src/effects/GrCircleBlurFragmentProcessor.cpp

/*
 * Copyright 2015 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "GrCircleBlurFragmentProcessor.h"

#if SK_SUPPORT_GPU

#include "GrContext.h"
#include "GrTextureProvider.h"
#include "glsl/GrGLSLFragmentProcessor.h"
#include "glsl/GrGLSLFragmentShaderBuilder.h"
#include "glsl/GrGLSLProgramDataManager.h"
#include "glsl/GrGLSLUniformHandler.h"

#include "SkFixed.h"

class GrCircleBlurFragmentProcessor::GLSLProcessor : public GrGLSLFragmentProcessor {
public:
    void emitCode(EmitArgs&) override;

protected:
    void onSetData(const GrGLSLProgramDataManager&, const GrProcessor&) override;

private:
    GrGLSLProgramDataManager::UniformHandle fDataUniform;

    typedef GrGLSLFragmentProcessor INHERITED;
};

void GrCircleBlurFragmentProcessor::GLSLProcessor::emitCode(EmitArgs& args) {
    const char *dataName;

    // The data is formatted as:
    // x,y  - the center of the circle
    // z    - inner radius that should map to 0th entry in the texture.
    // w    - the inverse of the distance over which the texture is stretched.
    fDataUniform = args.fUniformHandler->addUniform(kFragment_GrShaderFlag,
                                                    kVec4f_GrSLType,
                                                    kDefault_GrSLPrecision,
                                                    "data",
                                                    &dataName);

    GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;

    if (args.fInputColor) {
        fragBuilder->codeAppendf("vec4 src=%s;", args.fInputColor);
    } else {
        fragBuilder->codeAppendf("vec4 src=vec4(1);");
    }

    // We just want to compute "(length(vec) - %s.z + 0.5) * %s.w" but need to rearrange
    // for precision.
    fragBuilder->codeAppendf("vec2 vec = vec2( (sk_FragCoord.x - %s.x) * %s.w, "
                                              "(sk_FragCoord.y - %s.y) * %s.w );",
                             dataName, dataName, dataName, dataName);
    fragBuilder->codeAppendf("float dist = length(vec) + (0.5 - %s.z) * %s.w;",
                             dataName, dataName);

    fragBuilder->codeAppendf("float intensity = ");
    fragBuilder->appendTextureLookup(args.fTexSamplers[0], "vec2(dist, 0.5)");
    fragBuilder->codeAppend(".a;");

    fragBuilder->codeAppendf("%s = src * intensity;\n", args.fOutputColor );
}

void GrCircleBlurFragmentProcessor::GLSLProcessor::onSetData(const GrGLSLProgramDataManager& pdman,
                                                             const GrProcessor& proc) {
    const GrCircleBlurFragmentProcessor& cbfp = proc.cast<GrCircleBlurFragmentProcessor>();
    const SkRect& circle = cbfp.fCircle;

    // The data is formatted as:
    // x,y  - the center of the circle
    // z    - inner radius that should map to 0th entry in the texture.
    // w    - the inverse of the distance over which the profile texture is stretched.
    pdman.set4f(fDataUniform, circle.centerX(), circle.centerY(), cbfp.fSolidRadius,
                1.f / cbfp.fTextureRadius);
}

///////////////////////////////////////////////////////////////////////////////

GrCircleBlurFragmentProcessor::GrCircleBlurFragmentProcessor(const SkRect& circle,
                                                             float textureRadius,
                                                             float solidRadius,
                                                             GrTexture* blurProfile)
        : INHERITED(kCompatibleWithCoverageAsAlpha_OptimizationFlag)
        , fCircle(circle)
        , fSolidRadius(solidRadius)
        , fTextureRadius(textureRadius)
        , fBlurProfileSampler(blurProfile, GrSamplerParams::kBilerp_FilterMode) {
    this->initClassID<GrCircleBlurFragmentProcessor>();
    this->addTextureSampler(&fBlurProfileSampler);
}

