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      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 "SkTypes.h"
     13 
     14 /**
     15  * Equations for alpha-blending.
     16  */
     17 enum 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 
     46 static const int kGrBlendEquationCnt = kLast_GrBlendEquation + 1;
     47 
     48 
     49 /**
     50  * Coefficients for alpha-blending.
     51  */
     52 enum 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     kConstA_GrBlendCoeff,  //<! constant color alpha
     66     kIConstA_GrBlendCoeff, //<! one minus constant color alpha
     67     kS2C_GrBlendCoeff,
     68     kIS2C_GrBlendCoeff,
     69     kS2A_GrBlendCoeff,
     70     kIS2A_GrBlendCoeff,
     71 
     72     kIllegal_GrBlendCoeff,
     73 
     74     kLast_GrBlendCoeff = kIllegal_GrBlendCoeff,
     75 };
     76 
     77 static const int kGrBlendCoeffCnt = kLast_GrBlendCoeff + 1;
     78 
     79 static constexpr bool GrBlendCoeffRefsSrc(const GrBlendCoeff coeff) {
     80     return kSC_GrBlendCoeff == coeff || kISC_GrBlendCoeff == coeff || kSA_GrBlendCoeff == coeff ||
     81            kISA_GrBlendCoeff == coeff;
     82 }
     83 
     84 static constexpr bool GrBlendCoeffRefsDst(const GrBlendCoeff coeff) {
     85     return kDC_GrBlendCoeff == coeff || kIDC_GrBlendCoeff == coeff || kDA_GrBlendCoeff == coeff ||
     86            kIDA_GrBlendCoeff == coeff;
     87 }
     88 
     89 static constexpr bool GrBlendCoeffRefsSrc2(const GrBlendCoeff coeff) {
     90     return kS2C_GrBlendCoeff == coeff || kIS2C_GrBlendCoeff == coeff ||
     91            kS2A_GrBlendCoeff == coeff || kIS2A_GrBlendCoeff == coeff;
     92 }
     93 
     94 static constexpr bool GrBlendCoeffsUseSrcColor(GrBlendCoeff srcCoeff, GrBlendCoeff dstCoeff) {
     95     return kZero_GrBlendCoeff != srcCoeff || GrBlendCoeffRefsSrc(dstCoeff);
     96 }
     97 
     98 static constexpr bool GrBlendCoeffsUseDstColor(GrBlendCoeff srcCoeff, GrBlendCoeff dstCoeff) {
     99     return GrBlendCoeffRefsDst(srcCoeff) || kZero_GrBlendCoeff != dstCoeff;
    100 }
    101 
    102 static constexpr bool GrBlendEquationIsAdvanced(GrBlendEquation equation) {
    103     return equation >= kFirstAdvancedGrBlendEquation
    104         && equation != kIllegal_GrBlendEquation;
    105 }
    106 
    107 static constexpr bool GrBlendModifiesDst(GrBlendEquation equation, GrBlendCoeff srcCoeff,
    108                                          GrBlendCoeff dstCoeff) {
    109     return (kAdd_GrBlendEquation != equation && kReverseSubtract_GrBlendEquation != equation) ||
    110            kZero_GrBlendCoeff != srcCoeff || kOne_GrBlendCoeff != dstCoeff;
    111 }
    112 
    113 /**
    114  * Advanced blend equations can always tweak alpha for coverage. (See GrCustomXfermode.cpp)
    115  *
    116  * For "add" and "reverse subtract" the blend equation with f=coverage is:
    117  *
    118  *   D' = f * (S * srcCoeff + D * dstCoeff) + (1-f) * D
    119  *      = f * S * srcCoeff + D * (f * dstCoeff + (1 - f))
    120  *
    121  * (Let srcCoeff be negative for reverse subtract.) We can tweak alpha for coverage when the
    122  * following relationship holds:
    123  *
    124  *   (f*S) * srcCoeff' + D * dstCoeff' == f * S * srcCoeff + D * (f * dstCoeff + (1 - f))
    125  *
    126  * (Where srcCoeff' and dstCoeff' have any reference to S pre-multiplied by f.)
    127  *
    128  * It's easy to see this works for the src term as long as srcCoeff' == srcCoeff (meaning srcCoeff
    129  * does not reference S). For the dst term, this will work as long as the following is true:
    130  *|
    131  *   dstCoeff' == f * dstCoeff + (1 - f)
    132  *   dstCoeff' == 1 - f * (1 - dstCoeff)
    133  *
    134  * By inspection we can see this will work as long as dstCoeff has a 1, and any other term in
    135  * dstCoeff references S.
    136  *
    137  * Moreover, if the blend doesn't modify the dst at all then it is ok to arbitrarily modify the src
    138  * color so folding in coverage is allowed.
    139  */
    140 static constexpr bool GrBlendAllowsCoverageAsAlpha(GrBlendEquation equation,
    141                                                    GrBlendCoeff srcCoeff,
    142                                                    GrBlendCoeff dstCoeff) {
    143     return GrBlendEquationIsAdvanced(equation) ||
    144            !GrBlendModifiesDst(equation, srcCoeff, dstCoeff) ||
    145            ((kAdd_GrBlendEquation == equation || kReverseSubtract_GrBlendEquation == equation) &&
    146             !GrBlendCoeffRefsSrc(srcCoeff) &&
    147             (kOne_GrBlendCoeff == dstCoeff || kISC_GrBlendCoeff == dstCoeff ||
    148              kISA_GrBlendCoeff == dstCoeff));
    149 }
    150 
    151 #endif
    152