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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.
This commit is contained in:
Tobias Christiansen 2021-07-25 00:33:02 +02:00 committed by Linus Groh
parent 48d4062b47
commit d1844e424d

View file

@ -0,0 +1,150 @@
/*
* Copyright (c) 2021, Tobias Christiansen <tobi@tobyase.de>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <LibCore/ElapsedTimer.h>
#include <LibGfx/Bitmap.h>
namespace Gfx {
class FastBoxBlurFilter {
public:
FastBoxBlurFilter(Bitmap& bitmap)
: m_bitmap(bitmap)
{
}
// Based on the super fast blur algorithm by Quasimondo, explored here: https://stackoverflow.com/questions/21418892/understanding-super-fast-blur-algorithm
void apply_single_pass(int radius)
{
VERIFY(radius >= 0);
VERIFY(m_bitmap.format() == BitmapFormat::BGRA8888);
int height = m_bitmap.height();
int width = m_bitmap.width();
int div = 2 * radius + 1;
size_t sum_red, sum_green, sum_blue, sum_alpha;
u8 intermediate_red[width * height];
u8 intermediate_green[width * height];
u8 intermediate_blue[width * height];
u8 intermediate_alpha[width * height];
// First pass: vertical
for (int y = 0; y < height; y++) {
sum_red = sum_green = sum_blue = sum_alpha = 0;
// Setup sliding window
for (int i = -radius; i <= radius; i++) {
auto color_at_px = m_bitmap.get_pixel(clamp(i, 0, width - 1), y);
sum_red += red_value(color_at_px);
sum_green += green_value(color_at_px);
sum_blue += blue_value(color_at_px);
sum_alpha += color_at_px.alpha();
}
// Slide horizontally
for (int x = 0; x < width; x++) {
intermediate_red[y * width + x] = (sum_red / div);
intermediate_green[y * width + x] = (sum_green / div);
intermediate_blue[y * width + x] = (sum_blue / div);
intermediate_alpha[y * width + x] = (sum_alpha / div);
auto leftmost_x_coord = max(x - radius, 0);
auto rightmost_x_coord = min(x + radius + 1, width - 1);
auto leftmost_x_color = m_bitmap.get_pixel(leftmost_x_coord, y);
auto rightmost_x_color = m_bitmap.get_pixel(rightmost_x_coord, y);
sum_red -= red_value(leftmost_x_color);
sum_red += red_value(rightmost_x_color);
sum_green -= green_value(leftmost_x_color);
sum_green += green_value(rightmost_x_color);
sum_blue -= blue_value(leftmost_x_color);
sum_blue += blue_value(rightmost_x_color);
sum_alpha -= leftmost_x_color.alpha();
sum_alpha += rightmost_x_color.alpha();
}
}
// Second pass: horizontal
for (int x = 0; x < width; x++) {
sum_red = sum_green = sum_blue = sum_alpha = 0;
// Setup sliding window
for (int i = -radius; i <= radius; i++) {
int offset = clamp(i, 0, height - 1) * width + x;
sum_red += intermediate_red[offset];
sum_green += intermediate_green[offset];
sum_blue += intermediate_blue[offset];
sum_alpha += intermediate_alpha[offset];
}
for (int y = 0; y < height; y++) {
auto color = Color(
sum_red / div,
sum_green / div,
sum_blue / div,
sum_alpha / div);
m_bitmap.set_pixel(x, y, color);
auto topmost_y_coord = max(y - radius, 0);
auto bottommost_y_coord = min(y + radius + 1, height - 1);
sum_red += intermediate_red[x + bottommost_y_coord * width];
sum_red -= intermediate_red[x + topmost_y_coord * width];
sum_green += intermediate_green[x + bottommost_y_coord * width];
sum_green -= intermediate_green[x + topmost_y_coord * width];
sum_blue += intermediate_blue[x + bottommost_y_coord * width];
sum_blue -= intermediate_blue[x + topmost_y_coord * width];
sum_alpha += intermediate_alpha[x + bottommost_y_coord * width];
sum_alpha -= intermediate_alpha[x + topmost_y_coord * width];
}
}
}
// Math from here: http://blog.ivank.net/fastest-gaussian-blur.html
void apply_three_passes(size_t radius)
{
if (!radius)
return;
size_t no_of_passes = 3;
double wIdeal = sqrt((12 * radius * radius / (double)no_of_passes) + 1);
int wl = floor(wIdeal);
if (wl % 2 == 0)
wl--;
int wu = wl - 2;
double mIdeal = (12 * radius * radius - no_of_passes * wl * wl - 4 * no_of_passes * wl - 3 * no_of_passes) / (double)(-4 * wl - 4);
int m = round(mIdeal);
for (size_t i = 0; i < no_of_passes; i++) {
int weighted_radius = (int)i < m ? wl : wu;
if (weighted_radius < 2)
continue;
apply_single_pass((weighted_radius - 1) / 2);
}
}
private:
ALWAYS_INLINE static u8 red_value(Color const& color)
{
return (color.alpha() == 0) ? 0xFF : color.red();
}
ALWAYS_INLINE static u8 green_value(Color const& color)
{
return (color.alpha() == 0) ? 0xFF : color.green();
}
ALWAYS_INLINE static u8 blue_value(Color const& color)
{
return (color.alpha() == 0) ? 0xFF : color.blue();
}
Bitmap& m_bitmap;
};
}