LibGfx+LibWeb: Wire up CanvasRenderingContext2D.ellipse()

Note that this is *extremely* naive, and not very good at being correct.

Userland/Libraries/LibGfx/Painter.cpp

@@ -1492,7 +1492,7 @@ void Painter::for_each_line_segment_on_bezier_curve(const FloatPoint& control_po
 
 static bool can_approximate_elliptical_arc(const FloatPoint& p1, const FloatPoint& p2, const FloatPoint& center, const FloatPoint radii, float x_axis_rotation, float theta_1, float theta_delta)
 {
-    constexpr static float tolerance = 0.5f;
+    constexpr static float tolerance = 0.3f;
 
     auto half_theta_delta = theta_delta / 2.0f;
 

Userland/Libraries/LibGfx/Path.cpp

@@ -30,11 +30,14 @@
 #include <AK/StringBuilder.h>
 #include <LibGfx/Painter.h>
 #include <LibGfx/Path.h>
+#include <math.h>
 
 namespace Gfx {
 
-void Path::elliptical_arc_to(const FloatPoint& next_point, const FloatPoint& radii, double x_axis_rotation, bool large_arc, bool sweep)
+void Path::elliptical_arc_to(const FloatPoint& point, const FloatPoint& radii, double x_axis_rotation, bool large_arc, bool sweep)
 {
+    auto next_point = point;
+
     double rx = radii.x();
     double ry = radii.y();
 
@@ -58,6 +61,18 @@ void Path::elliptical_arc_to(const FloatPoint& next_point, const FloatPoint& rad
     if (ry < 0)
         ry *= -1.0;
 
+    // POSSIBLY HACK: Handle the case where both points are the same.
+    auto same_endpoints = next_point == last_point;
+    if (same_endpoints) {
+        if (!large_arc) {
+            // Nothing is going to be drawn anyway.
+            return;
+        }
+
+        // Move the endpoint by a small amount to avoid division by zero.
+        next_point.move_by(0.01f, 0.01f);
+    }
+
     // Find (cx, cy), theta_1, theta_delta
     // Step 1: Compute (x1', y1')
     auto x_avg = (last_point.x() - next_point.x()) / 2.0f;
@@ -104,20 +119,18 @@ void Path::elliptical_arc_to(const FloatPoint& next_point, const FloatPoint& rad
     auto theta_delta = theta_2 - theta_1;
 
     if (!sweep && theta_delta > 0.0f) {
-        theta_delta -= M_TAU;
+        theta_delta -= 2 * M_PI;
     } else if (sweep && theta_delta < 0) {
-        theta_delta += M_TAU;
+        theta_delta += 2 * M_PI;
     }
 
-    append_segment<EllipticalArcSegment>(
+    elliptical_arc_to(
         next_point,
-        FloatPoint(cx, cy),
-        FloatPoint(rx, ry),
-        static_cast<float>(x_axis_rotation),
-        static_cast<float>(theta_1),
-        static_cast<float>(theta_delta));
-
-    invalidate_split_lines();
+        { cx, cy },
+        { rx, ry },
+        x_axis_rotation,
+        theta_1,
+        theta_delta);
 }
 
 void Path::close()

Userland/Libraries/LibGfx/Path.h

@@ -160,6 +160,20 @@ public:
         elliptical_arc_to(point, { radius, radius }, 0, large_arc, sweep);
     }
 
+    // Note: This does not do any sanity checks!
+    void elliptical_arc_to(const FloatPoint& endpoint, const FloatPoint& center, const FloatPoint& radii, double x_axis_rotation, double theta, double theta_delta)
+    {
+        append_segment<EllipticalArcSegment>(
+            endpoint,
+            center,
+            radii,
+            x_axis_rotation,
+            theta,
+            theta_delta);
+
+        invalidate_split_lines();
+    }
+
     void close();
     void close_all_subpaths();
 

Userland/Libraries/LibWeb/HTML/CanvasRenderingContext2D.cpp

@@ -188,6 +188,65 @@ void CanvasRenderingContext2D::quadratic_curve_to(float cx, float cy, float x, f
     m_path.quadratic_bezier_curve_to({ cx, cy }, { x, y });
 }
 
