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