1/*
2 * Copyright (c) 2000, 2002, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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24 */
25
26#include "AlphaMacros.h"
27
28/*
29 * The following equation is used to blend each pixel in a compositing
30 * operation between two images (a and b). If we have Ca (Component of a)
31 * and Cb (Component of b) representing the alpha and color components
32 * of a given pair of corresponding pixels in the two source images,
33 * then Porter & Duff have defined blending factors Fa (Factor for a)
34 * and Fb (Factor for b) to represent the contribution of the pixel
35 * from the corresponding image to the pixel in the result.
36 *
37 * Cresult = Fa * Ca + Fb * Cb
38 *
39 * The blending factors Fa and Fb are computed from the alpha value of
40 * the pixel from the "other" source image. Thus, Fa is computed from
41 * the alpha of Cb and vice versa on a per-pixel basis.
42 *
43 * A given factor (Fa or Fb) is computed from the other alpha using
44 * one of the following blending factor equations depending on the
45 * blending rule and depending on whether we are computing Fa or Fb:
46 *
47 * Fblend = 0
48 * Fblend = ONE
49 * Fblend = alpha
50 * Fblend = (ONE - alpha)
51 *
52 * The value ONE in these equations represents the same numeric value
53 * as is used to represent "full coverage" in the alpha component. For
54 * example it is the value 0xff for 8-bit alpha channels and the value
55 * 0xffff for 16-bit alpha channels.
56 *
57 * Each Porter-Duff blending rule thus defines a pair of the above Fblend
58 * equations to define Fa and Fb independently and thus to control
59 * the contributions of the two source pixels to the destination pixel.
60 *
61 * Rather than use conditional tests per pixel in the inner loop,
62 * we note that the following 3 logical and mathematical operations
63 * can be applied to any alpha value to produce the result of one
64 * of the 4 Fblend equations:
65 *
66 * Fcomp = ((alpha AND Fk1) XOR Fk2) PLUS Fk3
67 *
68 * Through appropriate choices for the 3 Fk values we can cause
69 * the result of this Fcomp equation to always match one of the
70 * defined Fblend equations. More importantly, the Fcomp equation
71 * involves no conditional tests which can stall pipelined processor
72 * execution and typically compiles very tightly into 3 machine
73 * instructions.
74 *
75 * For each of the 4 Fblend equations the desired Fk values are
76 * as follows:
77 *
78 * Fblend Fk1 Fk2 Fk3
79 * ------ --- --- ---
80 * 0 0 0 0
81 * ONE 0 0 ONE
82 * alpha ONE 0 0
83 * ONE-alpha ONE -1 ONE+1
84 *
85 * This gives us the following derivations for Fcomp. Note that
86 * the derivation of the last equation is less obvious so it is
87 * broken down into steps and uses the well-known equality for
88 * two's-complement arithmetic "((n XOR -1) PLUS 1) == -n":
89 *
90 * ((alpha AND 0 ) XOR 0) PLUS 0 == 0
91 *
92 * ((alpha AND 0 ) XOR 0) PLUS ONE == ONE
93 *
94 * ((alpha AND ONE) XOR 0) PLUS 0 == alpha
95 *
96 * ((alpha AND ONE) XOR -1) PLUS ONE+1 ==
97 * ((alpha XOR -1) PLUS 1) PLUS ONE ==
98 * (-alpha) PLUS ONE == ONE - alpha
99 *
100 * We have assigned each Porter-Duff rule an implicit index for
101 * simplicity of referring to the rule in parameter lists. For
102 * a given blending operation which uses a specific rule, we simply
103 * use the index of that rule to index into a table and load values
104 * from that table which help us construct the 2 sets of 3 Fk values
105 * needed for applying that blending rule (one set for Fa and the
106 * other set for Fb). Since these Fk values depend only on the
107 * rule we can set them up at the start of the outer loop and only
108 * need to do the 3 operations in the Fcomp equation twice per
109 * pixel (once for Fa and again for Fb).
