LibGfx: Add BitmapMixer

With this BitmapMixer one can draw one Bitmap onto another with
different modes.
For now the only supported mixing methods implemented are Add and
Lightest (which is very naive).

Userland/Libraries/LibGfx/BitmapMixer.cpp

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2022, Tobias Christiansen <tobyase@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "BitmapMixer.h"
+
+namespace Gfx {
+
+void BitmapMixer::mix_with(Bitmap& other_bitmap, MixingMethod mixing_method)
+{
+    VERIFY(m_bitmap.size() == other_bitmap.size());
+
+    int height = m_bitmap.height();
+    int width = m_bitmap.width();
+
+    for (int y = 0; y < height; ++y) {
+        for (int x = 0; x < width; ++x) {
+            auto original_color = m_bitmap.get_pixel(x, y);
+            auto other_color = other_bitmap.get_pixel(x, y);
+
+            Color output_color = { 0, 0, 0, original_color.alpha() };
+            switch (mixing_method) {
+            case MixingMethod::Add:
+                output_color.set_red(original_color.red() + other_color.red());
+                output_color.set_green(original_color.green() + other_color.green());
+                output_color.set_blue(original_color.blue() + other_color.blue());
+                break;
+            case MixingMethod::Lightest:
+                auto original_lightness = original_color.red() + original_color.green() + original_color.blue();
+                auto other_lightness = other_color.red() + other_color.green() + other_color.blue();
+                if (original_lightness > other_lightness) {
+                    output_color = original_color;
+                } else {
+                    output_color.set_red(other_color.red());
+                    output_color.set_green(other_color.green());
+                    output_color.set_blue(other_color.blue());
+                }
+                break;
+            }
+            if (original_color != output_color)
+                m_bitmap.set_pixel(x, y, output_color);
+        }
+    }
+}
+
+}

Userland/Libraries/LibGfx/BitmapMixer.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2022, Tobias Christiansen <tobyase@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Bitmap.h"
+
+namespace Gfx {
+
+class BitmapMixer {
+public:
+    enum class MixingMethod {
+        Add,
+        Lightest,
+    };
+
+    BitmapMixer(Bitmap& bitmap)
+        : m_bitmap(bitmap) {};
+
+    void mix_with(Bitmap&, MixingMethod);
+
+private:
+    Bitmap& m_bitmap;
+};
+
+}

Userland/Libraries/LibGfx/CMakeLists.txt

@@ -2,6 +2,7 @@ set(SOURCES
     AffineTransform.cpp
     AntiAliasingPainter.cpp
     Bitmap.cpp
+    BitmapMixer.cpp
     BitmapFont.cpp
     BMPLoader.cpp
     BMPWriter.cpp