GrGLSLFragmentProcessor* GrCircleBlurFragmentProcessor::onCreateGLSLInstance() const {
    return new GLSLProcessor;
}

void GrCircleBlurFragmentProcessor::onGetGLSLProcessorKey(const GrShaderCaps& caps,
                                                          GrProcessorKeyBuilder* b) const {
    // The code for this processor is always the same so there is nothing to add to the key.
    return;
}

// Computes an unnormalized half kernel (right side). Returns the summation of all the half kernel
// values.
static float make_unnormalized_half_kernel(float* halfKernel, int halfKernelSize, float sigma) {
    const float invSigma = 1.f / sigma;
    const float b = -0.5f * invSigma * invSigma;
    float tot = 0.0f;
    // Compute half kernel values at half pixel steps out from the center.
    float t = 0.5f;
    for (int i = 0; i < halfKernelSize; ++i) {
        float value = expf(t * t * b);
        tot += value;
        halfKernel[i] = value;
        t += 1.f;
    }
    return tot;
}

// Create a Gaussian half-kernel (right side) and a summed area table given a sigma and number of
// discrete steps. The half kernel is normalized to sum to 0.5.
static void make_half_kernel_and_summed_table(float* halfKernel, float* summedHalfKernel,
                                              int halfKernelSize, float sigma) {
    // The half kernel should sum to 0.5 not 1.0.
    const float tot = 2.f * make_unnormalized_half_kernel(halfKernel, halfKernelSize, sigma);
    float sum = 0.f;
    for (int i = 0; i < halfKernelSize; ++i) {
        halfKernel[i] /= tot;
        sum += halfKernel[i];
        summedHalfKernel[i] = sum;
    }
}

// Applies the 1D half kernel vertically at points along the x axis to a circle centered at the
// origin with radius circleR.
void apply_kernel_in_y(float* results, int numSteps, float firstX, float circleR,
                       int halfKernelSize, const float* summedHalfKernelTable) {
    float x = firstX;
    for (int i = 0; i < numSteps; ++i, x += 1.f) {
        if (x < -circleR || x > circleR) {
            results[i] = 0;
            continue;
        }
        float y = sqrtf(circleR * circleR - x * x);
        // In the column at x we exit the circle at +y and -y
        // The summed table entry j is actually reflects an offset of j + 0.5.
        y -= 0.5f;
        int yInt = SkScalarFloorToInt(y);
        SkASSERT(yInt >= -1);
        if (y < 0) {
            results[i] = (y + 0.5f) * summedHalfKernelTable[0];
        } else if (yInt >= halfKernelSize - 1) {
            results[i] = 0.5f;
        } else {
            float yFrac = y - yInt;
            results[i] = (1.f - yFrac) * summedHalfKernelTable[yInt] +
                         yFrac * summedHalfKernelTable[yInt + 1];
        }
    }
}

// Apply a Gaussian at point (evalX, 0) to a circle centered at the origin with radius circleR.
// This relies on having a half kernel computed for the Gaussian and a table of applications of
// the half kernel in y to columns at (evalX - halfKernel, evalX - halfKernel + 1, ..., evalX +
// halfKernel) passed in as yKernelEvaluations.
static uint8_t eval_at(float evalX, float circleR, const float* halfKernel, int halfKernelSize,
                       const float* yKernelEvaluations) {
    float acc = 0;

    float x = evalX - halfKernelSize;
    for (int i = 0; i < halfKernelSize; ++i, x += 1.f) {
        if (x < -circleR || x > circleR) {
            continue;
        }
        float verticalEval = yKernelEvaluations[i];
        acc += verticalEval * halfKernel[halfKernelSize - i - 1];
    }
    for (int i = 0; i < halfKernelSize; ++i, x += 1.f) {
        if (x < -circleR || x > circleR) {
            continue;
        }
        float verticalEval = yKernelEvaluations[i + halfKernelSize];
        acc += verticalEval * halfKernel[i];
    }
    // Since we applied a half kernel in y we multiply acc by 2 (the circle is symmetric about the
    // x axis).
    return SkUnitScalarClampToByte(2.f * acc);
}

