src/gpu/effects/GrRectBlurEffect.fp

/*
 * Copyright 2018 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

@header {
#include <cmath>
#include "include/core/SkRect.h"
#include "include/core/SkScalar.h"
#include "include/gpu/GrContext.h"
#include "include/private/GrRecordingContext.h"
#include "src/core/SkBlurMask.h"
#include "src/core/SkMathPriv.h"
#include "src/gpu/GrBitmapTextureMaker.h"
#include "src/gpu/GrProxyProvider.h"
#include "src/gpu/GrRecordingContextPriv.h"
#include "src/gpu/GrShaderCaps.h"
#include "src/gpu/effects/GrTextureEffect.h"
}

in fragmentProcessor? inputFP;
in float4 rect;

layout(key) bool highp = abs(rect.x) > 16000 || abs(rect.y) > 16000 ||
                         abs(rect.z) > 16000 || abs(rect.w) > 16000;

layout(when= highp) uniform float4 rectF;
layout(when=!highp) uniform half4  rectH;

// Effect that is a LUT for integral of normal distribution. The value at x:[0,6*sigma] is the
// integral from -inf to (3*sigma - x). I.e. x is mapped from [0, 6*sigma] to [3*sigma to -3*sigma].
// The flip saves a reversal in the shader.
in fragmentProcessor integral;

// There is a fast variant of the effect that does 2 texture lookups and a more general one for
// wider blurs relative to rect sizes that does 4.
layout(key) in bool isFast;

@optimizationFlags {
    (inputFP ? ProcessorOptimizationFlags(inputFP.get()) : kAll_OptimizationFlags) &
            kCompatibleWithCoverageAsAlpha_OptimizationFlag
}

@constructorParams {
    GrSamplerState samplerParams
}

@samplerParams(integral) {
    samplerParams
}

@class {
static std::unique_ptr<GrFragmentProcessor> MakeIntegralFP(GrRecordingContext* context,
                                                           float sixSigma) {
    // The texture we're producing represents the integral of a normal distribution over a six-sigma
    // range centered at zero. We want enough resolution so that the linear interpolation done in
    // texture lookup doesn't introduce noticeable artifacts. We conservatively choose to have 2
    // texels for each dst pixel.
    int minWidth = 2 * sk_float_ceil2int(sixSigma);
    // Bin by powers of 2 with a minimum so we get good profile reuse.
    int width = std::max(SkNextPow2(minWidth), 32);

    static const GrUniqueKey::Domain kDomain = GrUniqueKey::GenerateDomain();
    GrUniqueKey key;
    GrUniqueKey::Builder builder(&key, kDomain, 1, "Rect Blur Mask");
    builder[0] = width;
    builder.finish();

    SkMatrix m = SkMatrix::Scale(width/sixSigma, 1.f);

    GrProxyProvider* proxyProvider = context->priv().proxyProvider();
    if (sk_sp<GrTextureProxy> proxy = proxyProvider->findOrCreateProxyByUniqueKey(key)) {
        GrSwizzle swizzle = context->priv().caps()->getReadSwizzle(proxy->backendFormat(),
                                                                   GrColorType::kAlpha_8);
        GrSurfaceProxyView view{std::move(proxy), kTopLeft_GrSurfaceOrigin, swizzle};
        return GrTextureEffect::Make(
                std::move(view), kPremul_SkAlphaType, m, GrSamplerState::Filter::kBilerp);
    }

    SkBitmap bitmap;
    if (!bitmap.tryAllocPixels(SkImageInfo::MakeA8(width, 1))) {
        return {};
    }
    *bitmap.getAddr8(0, 0) = 255;
    const float invWidth = 1.f / width;
    for (int i = 1; i < width - 1; ++i) {
        float x = (i + 0.5f) * invWidth;
        x = (-6 * x + 3) * SK_ScalarRoot2Over2;
        float integral = 0.5f * (std::erf(x) + 1.f);
        *bitmap.getAddr8(i, 0) = SkToU8(sk_float_round2int(255.f * integral));
    }
    *bitmap.getAddr8(width - 1, 0) = 0;
    bitmap.setImmutable();

    GrBitmapTextureMaker maker(context, bitmap, GrImageTexGenPolicy::kNew_Uncached_Budgeted);
    auto view = maker.view(GrMipMapped::kNo);
    if (!view) {
        return {};
    }
    SkASSERT(view.origin() == kTopLeft_GrSurfaceOrigin);
    proxyProvider->assignUniqueKeyToProxy(key, view.asTextureProxy());
    return GrTextureEffect::Make(
            std::move(view), kPremul_SkAlphaType, m, GrSamplerState::Filter::kBilerp);
}
}

