1
2/*
3 * Copyright 2013 Google Inc.
4 *
5 * Use of this source code is governed by a BSD-style license that can be
6 * found in the LICENSE file.
7 */
8
9#ifndef GrBlend_DEFINED
10#define GrBlend_DEFINED
11
12#include "include/core/SkTypes.h"
13
14/**
15 * Equations for alpha-blending.
16 */
17enum GrBlendEquation {
18 // Basic blend equations.
19 kAdd_GrBlendEquation, //<! Cs*S + Cd*D
20 kSubtract_GrBlendEquation, //<! Cs*S - Cd*D
21 kReverseSubtract_GrBlendEquation, //<! Cd*D - Cs*S
22
23 // Advanced blend equations. These are described in the SVG and PDF specs.
24 kScreen_GrBlendEquation,
25 kOverlay_GrBlendEquation,
26 kDarken_GrBlendEquation,
27 kLighten_GrBlendEquation,
28 kColorDodge_GrBlendEquation,
29 kColorBurn_GrBlendEquation,
30 kHardLight_GrBlendEquation,
31 kSoftLight_GrBlendEquation,
32 kDifference_GrBlendEquation,
33 kExclusion_GrBlendEquation,
34 kMultiply_GrBlendEquation,
35 kHSLHue_GrBlendEquation,
36 kHSLSaturation_GrBlendEquation,
37 kHSLColor_GrBlendEquation,
38 kHSLLuminosity_GrBlendEquation,
39
40 kIllegal_GrBlendEquation,
41
42 kFirstAdvancedGrBlendEquation = kScreen_GrBlendEquation,
43 kLast_GrBlendEquation = kIllegal_GrBlendEquation,
44};
45
46static const int kGrBlendEquationCnt = kLast_GrBlendEquation + 1;
47
48
49/**
50 * Coefficients for alpha-blending.
51 */
52enum GrBlendCoeff {
53 kZero_GrBlendCoeff, //<! 0
54 kOne_GrBlendCoeff, //<! 1
55 kSC_GrBlendCoeff, //<! src color
56 kISC_GrBlendCoeff, //<! one minus src color
57 kDC_GrBlendCoeff, //<! dst color
58 kIDC_GrBlendCoeff, //<! one minus dst color
59 kSA_GrBlendCoeff, //<! src alpha
60 kISA_GrBlendCoeff, //<! one minus src alpha
61 kDA_GrBlendCoeff, //<! dst alpha
62 kIDA_GrBlendCoeff, //<! one minus dst alpha
63 kConstC_GrBlendCoeff, //<! constant color
64 kIConstC_GrBlendCoeff, //<! one minus constant color
65 kS2C_GrBlendCoeff,
66 kIS2C_GrBlendCoeff,
67 kS2A_GrBlendCoeff,
68 kIS2A_GrBlendCoeff,
69
70 kIllegal_GrBlendCoeff,
71
72 kLast_GrBlendCoeff = kIllegal_GrBlendCoeff,
73};
74
75static const int kGrBlendCoeffCnt = kLast_GrBlendCoeff + 1;
76
77static constexpr bool GrBlendCoeffRefsSrc(const GrBlendCoeff coeff) {
78 return kSC_GrBlendCoeff == coeff || kISC_GrBlendCoeff == coeff || kSA_GrBlendCoeff == coeff ||
79 kISA_GrBlendCoeff == coeff;
80}
81
82static constexpr bool GrBlendCoeffRefsDst(const GrBlendCoeff coeff) {
83 return kDC_GrBlendCoeff == coeff || kIDC_GrBlendCoeff == coeff || kDA_GrBlendCoeff == coeff ||
84 kIDA_GrBlendCoeff == coeff;
85}
86
87static constexpr bool GrBlendCoeffRefsSrc2(const GrBlendCoeff coeff) {
88 return kS2C_GrBlendCoeff == coeff || kIS2C_GrBlendCoeff == coeff ||
89 kS2A_GrBlendCoeff == coeff || kIS2A_GrBlendCoeff == coeff;
90}
91
92static constexpr bool GrBlendCoeffsUseSrcColor(GrBlendCoeff srcCoeff, GrBlendCoeff dstCoeff) {
93 return kZero_GrBlendCoeff != srcCoeff || GrBlendCoeffRefsSrc(dstCoeff);