// This function creates a profile of a blurred circle. It does this by computing a kernel for
// half the Gaussian and a matching summed area table. The summed area table is used to compute
// an array of vertical applications of the half kernel to the circle along the x axis. The table
// of y evaluations has 2 * k + n entries where k is the size of the half kernel and n is the size
// of the profile being computed. Then for each of the n profile entries we walk out k steps in each
// horizontal direction multiplying the corresponding y evaluation by the half kernel entry and
// sum these values to compute the profile entry.
static uint8_t* create_circle_profile(float sigma, float circleR, int profileTextureWidth) {
    const int numSteps = profileTextureWidth;
    uint8_t* weights = new uint8_t[numSteps];

    // The full kernel is 6 sigmas wide.
    int halfKernelSize = SkScalarCeilToInt(6.0f*sigma);
    // round up to next multiple of 2 and then divide by 2
    halfKernelSize = ((halfKernelSize + 1) & ~1) >> 1;

    // Number of x steps at which to apply kernel in y to cover all the profile samples in x.
    int numYSteps = numSteps + 2 * halfKernelSize;

    SkAutoTArray<float> bulkAlloc(halfKernelSize + halfKernelSize + numYSteps);
    float* halfKernel = bulkAlloc.get();
    float* summedKernel = bulkAlloc.get() + halfKernelSize;
    float* yEvals = bulkAlloc.get() + 2 * halfKernelSize;
    make_half_kernel_and_summed_table(halfKernel, summedKernel, halfKernelSize, sigma);

    float firstX = -halfKernelSize + 0.5f;
    apply_kernel_in_y(yEvals, numYSteps, firstX, circleR, halfKernelSize, summedKernel);

    for (int i = 0; i < numSteps - 1; ++i) {
        float evalX = i + 0.5f;
        weights[i] = eval_at(evalX, circleR, halfKernel, halfKernelSize, yEvals + i);
    }
    // Ensure the tail of the Gaussian goes to zero.
    weights[numSteps - 1] = 0;
    return weights;
}

static uint8_t* create_half_plane_profile(int profileWidth) {
    SkASSERT(!(profileWidth & 0x1));
    // The full kernel is 6 sigmas wide.
    float sigma = profileWidth / 6.f;
    int halfKernelSize = profileWidth / 2;

    SkAutoTArray<float> halfKernel(halfKernelSize);
    uint8_t* profile = new uint8_t[profileWidth];

    // The half kernel should sum to 0.5.
    const float tot = 2.f * make_unnormalized_half_kernel(halfKernel.get(), halfKernelSize, sigma);
    float sum = 0.f;
    // Populate the profile from the right edge to the middle.
    for (int i = 0; i < halfKernelSize; ++i) {
        halfKernel[halfKernelSize - i - 1] /= tot;
        sum += halfKernel[halfKernelSize - i - 1];
        profile[profileWidth - i - 1] = SkUnitScalarClampToByte(sum);
    }
    // Populate the profile from the middle to the left edge (by flipping the half kernel and
    // continuing the summation).
    for (int i = 0; i < halfKernelSize; ++i) {
        sum += halfKernel[i];
        profile[halfKernelSize - i - 1] = SkUnitScalarClampToByte(sum);
    }
    // Ensure tail goes to 0.
    profile[profileWidth - 1] = 0;
    return profile;
}

