src/gpu/ganesh/effects/GrDisableColorXP.cpp

/*
 * Copyright 2014 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "src/gpu/ganesh/effects/GrDisableColorXP.h"

#include "src/gpu/Blend.h"
#include "src/gpu/ganesh/GrShaderCaps.h"
#include "src/gpu/ganesh/GrXferProcessor.h"
#include "src/gpu/ganesh/glsl/GrGLSLFragmentShaderBuilder.h"

#include <memory>

namespace skgpu {
class KeyBuilder;
class Swizzle;
}

/**
 * This xfer processor disables color writing. Thus color and coverage and ignored and no blending
 * occurs. This XP is usful for things like stenciling.
 */
class DisableColorXP : public GrXferProcessor {
public:
    DisableColorXP() : INHERITED(kDisableColorXP_ClassID) {}

private:
    const char* name() const override { return "Disable Color"; }
    bool onIsEqual(const GrXferProcessor& xpBase) const override { return true; }
    void onAddToKey(const GrShaderCaps&, skgpu::KeyBuilder*) const override {}
    void onGetBlendInfo(skgpu::BlendInfo* blendInfo) const override {
        blendInfo->fWritesColor = false;
    }
    std::unique_ptr<ProgramImpl> makeProgramImpl() const override;

    using INHERITED = GrXferProcessor;
};

std::unique_ptr<GrXferProcessor::ProgramImpl> DisableColorXP::makeProgramImpl() const {
    class Impl : public ProgramImpl {
    private:
        void emitOutputsForBlendState(const EmitArgs& args) override {
            if (args.fShaderCaps->fMustWriteToFragColor) {
                // This emit code should be empty. However, on the nexus 6 there is a driver bug
                // where if you do not give gl_FragColor a value, the gl context is lost and we end
                // up drawing nothing. So this fix just sets the gl_FragColor arbitrarily to 0.
                // https://bugs.chromium.org/p/chromium/issues/detail?id=445377
                GrGLSLXPFragmentBuilder* fragBuilder = args.fXPFragBuilder;
                fragBuilder->codeAppendf("%s = half4(0);", args.fOutputPrimary);
            }
        }

        void emitWriteSwizzle(GrGLSLXPFragmentBuilder*,
                              const skgpu::Swizzle&,
                              const char*,
                              const char*) const override {
            // Don't write any swizzling. This makes sure the final shader does not output a color.
        }
    };

    return std::make_unique<Impl>();
}

sk_sp<const GrXferProcessor> GrDisableColorXPFactory::MakeXferProcessor() {
    return sk_make_sp<DisableColorXP>();
}

GR_DEFINE_XP_FACTORY_TEST(GrDisableColorXPFactory)

#if defined(GPU_TEST_UTILS)
const GrXPFactory* GrDisableColorXPFactory::TestGet(GrProcessorTestData*) {
    return GrDisableColorXPFactory::Get();
}
#endif