Source/core/platform/graphics/ShadowBlur.cpp

/*
 * Copyright (C) 2011 Apple Inc. All rights reserved.
 * Copyright (C) 2010 Sencha, Inc. All rights reserved.
 * Copyright (C) 2010 Igalia S.L. All rights reserved.
 * Copyright (C) Research In Motion Limited 2011. All rights reserved.
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
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#include "config.h"
#include "core/platform/graphics/ShadowBlur.h"

#include "wtf/MathExtras.h"
#include <algorithm>

using namespace std;

namespace WebCore {

enum {
    leftLobe = 0,
    rightLobe = 1
};

ShadowBlur::ShadowBlur(const FloatSize& radius, const FloatSize& offset, const Color& color)
    : m_color(color)
    , m_blurRadius(radius)
    , m_offset(offset)
    , m_shadowsIgnoreTransforms(false)
{
    updateShadowBlurValues();
}

void ShadowBlur::updateShadowBlurValues()
{
    // Limit blur radius to 128 to avoid lots of very expensive blurring.
    m_blurRadius = m_blurRadius.shrunkTo(FloatSize(128, 128));

    // The type of shadow is decided by the blur radius, shadow offset, and shadow color.
    if (!m_color.isValid() || !m_color.alpha()) {
        // Can't paint the shadow with invalid or invisible color.
        m_type = NoShadow;
    } else if (m_blurRadius.width() > 0 || m_blurRadius.height() > 0) {
        // Shadow is always blurred, even the offset is zero.
        m_type = BlurShadow;
    } else if (!m_offset.width() && !m_offset.height()) {
        // Without blur and zero offset means the shadow is fully hidden.
        m_type = NoShadow;
    } else
        m_type = SolidShadow;
}

// Instead of integer division, we use 17.15 for fixed-point division.
static const int blurSumShift = 15;

// Takes a two dimensional array with three rows and two columns for the lobes.
static void calculateLobes(int lobes[][2], float blurRadius, bool shadowsIgnoreTransforms)
{
    int diameter;
    if (shadowsIgnoreTransforms)
        diameter = max(2, static_cast<int>(floorf((2 / 3.f) * blurRadius))); // Canvas shadow. FIXME: we should adjust the blur radius higher up.
    else {
        // http://dev.w3.org/csswg/css3-background/#box-shadow
        // Approximate a Gaussian blur with a standard deviation equal to half the blur radius,
        // which http://www.w3.org/TR/SVG/filters.html#feGaussianBlurElement tell us how to do.
        // However, shadows rendered according to that spec will extend a little further than m_blurRadius,
        // so we apply a fudge factor to bring the radius down slightly.
        float stdDev = blurRadius / 2;
        const float gaussianKernelFactor = 3 / 4.f * sqrtf(2 * piFloat);
        const float fudgeFactor = 0.88f;
        diameter = max(2, static_cast<int>(floorf(stdDev * gaussianKernelFactor * fudgeFactor + 0.5f)));
    }

    if (diameter & 1) {
        // if d is odd, use three box-blurs of size 'd', centered on the output pixel.
        int lobeSize = (diameter - 1) / 2;
        lobes[0][leftLobe] = lobeSize;
        lobes[0][rightLobe] = lobeSize;
        lobes[1][leftLobe] = lobeSize;
        lobes[1][rightLobe] = lobeSize;
        lobes[2][leftLobe] = lobeSize;
        lobes[2][rightLobe] = lobeSize;
    } else {
        // if d is even, two box-blurs of size 'd' (the first one centered on the pixel boundary
        // between the output pixel and the one to the left, the second one centered on the pixel
        // boundary between the output pixel and the one to the right) and one box blur of size 'd+1' centered on the output pixel
        int lobeSize = diameter / 2;
        lobes[0][leftLobe] = lobeSize;
        lobes[0][rightLobe] = lobeSize - 1;
        lobes[1][leftLobe] = lobeSize - 1;
        lobes[1][rightLobe] = lobeSize;
        lobes[2][leftLobe] = lobeSize;
        lobes[2][rightLobe] = lobeSize;
    }
}

void ShadowBlur::blurLayerImage(unsigned char* imageData, const IntSize& size, int rowStride)
{
    const int channels[4] = { 3, 0, 1, 3 };

    int lobes[3][2]; // indexed by pass, and left/right lobe
    calculateLobes(lobes, m_blurRadius.width(), m_shadowsIgnoreTransforms);

    // First pass is horizontal.
    int stride = 4;
    int delta = rowStride;
    int final = size.height();
    int dim = size.width();

    // Two stages: horizontal and vertical
    for (int pass = 0; pass < 2; ++pass) {
        unsigned char* pixels = imageData;

        if (!pass && !m_blurRadius.width())
            final = 0; // Do no work if horizonal blur is zero.

        for (int j = 0; j < final; ++j, pixels += delta) {
            // For each step, we blur the alpha in a channel and store the result
            // in another channel for the subsequent step.
            // We use sliding window algorithm to accumulate the alpha values.
            // This is much more efficient than computing the sum of each pixels
            // covered by the box kernel size for each x.
            for (int step = 0; step < 3; ++step) {
                int side1 = lobes[step][leftLobe];
                int side2 = lobes[step][rightLobe];
                int pixelCount = side1 + 1 + side2;
                int invCount = ((1 << blurSumShift) + pixelCount - 1) / pixelCount;
                int ofs = 1 + side2;
                int alpha1 = pixels[channels[step]];
                int alpha2 = pixels[(dim - 1) * stride + channels[step]];

                unsigned char* ptr = pixels + channels[step + 1];
                unsigned char* prev = pixels + stride + channels[step];
                unsigned char* next = pixels + ofs * stride + channels[step];

                int i;
                int sum = side1 * alpha1 + alpha1;
                int limit = (dim < side2 + 1) ? dim : side2 + 1;

                for (i = 1; i < limit; ++i, prev += stride)
                    sum += *prev;

                if (limit <= side2)
                    sum += (side2 - limit + 1) * alpha2;

                limit = (side1 < dim) ? side1 : dim;
                for (i = 0; i < limit; ptr += stride, next += stride, ++i, ++ofs) {
                    *ptr = (sum * invCount) >> blurSumShift;
                    sum += ((ofs < dim) ? *next : alpha2) - alpha1;
                }

                prev = pixels + channels[step];
                for (; ofs < dim; ptr += stride, prev += stride, next += stride, ++i, ++ofs) {
                    *ptr = (sum * invCount) >> blurSumShift;
                    sum += (*next) - (*prev);
                }

                for (; i < dim; ptr += stride, prev += stride, ++i) {
                    *ptr = (sum * invCount) >> blurSumShift;
                    sum += alpha2 - (*prev);
                }
            }
        }

        // Last pass is vertical.
        stride = rowStride;
        delta = 4;
        final = size.width();
        dim = size.height();

        if (!m_blurRadius.height())
            break;

        if (m_blurRadius.width() != m_blurRadius.height())
            calculateLobes(lobes, m_blurRadius.height(), m_shadowsIgnoreTransforms);
    }
}

} // namespace WebCore