Add support for counterclockwise winding to arcs

canvas/src/lib.rs

@@ -19,7 +19,7 @@ use pathfinder_geometry::basic::point::Point2DF;
 use pathfinder_geometry::basic::rect::RectF;
 use pathfinder_geometry::basic::transform2d::Transform2DF;
 use pathfinder_geometry::color::ColorU;
-use pathfinder_geometry::outline::{Contour, Outline};
+use pathfinder_geometry::outline::{ArcDirection, Contour, Outline};
 use pathfinder_geometry::stroke::{LineCap, LineJoin as StrokeLineJoin};
 use pathfinder_geometry::stroke::{OutlineStrokeToFill, StrokeStyle};
 use pathfinder_renderer::paint::{Paint, PaintId};
@@ -246,7 +246,7 @@ impl CanvasRenderingContext2D {
 }
 
 #[derive(Clone)]
-pub struct State {
+struct State {
     transform: Transform2DF,
     font_collection: Arc<FontCollection>,
     font_size: f32,
@@ -336,10 +336,15 @@ impl Path2D {
     }
 
     #[inline]
-    pub fn arc(&mut self, center: Point2DF, radius: f32, start_angle: f32, end_angle: f32) {
+    pub fn arc(&mut self,
+               center: Point2DF,
+               radius: f32,
+               start_angle: f32,
+               end_angle: f32,
+               direction: ArcDirection) {
         let mut transform = Transform2DF::from_scale(Point2DF::splat(radius));
         transform = transform.post_mul(&Transform2DF::from_translation(center));
-        self.current_contour.push_arc(&transform, start_angle, end_angle);
+        self.current_contour.push_arc(&transform, start_angle, end_angle, direction);
     }
 
     #[inline]
@@ -357,7 +362,9 @@ impl Path2D {
 
         let chord = LineSegmentF::new(vu0.yx().scale_xy(Point2DF::new(-1.0, 1.0)),
                                       vu1.yx().scale_xy(Point2DF::new(1.0, -1.0)));
-        self.current_contour.push_arc_from_unit_chord(&transform, chord);
+
+        // FIXME(pcwalton): Is clockwise direction correct?
+        self.current_contour.push_arc_from_unit_chord(&transform, chord, ArcDirection::CW);
     }
 
     pub fn rect(&mut self, rect: RectF) {
@@ -380,7 +387,7 @@ impl Path2D {
         let mut transform = Transform2DF::from_rotation(rotation);
         transform = transform.post_mul(&Transform2DF::from_scale(axes));
         transform = transform.post_mul(&Transform2DF::from_translation(center));
-        self.current_contour.push_arc(&transform, start_angle, end_angle);
+        self.current_contour.push_arc(&transform, start_angle, end_angle, ArcDirection::CW);
 
         if end_angle - start_angle >= 2.0 * PI {
             self.current_contour.close();

geometry/src/basic/line_segment.rs

@@ -97,6 +97,11 @@ impl LineSegmentF {
         LineSegmentF(self.0 * F32x4::splat(factor))
     }
 
+    #[inline]
+    pub fn scale_xy(&self, factors: Point2DF) -> LineSegmentF {
+        LineSegmentF(self.0 * factors.0.xyxy())
+    }
+
     #[inline]
     pub fn split(&self, t: f32) -> (LineSegmentF, LineSegmentF) {
         debug_assert!(t >= 0.0 && t <= 1.0);

geometry/src/outline.rs

@@ -352,17 +352,30 @@ impl Contour {
         self.push_point(segment.baseline.to(), PointFlags::empty(), update_bounds);
     }
 
