PixelPaint: Add a Mask class

The Mask class represents an opacity mask over a rectangular section
of an image, linking every pixel to an alpha value ranging from 0 (not
selected) to 255 (fully selected). "Partially selected" pixels can be
used to simulate anti-aliased curves.

This class will be used as the basis for the new non-rectangular
selection feature.

Userland/Applications/PixelPaint/Mask.cpp

@@ -0,0 +1,118 @@
+/*
+ * Copyright (c) 2021, Davipb <daviparca@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Mask.h"
+
+namespace PixelPaint {
+
+Mask::Mask(Gfx::IntRect bounding_rect, u8 default_value)
+    : m_bounding_rect(bounding_rect)
+{
+    auto data_size = bounding_rect.size().area();
+    m_data.resize(data_size);
+
+    for (auto& x : m_data) {
+        x = default_value;
+    }
+}
+
+size_t Mask::to_index(int x, int y) const
+{
+    VERIFY(m_bounding_rect.contains(x, y));
+
+    int dx = x - m_bounding_rect.x();
+    int dy = y - m_bounding_rect.y();
+    return dy * m_bounding_rect.width() + dx;
+}
+
+u8 Mask::get(int x, int y) const
+{
+    if (is_null() || !m_bounding_rect.contains(x, y)) {
+        return 0;
+    }
+
+    return m_data[to_index(x, y)];
+}
+
+void Mask::set(int x, int y, u8 value)
+{
+    VERIFY(!is_null());
+    VERIFY(m_bounding_rect.contains(x, y));
+
+    m_data[to_index(x, y)] = value;
+}
+
+Mask Mask::with_bounding_rect(Gfx::IntRect inner_rect) const
+{
+    auto result = Mask::empty(inner_rect);
+
+    result.for_each_pixel([&](int x, int y) {
+        result.set(x, y, get(x, y));
+    });
+
+    return result;
+}
+
+void Mask::shrink_to_fit()
+{
+    int topmost = NumericLimits<int>::max();
+    int bottommost = NumericLimits<int>::min();
+    int leftmost = NumericLimits<int>::max();
+    int rightmost = NumericLimits<int>::min();
+
+    bool empty = true;
+    for_each_pixel([&](auto x, auto y) {
+        if (get(x, y) == 0) {
+            return;
+        }
+
+        empty = false;
+
+        topmost = min(topmost, y);
+        bottommost = max(bottommost, y);
+
+        leftmost = min(leftmost, x);
+        rightmost = max(rightmost, x);
+    });
+
+    if (empty) {
+        m_bounding_rect = {};
+        m_data.clear();
+        return;
+    }
+
+    Gfx::IntRect new_bounding_rect(
+        leftmost,
+        topmost,
+        rightmost - leftmost + 1,
+        bottommost - topmost + 1);
+
+    *this = with_bounding_rect(new_bounding_rect);
+}
+
+void Mask::invert()
+{
+    for_each_pixel([&](int x, int y) {
+        set(x, y, 0xFF - get(x, y));
+    });
+}
+
+void Mask::add(Mask const& other)
+{
+    combine(other, [](int a, int b) { return a + b; });
+}
+
+void Mask::subtract(Mask const& other)
+{
+    combine(other, [](int a, int b) { return a - b; });
+}
+
+void Mask::intersect(Mask const& other)
+{
+    combinef(other, [](float a, float b) { return a * b; });
+}
+
+}