GrBezierEffect.h

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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 GrBezierEffect_DEFINED
#define GrBezierEffect_DEFINED
 
#include "include/core/SkMatrix.h"
#include "include/private/SkColorData.h"
#include "include/private/gpu/ganesh/GrTypesPriv.h"
#include "src/base/SkArenaAlloc.h"
#include "src/core/SkSLTypeShared.h"
#include "src/gpu/ganesh/GrCaps.h"
#include "src/gpu/ganesh/GrGeometryProcessor.h"
#include "src/gpu/ganesh/GrProcessorUnitTest.h"
#include "src/gpu/ganesh/GrShaderCaps.h"
 
#include <cstdint>
#include <memory>
 
namespace skgpu { class KeyBuilder; }
 
/**
 * Shader is based off of Loop-Blinn Quadratic GPU Rendering
 * The output of this effect is a hairline edge for conics.
 * Conics specified by implicit equation K^2 - LM.
 * K, L, and M, are the first three values of the vertex attribute,
 * the fourth value is not used. Distance is calculated using a
 * first order approximation from the taylor series.
 * Coverage for AA is max(0, 1-distance).
 *
 * Test were also run using a second order distance approximation.
 * There were two versions of the second order approx. The first version
 * is of roughly the form:
 * f(q) = |f(p)| - ||f'(p)||*||q-p|| - ||f''(p)||*||q-p||^2.
 * The second is similar:
 * f(q) = |f(p)| + ||f'(p)||*||q-p|| + ||f''(p)||*||q-p||^2.
 * The exact version of the equations can be found in the paper
 * "Distance Approximations for Rasterizing Implicit Curves" by Gabriel Taubin
 *
 * In both versions we solve the quadratic for ||q-p||.
 * Version 1:
 * gFM is magnitude of first partials and gFM2 is magnitude of 2nd partials (as derived from paper)
 * builder->fsCodeAppend("\t\tedgeAlpha = (sqrt(gFM*gFM+4.0*func*gF2M) - gFM)/(2.0*gF2M);\n");
 * Version 2:
 * builder->fsCodeAppend("\t\tedgeAlpha = (gFM - sqrt(gFM*gFM-4.0*func*gF2M))/(2.0*gF2M);\n");
 *
 * Also note that 2nd partials of k,l,m are zero
 *
 * When comparing the two second order approximations to the first order approximations,
 * the following results were found. Version 1 tends to underestimate the distances, thus it
 * basically increases all the error that we were already seeing in the first order
 * approx. So this version is not the one to use. Version 2 has the opposite effect
 * and tends to overestimate the distances. This is much closer to what we are
 * looking for. It is able to render ellipses (even thin ones) without the need to chop.
 * However, it can not handle thin hyperbolas well and thus would still rely on
 * chopping to tighten the clipping. Another side effect of the overestimating is
 * that the curves become much thinner and "ropey". If all that was ever rendered
 * were "not too thin" curves and ellipses then 2nd order may have an advantage since
 * only one geometry would need to be rendered. However no benches were run comparing
 * chopped first order and non chopped 2nd order.
 */
class GrConicEffect : public GrGeometryProcessor {
public:
    static GrGeometryProcessor* Make(SkArenaAlloc* arena,
                                     const SkPMColor4f& color,
                                     const SkMatrix& viewMatrix,
                                     const GrCaps& caps,
                                     const SkMatrix& localMatrix,
                                     bool usesLocalCoords,
                                     uint8_t coverage = 0xff) {
        if (!caps.shaderCaps()->fShaderDerivativeSupport) {
            return nullptr;
        }
 
        return arena->make([&](void* ptr) {
            return new (ptr) GrConicEffect(color, viewMatrix, coverage, localMatrix,
                                           usesLocalCoords);
        });
    }
 
    ~GrConicEffect() override;
 
    const char* name() const override { return "Conic"; }
 
    void addToKey(const GrShaderCaps& caps, skgpu::KeyBuilder* b) const override;
 
    std::unique_ptr<ProgramImpl> makeProgramImpl(const GrShaderCaps&) const override;
 
private:
    class Impl;
 
    GrConicEffect(const SkPMColor4f&, const SkMatrix& viewMatrix, uint8_t coverage,
                  const SkMatrix& localMatrix, bool usesLocalCoords);
 
    inline const Attribute& inPosition() const { return kAttributes[0]; }
    inline const Attribute& inConicCoeffs() const { return kAttributes[1]; }
 
    SkPMColor4f         fColor;
    SkMatrix            fViewMatrix;
    SkMatrix            fLocalMatrix;
    bool                fUsesLocalCoords;
    uint8_t             fCoverageScale;
    inline static constexpr Attribute kAttributes[] = {
        {"inPosition", kFloat2_GrVertexAttribType, SkSLType::kFloat2},
        {"inConicCoeffs", kFloat4_GrVertexAttribType, SkSLType::kHalf4}
    };
 
    GR_DECLARE_GEOMETRY_PROCESSOR_TEST
 
    using INHERITED = GrGeometryProcessor;
};
 
///////////////////////////////////////////////////////////////////////////////
/**
 * The output of this effect is a hairline edge for quadratics.
 * Quadratic specified by 0=u^2-v canonical coords. u and v are the first
 * two components of the vertex attribute. At the three control points that define
 * the Quadratic, u, v have the values {0,0}, {1/2, 0}, and {1, 1} respectively.
 * Coverage for AA is min(0, 1-distance). 3rd & 4th cimponent unused.
 * Requires shader derivative instruction support.
 */
class GrQuadEffect : public GrGeometryProcessor {
public:
    static GrGeometryProcessor* Make(SkArenaAlloc* arena,
                                     const SkPMColor4f& color,
                                     const SkMatrix& viewMatrix,
                                     const GrCaps& caps,
                                     const SkMatrix& localMatrix,
                                     bool usesLocalCoords,
                                     uint8_t coverage = 0xff) {
        if (!caps.shaderCaps()->fShaderDerivativeSupport) {
            return nullptr;
        }
 
        return arena->make([&](void* ptr) {
            return new (ptr) GrQuadEffect(color, viewMatrix, coverage, localMatrix,
                                          usesLocalCoords);
        });
    }
 
    ~GrQuadEffect() override;
 
    const char* name() const override { return "Quad"; }
 
    void addToKey(const GrShaderCaps& caps, skgpu::KeyBuilder* b) const override;
 
    std::unique_ptr<ProgramImpl> makeProgramImpl(const GrShaderCaps&) const override;
 
private:
    class Impl;
 
    GrQuadEffect(const SkPMColor4f&, const SkMatrix& viewMatrix, uint8_t coverage,
                 const SkMatrix& localMatrix, bool usesLocalCoords);
 
    inline const Attribute& inPosition() const { return kAttributes[0]; }
    inline const Attribute& inHairQuadEdge() const { return kAttributes[1]; }
 
    SkPMColor4f fColor;
    SkMatrix fViewMatrix;
    SkMatrix fLocalMatrix;
    bool fUsesLocalCoords;
    uint8_t fCoverageScale;
 
    inline static constexpr Attribute kAttributes[] = {
        {"inPosition", kFloat2_GrVertexAttribType, SkSLType::kFloat2},
        {"inHairQuadEdge", kFloat4_GrVertexAttribType, SkSLType::kHalf4}
    };
 
    GR_DECLARE_GEOMETRY_PROCESSOR_TEST
 
    using INHERITED = GrGeometryProcessor;
};
 
#endif