+void CanvasRenderingContext2D::arc(float x, float y, float radius, float start_angle, float end_angle, bool counter_clockwise)
+{
+    ellipse(x, y, radius, radius, 0, start_angle, end_angle, counter_clockwise);
+}
+
+void CanvasRenderingContext2D::ellipse(float x, float y, float radius_x, float radius_y, float rotation, float start_angle, float end_angle, bool counter_clockwise)
+{
+    if ((!counter_clockwise && (end_angle - start_angle) >= M_TAU)
+        || (counter_clockwise && (start_angle - end_angle) >= M_TAU)) {
+        start_angle = 0;
+        end_angle = M_TAU;
+    } else {
+        start_angle = fmodf(start_angle, M_TAU);
+        end_angle = fmodf(end_angle, M_TAU);
+    }
+
+    // Then, figure out where the ends of the arc are.
+    // To do so, we can pretend that the center of this ellipse is at (0, 0),
+    // and the whole coordinate system is rotated `rotation` radians around the x axis, centered on `center`.
+    // The sign of the resulting relative positions is just whether our angle is on one of the left quadrants.
+    auto sin_rotation = sinf(rotation);
+    auto cos_rotation = cosf(rotation);
+
+    auto resolve_point_with_angle = [&](float angle) {
+        auto tan_relative = tanf(angle);
+        auto tan2 = tan_relative * tan_relative;
+
+        auto ab = radius_x * radius_y;
+        auto a2 = radius_x * radius_x;
+        auto b2 = radius_y * radius_y;
+        auto sqrt = sqrtf(b2 + a2 * tan2);
+
+        auto relative_x_position = ab / sqrt;
+        auto relative_y_position = ab * tan_relative / sqrt;
+
+        // Make sure to set the correct sign
+        float sn = sinf(angle) >= 0 ? 1 : -1;
+        relative_x_position *= sn;
+        relative_y_position *= sn;
+
+        // Now rotate it (back) around the center point by 'rotation' radians, then move it back to our actual origin.
+        auto relative_rotated_x_position = relative_x_position * cos_rotation - relative_y_position * sin_rotation;
+        auto relative_rotated_y_position = relative_x_position * sin_rotation + relative_y_position * cos_rotation;
+        return Gfx::FloatPoint { relative_rotated_x_position + x, relative_rotated_y_position + y };
+    };
+
+    auto start_point = resolve_point_with_angle(start_angle);
+    auto end_point = resolve_point_with_angle(end_angle);
+
+    m_path.move_to(start_point);
+
+    auto delta_theta = end_angle - start_angle;
+
+    // FIXME: This is still goofy for some values.
+    m_path.elliptical_arc_to(end_point, { radius_x, radius_y }, rotation, delta_theta > M_PI, !counter_clockwise);
+
+    m_path.close();
+}
+
 void CanvasRenderingContext2D::rect(float x, float y, float width, float height)
 {
     m_path.move_to({ x, y });

Userland/Libraries/LibWeb/HTML/CanvasRenderingContext2D.h

@@ -74,6 +74,9 @@ public:
     void move_to(float x, float y);
     void line_to(float x, float y);
     void quadratic_curve_to(float cx, float cy, float x, float y);
+
+    void arc(float x, float y, float radius, float start_angle, float end_angle, bool counter_clockwise);
+    void ellipse(float x, float y, float radius_x, float radius_y, float rotation, float start_angle, float end_angle, bool counter_clockwise);
     void rect(float x, float y, float width, float height);
     void stroke();
 

Userland/Libraries/LibWeb/HTML/CanvasRenderingContext2D.idl

@@ -15,6 +15,9 @@ interface CanvasRenderingContext2D {
     undefined moveTo(double x, double y);
     undefined lineTo(double x, double y);
     undefined quadraticCurveTo(double cpx, double cpy, double x, double y);
+
+    undefined arc(double x, double y, double radius, double startAngle, double endAngle, optional boolean counterclockwise = false);
+    undefined ellipse(double x, double y, double radiusX, double radiusY, double rotation, double startAngle, double endAngle, optional boolean counterclockwise = false);
     undefined rect(double x, double y, double width, double height);
 
     undefined drawImage(HTMLImageElement image, double dx, double dy);