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LibGfx: Add FastBoxBlurFilter
This patch adds a FastBoxBlurFilter to the system. It can be created by specifying a Bitmap it will work on. There are two uses implemented: - apply_single_pass() applys an implementation of a linear-time box-blur algorithm with the specified radius using a horizontal and a vertical pass and utilizinga sliding window. - apply_three_passes() gets a better Gaussian approximation by applying the filter three times. For this to work the radius of each pass is calculated to fit Gauss the best.
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Userland/Libraries/LibGfx/Filters/FastBoxBlurFilter.h
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Userland/Libraries/LibGfx/Filters/FastBoxBlurFilter.h
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/*
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* Copyright (c) 2021, Tobias Christiansen <tobi@tobyase.de>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <LibCore/ElapsedTimer.h>
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#include <LibGfx/Bitmap.h>
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namespace Gfx {
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class FastBoxBlurFilter {
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public:
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FastBoxBlurFilter(Bitmap& bitmap)
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: m_bitmap(bitmap)
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{
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}
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// Based on the super fast blur algorithm by Quasimondo, explored here: https://stackoverflow.com/questions/21418892/understanding-super-fast-blur-algorithm
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void apply_single_pass(int radius)
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{
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VERIFY(radius >= 0);
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VERIFY(m_bitmap.format() == BitmapFormat::BGRA8888);
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int height = m_bitmap.height();
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int width = m_bitmap.width();
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int div = 2 * radius + 1;
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size_t sum_red, sum_green, sum_blue, sum_alpha;
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u8 intermediate_red[width * height];
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u8 intermediate_green[width * height];
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u8 intermediate_blue[width * height];
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u8 intermediate_alpha[width * height];
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// First pass: vertical
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for (int y = 0; y < height; y++) {
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sum_red = sum_green = sum_blue = sum_alpha = 0;
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// Setup sliding window
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for (int i = -radius; i <= radius; i++) {
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auto color_at_px = m_bitmap.get_pixel(clamp(i, 0, width - 1), y);
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sum_red += red_value(color_at_px);
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sum_green += green_value(color_at_px);
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sum_blue += blue_value(color_at_px);
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sum_alpha += color_at_px.alpha();
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}
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// Slide horizontally
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for (int x = 0; x < width; x++) {
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intermediate_red[y * width + x] = (sum_red / div);
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intermediate_green[y * width + x] = (sum_green / div);
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intermediate_blue[y * width + x] = (sum_blue / div);
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intermediate_alpha[y * width + x] = (sum_alpha / div);
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auto leftmost_x_coord = max(x - radius, 0);
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auto rightmost_x_coord = min(x + radius + 1, width - 1);
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auto leftmost_x_color = m_bitmap.get_pixel(leftmost_x_coord, y);
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auto rightmost_x_color = m_bitmap.get_pixel(rightmost_x_coord, y);
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sum_red -= red_value(leftmost_x_color);
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sum_red += red_value(rightmost_x_color);
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sum_green -= green_value(leftmost_x_color);
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sum_green += green_value(rightmost_x_color);
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sum_blue -= blue_value(leftmost_x_color);
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sum_blue += blue_value(rightmost_x_color);
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sum_alpha -= leftmost_x_color.alpha();
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sum_alpha += rightmost_x_color.alpha();
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}
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}
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// Second pass: horizontal
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for (int x = 0; x < width; x++) {
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sum_red = sum_green = sum_blue = sum_alpha = 0;
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// Setup sliding window
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for (int i = -radius; i <= radius; i++) {
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int offset = clamp(i, 0, height - 1) * width + x;
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sum_red += intermediate_red[offset];
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sum_green += intermediate_green[offset];
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sum_blue += intermediate_blue[offset];
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sum_alpha += intermediate_alpha[offset];
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}
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for (int y = 0; y < height; y++) {
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auto color = Color(
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sum_red / div,
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sum_green / div,
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sum_blue / div,
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sum_alpha / div);
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m_bitmap.set_pixel(x, y, color);
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auto topmost_y_coord = max(y - radius, 0);
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auto bottommost_y_coord = min(y + radius + 1, height - 1);
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sum_red += intermediate_red[x + bottommost_y_coord * width];
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sum_red -= intermediate_red[x + topmost_y_coord * width];
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sum_green += intermediate_green[x + bottommost_y_coord * width];
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sum_green -= intermediate_green[x + topmost_y_coord * width];
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sum_blue += intermediate_blue[x + bottommost_y_coord * width];
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sum_blue -= intermediate_blue[x + topmost_y_coord * width];
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sum_alpha += intermediate_alpha[x + bottommost_y_coord * width];
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sum_alpha -= intermediate_alpha[x + topmost_y_coord * width];
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}
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}
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}
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// Math from here: http://blog.ivank.net/fastest-gaussian-blur.html
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void apply_three_passes(size_t radius)
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{
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if (!radius)
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return;
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size_t no_of_passes = 3;
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double wIdeal = sqrt((12 * radius * radius / (double)no_of_passes) + 1);
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int wl = floor(wIdeal);
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if (wl % 2 == 0)
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wl--;
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int wu = wl - 2;
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double mIdeal = (12 * radius * radius - no_of_passes * wl * wl - 4 * no_of_passes * wl - 3 * no_of_passes) / (double)(-4 * wl - 4);
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int m = round(mIdeal);
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for (size_t i = 0; i < no_of_passes; i++) {
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int weighted_radius = (int)i < m ? wl : wu;
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if (weighted_radius < 2)
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continue;
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apply_single_pass((weighted_radius - 1) / 2);
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}
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}
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private:
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ALWAYS_INLINE static u8 red_value(Color const& color)
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{
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return (color.alpha() == 0) ? 0xFF : color.red();
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}
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ALWAYS_INLINE static u8 green_value(Color const& color)
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{
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return (color.alpha() == 0) ? 0xFF : color.green();
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}
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ALWAYS_INLINE static u8 blue_value(Color const& color)
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{
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return (color.alpha() == 0) ? 0xFF : color.blue();
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}
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Bitmap& m_bitmap;
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};
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}
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