From 2131724a2a7213e4ae7958f5da19a4a10924c8fe Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Tue, 21 May 2019 20:21:46 -0700
Subject: [PATCH] Implement round line caps

---
 geometry/src/outline.rs | 18 +++++++++++++++---
 geometry/src/stroke.rs  | 41 +++++++++++++++++++++++++++++------------
 2 files changed, 44 insertions(+), 15 deletions(-)

diff --git a/geometry/src/outline.rs b/geometry/src/outline.rs
index 72bcb5cb..75c1cb1f 100644
--- a/geometry/src/outline.rs
+++ b/geometry/src/outline.rs
@@ -19,10 +19,12 @@ use crate::clip::{self, ContourPolygonClipper, ContourRectClipper};
 use crate::dilation::ContourDilator;
 use crate::orientation::Orientation;
 use crate::segment::{Segment, SegmentFlags, SegmentKind};
-use std::f32::consts::FRAC_PI_2;
+use std::f32::consts::{FRAC_PI_2, PI};
 use std::fmt::{self, Debug, Formatter};
 use std::mem;
 
+const TWO_PI: f32 = PI * 2.0;
+
 #[derive(Clone)]
 pub struct Outline {
     pub(crate) contours: Vec<Contour>,
@@ -367,7 +369,17 @@ impl Contour {
         self.push_point(segment.baseline.to(), PointFlags::empty(), update_bounds);
     }
 
-    pub fn push_arc(&mut self, center: Point2DF32, radius: f32, start_angle: f32, end_angle: f32) {
+    pub fn push_arc(&mut self,
+                    center: Point2DF32,
+                    radius: f32,
+                    mut start_angle: f32,
+                    mut end_angle: f32) {
+        start_angle %= TWO_PI;
+        end_angle %= TWO_PI;
+        if end_angle < start_angle {
+            end_angle += TWO_PI;
+        }
+
         let scale = Transform2DF32::from_scale(Point2DF32::splat(radius));
         let translation = Transform2DF32::from_translation(center);
 
@@ -375,7 +387,7 @@ impl Contour {
         while angle < end_angle {
             let sweep_angle = f32::min(FRAC_PI_2, end_angle - angle);
             let mut segment = Segment::arc(sweep_angle);
-            let rotation = Transform2DF32::from_rotation(angle);
+            let rotation = Transform2DF32::from_rotation(sweep_angle * 0.5 + angle);
             segment = segment.transform(&scale.post_mul(&rotation).post_mul(&translation));
 
             debug!("angle={} start_angle={} end_angle={} sweep_angle={} segment={:?}",
diff --git a/geometry/src/stroke.rs b/geometry/src/stroke.rs
index 02d6a282..75e51019 100644
--- a/geometry/src/stroke.rs
+++ b/geometry/src/stroke.rs
@@ -15,6 +15,8 @@ use crate::basic::point::Point2DF32;
 use crate::basic::rect::RectF32;
 use crate::outline::{Contour, Outline};
 use crate::segment::Segment;
+use std::f32::consts::FRAC_PI_2;
+use std::f32;
 use std::mem;
 
 const TOLERANCE: f32 = 0.01;
@@ -35,6 +37,7 @@ pub struct StrokeStyle {
 pub enum LineCap {
     Butt,
     Square,
+    Round,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq)]
@@ -70,9 +73,7 @@ impl OutlineStrokeToFill {
             }
 
             stroker.offset_backward();
-            if closed {
-                // TODO(pcwalton): Line join.
-            } else {
+            if !closed {
                 self.add_cap(&mut stroker.output);
             }
 
@@ -108,15 +109,27 @@ impl OutlineStrokeToFill {
         let width = self.style.line_width;
         let (p0, p1) = (contour.position_of_last(2), contour.position_of_last(1));
         let gradient = (p1 - p0).normalize();
-        let offset = gradient.scale(width * 0.5);
 
-        let p2 = p1 + offset;
-        let p3 = p2 + gradient.yx().scale_xy(Point2DF32::new(width, -width));
-        let p4 = p3 - offset;
+        match self.style.line_cap {
+            LineCap::Butt => unreachable!(),
+            LineCap::Square => {
+                let offset = gradient.scale(width * 0.5);
 
-        contour.push_endpoint(p2);
-        contour.push_endpoint(p3);
-        contour.push_endpoint(p4);
+                let p2 = p1 + offset;
+                let p3 = p2 + gradient.yx().scale_xy(Point2DF32::new(-width, width));
+                let p4 = p3 - offset;
+
+                contour.push_endpoint(p2);
+                contour.push_endpoint(p3);
+                contour.push_endpoint(p4);
+            }
+            LineCap::Round => {
+                // FIXME(pcwalton): Should we really be using angles here at all?
+                let offset = gradient.yx().scale_xy(Point2DF32::new(-width * 0.5, width * 0.5));
+                let angle = f32::atan2(gradient.y(), gradient.x());
+                contour.push_arc(p1 + offset, width * 0.5, angle - FRAC_PI_2, angle + FRAC_PI_2);
+            }
+        }
     }
 }
 
@@ -135,8 +148,10 @@ impl ContourStrokeToFill {
 
     fn offset_forward(&mut self) {
         for (segment_index, segment) in self.input.iter().enumerate() {
+            // FIXME(pcwalton): We negate the radius here so that round end caps can be drawn
+            // clockwise. Of course, we should just implement anticlockwise arcs to begin with...
             let join = if segment_index == 0 { LineJoin::Bevel } else { self.join };
-            segment.offset(self.radius, join, &mut self.output);
+            segment.offset(-self.radius, join, &mut self.output);
         }
     }
 
@@ -148,8 +163,10 @@ impl ContourStrokeToFill {
             .collect();
         segments.reverse();
         for (segment_index, segment) in segments.iter().enumerate() {
+            // FIXME(pcwalton): We negate the radius here so that round end caps can be drawn
+            // clockwise. Of course, we should just implement anticlockwise arcs to begin with...
             let join = if segment_index == 0 { LineJoin::Bevel } else { self.join };
-            segment.offset(self.radius, join, &mut self.output);
+            segment.offset(-self.radius, join, &mut self.output);
         }
     }
 }