94}
95
96static constexpr bool GrBlendCoeffsUseDstColor(GrBlendCoeff srcCoeff, GrBlendCoeff dstCoeff) {
97 return GrBlendCoeffRefsDst(srcCoeff) || kZero_GrBlendCoeff != dstCoeff;
98}
99
100static constexpr bool GrBlendEquationIsAdvanced(GrBlendEquation equation) {
101 return equation >= kFirstAdvancedGrBlendEquation
102 && equation != kIllegal_GrBlendEquation;
103}
104
105static constexpr bool GrBlendModifiesDst(GrBlendEquation equation, GrBlendCoeff srcCoeff,
106 GrBlendCoeff dstCoeff) {
107 return (kAdd_GrBlendEquation != equation && kReverseSubtract_GrBlendEquation != equation) ||
108 kZero_GrBlendCoeff != srcCoeff || kOne_GrBlendCoeff != dstCoeff;
109}
110
111static constexpr bool GrBlendCoeffRefsConstant(const GrBlendCoeff coeff) {
112 return coeff == kConstC_GrBlendCoeff || coeff == kIConstC_GrBlendCoeff;
113}
114
115static constexpr bool GrBlendShouldDisable(GrBlendEquation equation, GrBlendCoeff srcCoeff,
116 GrBlendCoeff dstCoeff) {
117 return (kAdd_GrBlendEquation == equation || kSubtract_GrBlendEquation == equation) &&
118 kOne_GrBlendCoeff == srcCoeff && kZero_GrBlendCoeff == dstCoeff;
119}
120
121/**
122 * Advanced blend equations can always tweak alpha for coverage. (See GrCustomXfermode.cpp)
123 *
124 * For "add" and "reverse subtract" the blend equation with f=coverage is:
125 *
126 * D' = f * (S * srcCoeff + D * dstCoeff) + (1-f) * D
127 * = f * S * srcCoeff + D * (f * dstCoeff + (1 - f))
128 *
129 * (Let srcCoeff be negative for reverse subtract.) We can tweak alpha for coverage when the
130 * following relationship holds:
131 *
132 * (f*S) * srcCoeff' + D * dstCoeff' == f * S * srcCoeff + D * (f * dstCoeff + (1 - f))
133 *
134 * (Where srcCoeff' and dstCoeff' have any reference to S pre-multiplied by f.)
135 *
136 * It's easy to see this works for the src term as long as srcCoeff' == srcCoeff (meaning srcCoeff
137 * does not reference S). For the dst term, this will work as long as the following is true:
138 *|
139 * dstCoeff' == f * dstCoeff + (1 - f)
140 * dstCoeff' == 1 - f * (1 - dstCoeff)
141 *
142 * By inspection we can see this will work as long as dstCoeff has a 1, and any other term in
143 * dstCoeff references S.
144 *
145 * Moreover, if the blend doesn't modify the dst at all then it is ok to arbitrarily modify the src
146 * color so folding in coverage is allowed.
147 */
148static constexpr bool GrBlendAllowsCoverageAsAlpha(GrBlendEquation equation,
149 GrBlendCoeff srcCoeff,
150 GrBlendCoeff dstCoeff) {
151 return GrBlendEquationIsAdvanced(equation) ||
152 !GrBlendModifiesDst(equation, srcCoeff, dstCoeff) ||
153 ((kAdd_GrBlendEquation == equation || kReverseSubtract_GrBlendEquation == equation) &&
154 !GrBlendCoeffRefsSrc(srcCoeff) &&
155 (kOne_GrBlendCoeff == dstCoeff || kISC_GrBlendCoeff == dstCoeff ||
156 kISA_GrBlendCoeff == dstCoeff));
157}
158
159#endif
160