-    pub fn push_arc(&mut self, transform: &Transform2DF, start_angle: f32, end_angle: f32) {
+    pub fn push_arc(&mut self,
+                    transform: &Transform2DF,
+                    start_angle: f32,
+                    end_angle: f32,
+                    direction: ArcDirection) {
         if end_angle - start_angle >= PI * 2.0 {
             self.push_ellipse(transform);
         } else {
             let start = Point2DF::new(f32::cos(start_angle), f32::sin(start_angle));
             let end = Point2DF::new(f32::cos(end_angle), f32::sin(end_angle));
-            self.push_arc_from_unit_chord(transform, LineSegmentF::new(start, end));
+            self.push_arc_from_unit_chord(transform, LineSegmentF::new(start, end), direction);
         }
     }
 
-    pub fn push_arc_from_unit_chord(&mut self, transform: &Transform2DF, chord: LineSegmentF) {
+    pub fn push_arc_from_unit_chord(&mut self,
+                                    transform: &Transform2DF,
+                                    mut chord: LineSegmentF,
+                                    direction: ArcDirection) {
+        let mut direction_transform = Transform2DF::default();
+        if direction == ArcDirection::CCW {
+            chord = chord.scale_xy(Point2DF::new(-1.0, 1.0));
+            direction_transform = Transform2DF::from_scale(Point2DF::new(-1.0, 1.0));
+        }
+
         let (mut vector, end_vector) = (UnitVector(chord.from()), UnitVector(chord.to()));
         let mut first_segment = true;
 
@@ -378,9 +391,10 @@ impl Contour {
                 segment = Segment::arc_from_cos(sweep_vector.0.x());
             }
 
-            let rotation =
+            let half_sweep_vector = sweep_vector.halve_angle();
-                Transform2DF::from_rotation_vector(sweep_vector.halve_angle().rotate_by(vector));
+            let rotation = Transform2DF::from_rotation_vector(half_sweep_vector.rotate_by(vector));
-            segment = segment.transform(&rotation.post_mul(&transform));
+            segment = segment.transform(&direction_transform.post_mul(&rotation)
+                                                            .post_mul(&transform));
 
             let mut push_segment_flags = PushSegmentFlags::UPDATE_BOUNDS;
             if first_segment {
@@ -816,6 +830,12 @@ impl<'a> Iterator for ContourIter<'a> {
     }
 }
 
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum ArcDirection {
+    CW,
+    CCW,
+}
+
 #[inline]
 pub(crate) fn union_rect(bounds: &mut RectF, new_point: Point2DF, first: bool) {
     if first {

geometry/src/stroke.rs

@@ -14,7 +14,7 @@ use crate::basic::line_segment::LineSegmentF;
 use crate::basic::point::Point2DF;
 use crate::basic::rect::RectF;
 use crate::basic::transform2d::Transform2DF;
-use crate::outline::{Contour, Outline, PushSegmentFlags};
+use crate::outline::{ArcDirection, Contour, Outline, PushSegmentFlags};
 use crate::segment::Segment;
 use std::f32;
 
@@ -141,7 +141,8 @@ impl<'a> OutlineStrokeToFill<'a> {
                 let mut transform = Transform2DF::from_scale(scale);
                 let translation = p1 + offset.scale(width * 0.5);
                 transform = transform.post_mul(&Transform2DF::from_translation(translation));
-                contour.push_arc_from_unit_chord(&transform, LineSegmentF::new(-offset, offset));
+                let chord = LineSegmentF::new(-offset, offset);
+                contour.push_arc_from_unit_chord(&transform, chord, ArcDirection::CW);
             }
         }
     }
@@ -377,7 +378,8 @@ impl Contour {
                 transform = transform.post_mul(&Transform2DF::from_translation(join_point));
                 let chord_from = (prev_tangent.to() - join_point).normalize();
                 let chord_to = (next_tangent.to() - join_point).normalize();
-                self.push_arc_from_unit_chord(&transform, LineSegmentF::new(chord_from, chord_to));
+                let chord = LineSegmentF::new(chord_from, chord_to);
+                self.push_arc_from_unit_chord(&transform, chord, ArcDirection::CW);
             }
         }
     }