110 * -------------------------------------------------------------
111 */
112
113/*
114 * The following definitions represent terms in the Fblend
115 * equations described above. One "term name" is chosen from
116 * each of the following 3 pairs of names to define the table
117 * values for the Fa or the Fb of a given Porter-Duff rule.
118 *
119 * AROP_ZERO the first operand is the constant zero
120 * AROP_ONE the first operand is the constant one
121 *
122 * AROP_PLUS the two operands are added together
123 * AROP_MINUS the second operand is subtracted from the first
124 *
125 * AROP_NAUGHT there is no second operand
126 * AROP_ALPHA the indicated alpha is used for the second operand
127 *
128 * These names expand to numeric values which can be conveniently
129 * combined to produce the 3 Fk values needed for the Fcomp equation.
130 *
131 * Note that the numeric values used here are most convenient for
132 * generating the 3 specific Fk values needed for manipulating images
133 * with 8-bits of alpha precision. But Fk values for manipulating
134 * images with other alpha precisions (such as 16-bits) can also be
135 * derived from these same values using a small amount of bit
136 * shifting and replication.
137 */
138#define AROP_ZERO 0x00
139#define AROP_ONE 0xff
140#define AROP_PLUS 0
141#define AROP_MINUS -1
142#define AROP_NAUGHT 0x00
143#define AROP_ALPHA 0xff
144
145/*
146 * This macro constructs a single Fcomp equation table entry from the
147 * term names for the 3 terms in the corresponding Fblend equation.
148 */
149#define MAKE_AROPS(add, xor, and) { AROP_ ## add, AROP_ ## and, AROP_ ## xor }
150
151/*
152 * These macros define the Fcomp equation table entries for each
153 * of the 4 Fblend equations described above.
154 *
155 * AROPS_ZERO Fblend = 0
156 * AROPS_ONE Fblend = 1
157 * AROPS_ALPHA Fblend = alpha
158 * AROPS_INVALPHA Fblend = (1 - alpha)
159 */
160#define AROPS_ZERO MAKE_AROPS( ZERO, PLUS, NAUGHT )
161#define AROPS_ONE MAKE_AROPS( ONE, PLUS, NAUGHT )
162#define AROPS_ALPHA MAKE_AROPS( ZERO, PLUS, ALPHA )
163#define AROPS_INVALPHA MAKE_AROPS( ONE, MINUS, ALPHA )
164
165/*
166 * This table maps a given Porter-Duff blending rule index to a
167 * pair of Fcomp equation table entries, one for computing the
168 * 3 Fk values needed for Fa and another for computing the 3
169 * Fk values needed for Fb.
170 */
171AlphaFunc AlphaRules[] = {
172 { {0, 0, 0}, {0, 0, 0} }, /* 0 - Nothing */
173 { AROPS_ZERO, AROPS_ZERO }, /* 1 - RULE_Clear */
174 { AROPS_ONE, AROPS_ZERO }, /* 2 - RULE_Src */
175 { AROPS_ONE, AROPS_INVALPHA }, /* 3 - RULE_SrcOver */
176 { AROPS_INVALPHA, AROPS_ONE }, /* 4 - RULE_DstOver */
177 { AROPS_ALPHA, AROPS_ZERO }, /* 5 - RULE_SrcIn */
178 { AROPS_ZERO, AROPS_ALPHA }, /* 6 - RULE_DstIn */
179 { AROPS_INVALPHA, AROPS_ZERO }, /* 7 - RULE_SrcOut */
180 { AROPS_ZERO, AROPS_INVALPHA }, /* 8 - RULE_DstOut */
181 { AROPS_ZERO, AROPS_ONE }, /* 9 - RULE_Dst */
182 { AROPS_ALPHA, AROPS_INVALPHA }, /*10 - RULE_SrcAtop */
183 { AROPS_INVALPHA, AROPS_ALPHA }, /*11 - RULE_DstAtop */
184 { AROPS_INVALPHA, AROPS_INVALPHA }, /*12 - RULE_Xor */
185};
186