LibGfx: Add fast path to AA line drawing with simple 2D transform

If the effective 2D transform is just a basic translation, we now simply
translate the underlying painter before & after drawing AA lines.

This avoids all the extra math that otherwise has to happen when mapping
points through an affine transform.

This noticeably increase "mousing around" performance on Wikipedia. :^)

Userland/Libraries/LibGfx/AntiAliasingPainter.cpp

@@ -12,8 +12,8 @@
 
 // Base algorithm from https://en.wikipedia.org/wiki/Xiaolin_Wu%27s_line_algorithm,
 // because there seems to be no other known method for drawing AA'd lines (?)
-template<Gfx::AntiAliasingPainter::AntiAliasPolicy policy>
-void Gfx::AntiAliasingPainter::draw_anti_aliased_line(FloatPoint const& actual_from, FloatPoint const& actual_to, Color color, float thickness, Gfx::Painter::LineStyle style, Color)
+template<Gfx::AntiAliasingPainter::AntiAliasPolicy policy, typename TransformPoint>
+void Gfx::AntiAliasingPainter::draw_anti_aliased_line(FloatPoint const& actual_from, FloatPoint const& actual_to, Color color, float thickness, Gfx::Painter::LineStyle style, Color, TransformPoint transform_point)
 {
     // FIXME: Implement this :P
     VERIFY(style == Painter::LineStyle::Solid);
@@ -21,8 +21,8 @@ void Gfx::AntiAliasingPainter::draw_anti_aliased_line(FloatPoint const& actual_f
     auto corrected_thickness = thickness > 1 ? thickness - 1 : thickness;
     auto size = IntSize(corrected_thickness, corrected_thickness);
     auto plot = [&](int x, int y, float c) {
-        auto center = m_transform.map(Gfx::IntPoint(x, y)).to_type<int>();
-        m_underlying_painter.fill_rect(IntRect::centered_on(center, size), color.with_alpha(color.alpha() * c));
+        transform_point(x, y, m_transform);
+        m_underlying_painter.fill_rect(IntRect::centered_on({ x, y }, size), color.with_alpha(color.alpha() * c));
     };
 
     auto integer_part = [](float x) { return floorf(x); };
@@ -112,14 +112,37 @@ void Gfx::AntiAliasingPainter::draw_anti_aliased_line(FloatPoint const& actual_f
     draw_line(actual_from.x(), actual_from.y(), actual_to.x(), actual_to.y());
 }
 
+static ALWAYS_INLINE void no_transform(int&, int&, Gfx::AffineTransform const&)
+{
+}
+
+static ALWAYS_INLINE void full_transform(int& x, int& y, Gfx::AffineTransform const& transform)
+{
+    auto mapped = transform.map(Gfx::IntPoint { x, y });
+    x = mapped.x();
+    y = mapped.y();
+}
+
 void Gfx::AntiAliasingPainter::draw_aliased_line(FloatPoint const& actual_from, FloatPoint const& actual_to, Color color, float thickness, Gfx::Painter::LineStyle style, Color alternate_color)
 {
-    draw_anti_aliased_line<AntiAliasPolicy::OnlyEnds>(actual_from, actual_to, color, thickness, style, alternate_color);
+    if (m_transform.is_identity_or_translation()) {
+        m_underlying_painter.translate(m_transform.e(), m_transform.f());
+        draw_anti_aliased_line<AntiAliasPolicy::OnlyEnds>(actual_from, actual_to, color, thickness, style, alternate_color, no_transform);
+        m_underlying_painter.translate(-m_transform.e(), -m_transform.f());
+    } else {
+        draw_anti_aliased_line<AntiAliasPolicy::OnlyEnds>(actual_from, actual_to, color, thickness, style, alternate_color, full_transform);
+    }
 }
 
 void Gfx::AntiAliasingPainter::draw_line(FloatPoint const& actual_from, FloatPoint const& actual_to, Color color, float thickness, Gfx::Painter::LineStyle style, Color alternate_color)
 {
-    draw_anti_aliased_line<AntiAliasPolicy::Full>(actual_from, actual_to, color, thickness, style, alternate_color);
+    if (m_transform.is_identity_or_translation()) {
+        m_underlying_painter.translate(m_transform.e(), m_transform.f());
+        draw_anti_aliased_line<AntiAliasPolicy::Full>(actual_from, actual_to, color, thickness, style, alternate_color, no_transform);
+        m_underlying_painter.translate(-m_transform.e(), -m_transform.f());
+    } else {
+        draw_anti_aliased_line<AntiAliasPolicy::Full>(actual_from, actual_to, color, thickness, style, alternate_color, full_transform);
+    }
 }
 
 void Gfx::AntiAliasingPainter::fill_path(Path& path, Color color, Painter::WindingRule rule)

Userland/Libraries/LibGfx/AntiAliasingPainter.h

@@ -37,8 +37,8 @@ private:
         OnlyEnds,
         Full,
     };
-    template<AntiAliasPolicy policy>
-    void draw_anti_aliased_line(FloatPoint const&, FloatPoint const&, Color, float thickness, Painter::LineStyle style, Color alternate_color);
+    template<AntiAliasPolicy policy, typename TransformPoint>
+    void draw_anti_aliased_line(FloatPoint const&, FloatPoint const&, Color, float thickness, Painter::LineStyle style, Color alternate_color, TransformPoint);
 
     Painter& m_underlying_painter;
     AffineTransform m_transform;