LibGfx: Add Painter::draw_circle_arc_intersecting()

This adds a function to draw a circle specified by a center point (
relative to the given Rect) and a radius. The circle arc is only drawn
inside the specified Rect as to allow for circle arc segments.
Technically this was already possible using draw_elliptical_arc(), but
the algorithm is quite involved and lead to wonky arcs when trying to
draw circle arc segments.

Userland/Libraries/LibGfx/Painter.cpp

@@ -384,6 +384,49 @@ void Painter::fill_rounded_corner(const IntRect& a_rect, int radius, Color color
  }
 }
 
+void Painter::draw_circle_arc_intersecting(const IntRect& a_rect, const IntPoint& center, int radius, Color color, int thickness)
+{
+ if (thickness <= 0)
+ return;
+
+ // Care about clipping
+ auto translated_a_rect = a_rect.translated(translation());
+ auto rect = translated_a_rect.intersected(clip_rect());
+
+ if (rect.is_empty())
+ return;
+ VERIFY(m_target->rect().contains(rect));
+
+ // We got cut on the top!
+ // FIXME: Also account for clipping on the x-axis
+ int clip_offset = 0;
+ if (translated_a_rect.y() < rect.y())
+ clip_offset = rect.y() - translated_a_rect.y();
+
+ if (thickness > radius)
+ thickness = radius;
+
+ int radius2 = radius * radius;
+ auto is_on_arc = [&](int x, int y) {
+ int distance2 = (center.x() - x) * (center.x() - x) + (center.y() - y) * (center.y() - y);
+ // Is within a circle of radius 1/2 around (x,y), so basically within the current pixel.
+ // Technically this is angle-dependent and should be between 1/2 and sqrt(2)/2, but this works.
+ return distance2 <= (radius2 + radius + 0.25) && distance2 >= (radius2 - radius + 0.25);
+ };
+
+ RGBA32* dst = m_target->scanline(rect.top()) + rect.left();
+ const size_t dst_skip = m_target->pitch() / sizeof(RGBA32);
+
+ for (int i = rect.height() - 1; i >= 0; --i) {
+ for (int j = 0; j < rect.width(); ++j)
+ if (is_on_arc(j, rect.height() - i + clip_offset))
+ dst[j] = color.value();
+ dst += dst_skip;
+ }
+
+ return draw_circle_arc_intersecting(a_rect, center, radius - 1, color, thickness - 1);
+}
+
 void Painter::fill_ellipse(const IntRect& a_rect, Color color)
 {
  VERIFY(scale() == 1); // FIXME: Add scaling support.

Userland/Libraries/LibGfx/Painter.h

@@ -71,6 +71,7 @@ class Painter {
  void draw_glyph(const IntPoint&, u32, const Font&, Color);
  void draw_emoji(const IntPoint&, const Gfx::Bitmap&, const Font&);
  void draw_glyph_or_emoji(const IntPoint&, u32 code_point, const Font&, Color);
+ void draw_circle_arc_intersecting(const IntRect&, const IntPoint&, int radius, Color, int thickness);
 
  enum class CornerOrientation {
  TopLeft,