 * ONE 0 0 ONE |
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82 * alpha ONE 0 0 |
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83 * ONE-alpha ONE -1 ONE+1 |
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84 * |
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85 * This gives us the following derivations for Fcomp. Note that |
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86 * the derivation of the last equation is less obvious so it is |
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87 * broken down into steps and uses the well-known equality for |
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88 * two's-complement arithmetic "((n XOR -1) PLUS 1) == -n": |
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89 * |
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90 * ((alpha AND 0 ) XOR 0) PLUS 0 == 0 |
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91 * |
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92 * ((alpha AND 0 ) XOR 0) PLUS ONE == ONE |
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93 * |
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94 * ((alpha AND ONE) XOR 0) PLUS 0 == alpha |
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95 * |
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96 * ((alpha AND ONE) XOR -1) PLUS ONE+1 == |
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97 * ((alpha XOR -1) PLUS 1) PLUS ONE == |
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98 * (-alpha) PLUS ONE == ONE - alpha |
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99 * |
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100 * We have assigned each Porter-Duff rule an implicit index for |
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101 * simplicity of referring to the rule in parameter lists. For |
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102 * a given blending operation which uses a specific rule, we simply |
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103 * use the index of that rule to index into a table and load values |
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104 * from that table which help us construct the 2 sets of 3 Fk values |
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105 * needed for applying that blending rule (one set for Fa and the |
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106 * other set for Fb). Since these Fk values depend only on the |
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107 * rule we can set them up at the start of the outer loop and only |
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108 * need to do the 3 operations in the Fcomp equation twice per |
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109 * pixel (once for Fa and again for Fb). |
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110 * ------------------------------------------------------------- |
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111 */ |
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112 |
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113 /* |
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114 * The following definitions represent terms in the Fblend |
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115 * equations described above. One "term name" is chosen from |
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116 * each of the following 3 pairs of names to define the table |
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117 * values for the Fa or the Fb of a given Porter-Duff rule. |
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118 * |
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119 * AROP_ZERO the first operand is the constant zero |
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120 * AROP_ONE the first operand is the constant one |
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121 * |
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122 * AROP_PLUS the two operands are added together |
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123 * AROP_MINUS the second operand is subtracted from the first |
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124 * |
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125 * AROP_NAUGHT there is no second operand |
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126 * AROP_ALPHA the indicated alpha is used for the second operand |
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127 * |
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128 * These names expand to numeric values which can be conveniently |
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129 * combined to produce the 3 Fk values needed for the Fcomp equation. |
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130 * |
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131 * Note that the numeric values used here are most convenient for |
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132 * generating the 3 specific Fk values needed for manipulating images |
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133 * with 8-bits of alpha precision. But Fk values for manipulating |
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134 * images with other alpha precisions (such as 16-bits) can also be |
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135 * derived from these same values using a small amount of bit |
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136 * shifting and replication. |
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137 */ |
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138 #define AROP_ZERO 0x00 |
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139 #define AROP_ONE 0xff |
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140 #define AROP_PLUS 0 |
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141 #define AROP_MINUS -1 |
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142 #define AROP_NAUGHT 0x00 |
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143 #define AROP_ALPHA 0xff |
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144 |
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145 /* |
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146 * This macro constructs a single Fcomp equation table entry from the |
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147 * term names for the 3 terms in the corresponding Fblend equation. |
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148 */ |
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149 #define MAKE_AROPS(add, xor, and) { AROP_ ## add, AROP_ ## and, AROP_ ## xor } |
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150 |
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151 /* |
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152 * These macros define the Fcomp equation table entries for each |
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153 * of the 4 Fblend equations described above. |
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154 * |
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155 * AROPS_ZERO Fblend = 0 |
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156 * AROPS_ONE Fblend = 1 |
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157 * AROPS_ALPHA Fblend = alpha |
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158 * AROPS_INVALPHA Fblend = (1 - alpha) |
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159 */ |
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160 #define AROPS_ZERO MAKE_AROPS( ZERO, PLUS, NAUGHT ) |
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161 #define AROPS_ONE MAKE_AROPS( ONE, PLUS, NAUGHT ) |
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162 #define AROPS_ALPHA MAKE_AROPS( ZERO, PLUS, ALPHA ) |
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163 #define AROPS_INVALPHA MAKE_AROPS( ONE, MINUS, ALPHA ) |
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164 |
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165 /* |
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166 * This table maps a given Porter-Duff blending rule index to a |
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167 * pair of Fcomp equation table entries, one for computing the |
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168 * 3 Fk values needed for Fa and another for computing the 3 |
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169 * Fk values needed for Fb. |
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170 */ |
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171 AlphaFunc AlphaRules[] = { |
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172 { {0, 0, 0}, {0, 0, 0} }, /* 0 - Nothing */ |
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173 { AROPS_ZERO, AROPS_ZERO }, /* 1 - RULE_Clear */ |
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174 { AROPS_ONE, AROPS_ZERO }, /* 2 - RULE_Src */ |
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175 { AROPS_ONE, AROPS_INVALPHA }, /* 3 - RULE_SrcOver */ |
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176 { AROPS_INVALPHA, AROPS_ONE }, /* 4 - RULE_DstOver */ |
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177 { AROPS_ALPHA, AROPS_ZERO }, /* 5 - RULE_SrcIn */ |
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178 { AROPS_ZERO, AROPS_ALPHA }, /* 6 - RULE_DstIn */ |
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179 { AROPS_INVALPHA, AROPS_ZERO }, /* 7 - RULE_SrcOut */ |
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180 { AROPS_ZERO, AROPS_INVALPHA }, /* 8 - RULE_DstOut */ |
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181 { AROPS_ZERO, AROPS_ONE }, /* 9 - RULE_Dst */ |
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182 { AROPS_ALPHA, AROPS_INVALPHA }, /*10 - RULE_SrcAtop */ |
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183 { AROPS_INVALPHA, AROPS_ALPHA }, /*11 - RULE_DstAtop */ |
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184 { AROPS_INVALPHA, AROPS_INVALPHA }, /*12 - RULE_Xor */ |
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185 }; |