@make {
     static std::unique_ptr<GrFragmentProcessor> Make(std::unique_ptr<GrFragmentProcessor> inputFP,
                                                      GrRecordingContext* context,
                                                      const GrShaderCaps& caps,
                                                      const SkRect& rect, float sigma) {
         SkASSERT(rect.isSorted());
         if (!caps.floatIs32Bits()) {
             // We promote the math that gets us into the Gaussian space to full float when the rect
             // coords are large. If we don't have full float then fail. We could probably clip the
             // rect to an outset device bounds instead.
             if (SkScalarAbs(rect.fLeft)  > 16000.f || SkScalarAbs(rect.fTop)    > 16000.f ||
                 SkScalarAbs(rect.fRight) > 16000.f || SkScalarAbs(rect.fBottom) > 16000.f) {
                    return nullptr;
             }
         }

         const float sixSigma = 6 * sigma;
         std::unique_ptr<GrFragmentProcessor> integral = MakeIntegralFP(context, sixSigma);
         if (!integral) {
             return nullptr;
         }

         // In the fast variant we think of the midpoint of the integral texture as aligning
         // with the closest rect edge both in x and y. To simplify texture coord calculation we
         // inset the rect so that the edge of the inset rect corresponds to t = 0 in the texture.
         // It actually simplifies things a bit in the !isFast case, too.
         float threeSigma = sixSigma / 2;
         SkRect insetRect = {rect.fLeft   + threeSigma,
                             rect.fTop    + threeSigma,
                             rect.fRight  - threeSigma,
                             rect.fBottom - threeSigma};

         // In our fast variant we find the nearest horizontal and vertical edges and for each
         // do a lookup in the integral texture for each and multiply them. When the rect is
         // less than 6 sigma wide then things aren't so simple and we have to consider both the
         // left and right edge of the rectangle (and similar in y).
         bool isFast = insetRect.isSorted();
         return std::unique_ptr<GrFragmentProcessor>(new GrRectBlurEffect(
                    std::move(inputFP), insetRect, std::move(integral),
                    isFast, GrSamplerState::Filter::kBilerp));
     }
}

void main() {
    half xCoverage, yCoverage;
    @if (isFast) {
        // Get the smaller of the signed distance from the frag coord to the left and right
        // edges and similar for y.
        // The integral texture goes "backwards" (from 3*sigma to -3*sigma), So, the below
        // computations align the left edge of the integral texture with the inset rect's edge
        // extending outward 6 * sigma from the inset rect.
        half2 xy;
        @if (highp) {
            xy = max(half2(rectF.LT - sk_FragCoord.xy),
                     half2(sk_FragCoord.xy - rectF.RB));
       } else {
            xy = max(half2(rectH.LT - sk_FragCoord.xy),
                     half2(sk_FragCoord.xy - rectH.RB));
        }
        xCoverage = sample(integral, half2(xy.x, 0.5)).a;
        yCoverage = sample(integral, half2(xy.y, 0.5)).a;
    } else {
        // We just consider just the x direction here. In practice we compute x and y separately
        // and multiply them together.
        // We define our coord system so that the point at which we're evaluating a kernel
        // defined by the normal distribution (K) at 0. In this coord system let L be left
        // edge and R be the right edge of the rectangle.
        // We can calculate C by integrating K with the half infinite ranges outside the L to R
        // range and subtracting from 1:
        //   C = 1 - <integral of K from from -inf to  L> - <integral of K from R to inf>
        // K is symmetric about x=0 so:
        //   C = 1 - <integral of K from from -inf to  L> - <integral of K from -inf to -R>

        // The integral texture goes "backwards" (from 3*sigma to -3*sigma) which is factored
        // in to the below calculations.
        // Also, our rect uniform was pre-inset by 3 sigma from the actual rect being blurred,
        // also factored in.
        half4 rect;
        @if (highp) {
            rect.LT = half2(rectF.LT - sk_FragCoord.xy);
            rect.RB = half2(sk_FragCoord.xy - rectF.RB);
        } else {
            rect.LT = half2(rectH.LT - sk_FragCoord.xy);
            rect.RB = half2(sk_FragCoord.xy - rectH.RB);
        }
        xCoverage = 1 - sample(integral, half2(rect.L, 0.5)).a
                      - sample(integral, half2(rect.R, 0.5)).a;
        yCoverage = 1 - sample(integral, half2(rect.T, 0.5)).a
                      - sample(integral, half2(rect.B, 0.5)).a;
    }
    half4 inputColor = sample(inputFP, sk_InColor);
    sk_OutColor = inputColor * xCoverage * yCoverage;
}

@setData(pdman) {
    float r[] {rect.fLeft, rect.fTop, rect.fRight, rect.fBottom};
    pdman.set4fv(highp ? rectF : rectH, 1, r);
}

@test(data) {
    float sigma = data->fRandom->nextRangeF(3,8);
    float width = data->fRandom->nextRangeF(200,300);
    float height = data->fRandom->nextRangeF(200,300);
    return GrRectBlurEffect::Make(/*inputFP=*/nullptr, data->context(), *data->caps()->shaderCaps(),
                                  SkRect::MakeWH(width, height), sigma);
}