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/*
* Copyright 2013 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef GrBlend_DEFINED
#define GrBlend_DEFINED
#include "GrColor.h"
#include "../private/SkTLogic.h"
/**
* Equations for alpha-blending.
*/
enum GrBlendEquation {
// Basic blend equations.
kAdd_GrBlendEquation, //<! Cs*S + Cd*D
kSubtract_GrBlendEquation, //<! Cs*S - Cd*D
kReverseSubtract_GrBlendEquation, //<! Cd*D - Cs*S
// Advanced blend equations. These are described in the SVG and PDF specs.
kScreen_GrBlendEquation,
kOverlay_GrBlendEquation,
kDarken_GrBlendEquation,
kLighten_GrBlendEquation,
kColorDodge_GrBlendEquation,
kColorBurn_GrBlendEquation,
kHardLight_GrBlendEquation,
kSoftLight_GrBlendEquation,
kDifference_GrBlendEquation,
kExclusion_GrBlendEquation,
kMultiply_GrBlendEquation,
kHSLHue_GrBlendEquation,
kHSLSaturation_GrBlendEquation,
kHSLColor_GrBlendEquation,
kHSLLuminosity_GrBlendEquation,
kFirstAdvancedGrBlendEquation = kScreen_GrBlendEquation,
kLast_GrBlendEquation = kHSLLuminosity_GrBlendEquation
};
static const int kGrBlendEquationCnt = kLast_GrBlendEquation + 1;
/**
* Coefficients for alpha-blending.
*/
enum GrBlendCoeff {
kZero_GrBlendCoeff, //<! 0
kOne_GrBlendCoeff, //<! 1
kSC_GrBlendCoeff, //<! src color
kISC_GrBlendCoeff, //<! one minus src color
kDC_GrBlendCoeff, //<! dst color
kIDC_GrBlendCoeff, //<! one minus dst color
kSA_GrBlendCoeff, //<! src alpha
kISA_GrBlendCoeff, //<! one minus src alpha
kDA_GrBlendCoeff, //<! dst alpha
kIDA_GrBlendCoeff, //<! one minus dst alpha
kConstC_GrBlendCoeff, //<! constant color
kIConstC_GrBlendCoeff, //<! one minus constant color
kConstA_GrBlendCoeff, //<! constant color alpha
kIConstA_GrBlendCoeff, //<! one minus constant color alpha
kS2C_GrBlendCoeff,
kIS2C_GrBlendCoeff,
kS2A_GrBlendCoeff,
kIS2A_GrBlendCoeff,
kLast_GrBlendCoeff = kIS2A_GrBlendCoeff
};
static const int kGrBlendCoeffCnt = kLast_GrBlendCoeff + 1;
static constexpr bool GrBlendCoeffRefsSrc(const GrBlendCoeff coeff) {
return kSC_GrBlendCoeff == coeff || kISC_GrBlendCoeff == coeff || kSA_GrBlendCoeff == coeff ||
kISA_GrBlendCoeff == coeff;
}
static constexpr bool GrBlendCoeffRefsDst(const GrBlendCoeff coeff) {
return kDC_GrBlendCoeff == coeff || kIDC_GrBlendCoeff == coeff || kDA_GrBlendCoeff == coeff ||
kIDA_GrBlendCoeff == coeff;
}
static constexpr bool GrBlendCoeffRefsSrc2(const GrBlendCoeff coeff) {
return kS2C_GrBlendCoeff == coeff || kIS2C_GrBlendCoeff == coeff ||
kS2A_GrBlendCoeff == coeff || kIS2A_GrBlendCoeff == coeff;
}
static constexpr bool GrBlendCoeffsUseSrcColor(GrBlendCoeff srcCoeff, GrBlendCoeff dstCoeff) {
return kZero_GrBlendCoeff != srcCoeff || GrBlendCoeffRefsSrc(dstCoeff);
}
static constexpr bool GrBlendCoeffsUseDstColor(GrBlendCoeff srcCoeff, GrBlendCoeff dstCoeff) {
return GrBlendCoeffRefsDst(srcCoeff) || kZero_GrBlendCoeff != dstCoeff;
}
static constexpr bool GrBlendEquationIsAdvanced(GrBlendEquation equation) {
return equation >= kFirstAdvancedGrBlendEquation;
}
static constexpr bool GrBlendModifiesDst(GrBlendEquation equation, GrBlendCoeff srcCoeff,
GrBlendCoeff dstCoeff) {
return (kAdd_GrBlendEquation != equation && kReverseSubtract_GrBlendEquation != equation) ||
kZero_GrBlendCoeff != srcCoeff || kOne_GrBlendCoeff != dstCoeff;
}
/**
* Advanced blend equations can always tweak alpha for coverage. (See GrCustomXfermode.cpp)
*
* For "add" and "reverse subtract" the blend equation with f=coverage is:
*
* D' = f * (S * srcCoeff + D * dstCoeff) + (1-f) * D
* = f * S * srcCoeff + D * (f * dstCoeff + (1 - f))
*
* (Let srcCoeff be negative for reverse subtract.) We can tweak alpha for coverage when the
* following relationship holds:
*
* (f*S) * srcCoeff' + D * dstCoeff' == f * S * srcCoeff + D * (f * dstCoeff + (1 - f))
*
* (Where srcCoeff' and dstCoeff' have any reference to S pre-multiplied by f.)
*
* It's easy to see this works for the src term as long as srcCoeff' == srcCoeff (meaning srcCoeff
* does not reference S). For the dst term, this will work as long as the following is true:
*|
* dstCoeff' == f * dstCoeff + (1 - f)
* dstCoeff' == 1 - f * (1 - dstCoeff)
*
* By inspection we can see this will work as long as dstCoeff has a 1, and any other term in
* dstCoeff references S.
*
* Moreover, if the blend doesn't modify the dst at all then it is ok to arbitrarily modify the src
* color so folding in coverage is allowed.
*/
static constexpr bool GrBlendAllowsCoverageAsAlpha(GrBlendEquation equation,
GrBlendCoeff srcCoeff,
GrBlendCoeff dstCoeff) {
return GrBlendEquationIsAdvanced(equation) ||
!GrBlendModifiesDst(equation, srcCoeff, dstCoeff) ||
((kAdd_GrBlendEquation == equation || kReverseSubtract_GrBlendEquation == equation) &&
!GrBlendCoeffRefsSrc(srcCoeff) &&
(kOne_GrBlendCoeff == dstCoeff || kISC_GrBlendCoeff == dstCoeff ||
kISA_GrBlendCoeff == dstCoeff));
}
#endif
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