static GrTexture* create_profile_texture(GrTextureProvider* textureProvider, const SkRect& circle,
                                         float sigma, float* solidRadius, float* textureRadius) {
    float circleR = circle.width() / 2.0f;
    // Profile textures are cached by the ratio of sigma to circle radius and by the size of the
    // profile texture (binned by powers of 2).
    SkScalar sigmaToCircleRRatio = sigma / circleR;
    // When sigma is really small this becomes a equivalent to convolving a Gaussian with a half-
    // plane. Similarly, in the extreme high ratio cases circle becomes a point WRT to the Guassian
    // and the profile texture is a just a Gaussian evaluation. However, we haven't yet implemented
    // this latter optimization.
    sigmaToCircleRRatio = SkTMin(sigmaToCircleRRatio, 8.f);
    SkFixed sigmaToCircleRRatioFixed;
    static const SkScalar kHalfPlaneThreshold = 0.1f;
    bool useHalfPlaneApprox = false;
    if (sigmaToCircleRRatio <= kHalfPlaneThreshold) {
        useHalfPlaneApprox = true;
        sigmaToCircleRRatioFixed = 0;
        *solidRadius = circleR - 3 * sigma;
        *textureRadius = 6 * sigma;
    } else {
        // Convert to fixed point for the key.
        sigmaToCircleRRatioFixed = SkScalarToFixed(sigmaToCircleRRatio);
        // We shave off some bits to reduce the number of unique entries. We could probably shave
        // off more than we do.
        sigmaToCircleRRatioFixed &= ~0xff;
        sigmaToCircleRRatio = SkFixedToScalar(sigmaToCircleRRatioFixed);
        sigma = circleR * sigmaToCircleRRatio;
        *solidRadius = 0;
        *textureRadius = circleR + 3 * sigma;
    }

    static const GrUniqueKey::Domain kDomain = GrUniqueKey::GenerateDomain();
    GrUniqueKey key;
    GrUniqueKey::Builder builder(&key, kDomain, 1);
    builder[0] = sigmaToCircleRRatioFixed;
    builder.finish();

    GrTexture *blurProfile = textureProvider->findAndRefTextureByUniqueKey(key);
    if (!blurProfile) {
        static constexpr int kProfileTextureWidth = 512;
        GrSurfaceDesc texDesc;
        texDesc.fWidth = kProfileTextureWidth;
        texDesc.fHeight = 1;
        texDesc.fConfig = kAlpha_8_GrPixelConfig;

        std::unique_ptr<uint8_t[]> profile(nullptr);
        if (useHalfPlaneApprox) {
            profile.reset(create_half_plane_profile(kProfileTextureWidth));
        } else {
            // Rescale params to the size of the texture we're creating.
            SkScalar scale = kProfileTextureWidth / *textureRadius;
            profile.reset(create_circle_profile(sigma * scale, circleR * scale,
                                                kProfileTextureWidth));
        }

        blurProfile = textureProvider->createTexture(texDesc, SkBudgeted::kYes, profile.get(), 0);
        if (blurProfile) {
            textureProvider->assignUniqueKeyToTexture(key, blurProfile);
        }
    }

    return blurProfile;
}

//////////////////////////////////////////////////////////////////////////////

sk_sp<GrFragmentProcessor> GrCircleBlurFragmentProcessor::Make(GrTextureProvider*textureProvider,
                                                               const SkRect& circle, float sigma) {
    float solidRadius;
    float textureRadius;
    sk_sp<GrTexture> profile(create_profile_texture(textureProvider, circle, sigma,
                                                    &solidRadius, &textureRadius));
    if (!profile) {
        return nullptr;
    }
    return sk_sp<GrFragmentProcessor>(
            new GrCircleBlurFragmentProcessor(circle, textureRadius, solidRadius, profile.get()));
}

//////////////////////////////////////////////////////////////////////////////

GR_DEFINE_FRAGMENT_PROCESSOR_TEST(GrCircleBlurFragmentProcessor);

#if GR_TEST_UTILS
sk_sp<GrFragmentProcessor> GrCircleBlurFragmentProcessor::TestCreate(GrProcessorTestData* d) {
    SkScalar wh = d->fRandom->nextRangeScalar(100.f, 1000.f);
    SkScalar sigma = d->fRandom->nextRangeF(1.f,10.f);
    SkRect circle = SkRect::MakeWH(wh, wh);
    return GrCircleBlurFragmentProcessor::Make(d->context()->textureProvider(), circle, sigma);
}
#endif

#endif