Fix stroke cap of shapes with partial stroke (#5688)

crates/typst-layout/src/shapes.rs

@@ -11,8 +11,8 @@ use typst_library::layout::{
 };
 use typst_library::visualize::{
     CircleElem, CloseMode, Curve, CurveComponent, CurveElem, EllipseElem, FillRule,
-    FixedStroke, Geometry, LineElem, Paint, PathElem, PathVertex, PolygonElem, RectElem,
+    FixedStroke, Geometry, LineCap, LineElem, Paint, PathElem, PathVertex, PolygonElem,
-    Shape, SquareElem, Stroke,
+    RectElem, Shape, SquareElem, Stroke,
 };
 use typst_syntax::Span;
 use typst_utils::{Get, Numeric};
@@ -889,7 +889,13 @@ fn segmented_rect(
             let end = current;
             last = current;
             let Some(stroke) = strokes.get_ref(start.side_cw()) else { continue };
-            let (shape, ontop) = segment(start, end, &corners, stroke);
+            let start_cap = stroke.cap;
+            let end_cap = match strokes.get_ref(end.side_ccw()) {
+                Some(stroke) => stroke.cap,
+                None => start_cap,
+            };
+            let (shape, ontop) =
+                segment(start, end, start_cap, end_cap, &corners, stroke);
             if ontop {
                 res.push(shape);
             } else {
@@ -899,7 +905,14 @@ fn segmented_rect(
         }
     } else if let Some(stroke) = &strokes.top {
         // single segment
-        let (shape, _) = segment(Corner::TopLeft, Corner::TopLeft, &corners, stroke);
+        let (shape, _) = segment(
+            Corner::TopLeft,
+            Corner::TopLeft,
+            stroke.cap,
+            stroke.cap,
+            &corners,
+            stroke,
+        );
         res.push(shape);
     }
     res
@@ -946,6 +959,8 @@ fn curve_segment(
 fn segment(
     start: Corner,
     end: Corner,
+    start_cap: LineCap,
+    end_cap: LineCap,
     corners: &Corners<ControlPoints>,
     stroke: &FixedStroke,
 ) -> (Shape, bool) {
@@ -979,7 +994,7 @@ fn segment(
 
     let use_fill = solid && fill_corners(start, end, corners);
     let shape = if use_fill {
-        fill_segment(start, end, corners, stroke)
+        fill_segment(start, end, start_cap, end_cap, corners, stroke)
     } else {
         stroke_segment(start, end, corners, stroke.clone())
     };
@@ -1010,6 +1025,8 @@ fn stroke_segment(
 fn fill_segment(
     start: Corner,
     end: Corner,
+    start_cap: LineCap,
+    end_cap: LineCap,
     corners: &Corners<ControlPoints>,
     stroke: &FixedStroke,
 ) -> Shape {
@@ -1035,8 +1052,7 @@ fn fill_segment(
         if c.arc_outer() {
             curve.arc_line(c.mid_outer(), c.center_outer(), c.end_outer());
         } else {
-            curve.line(c.outer());
+            c.start_cap(&mut curve, start_cap);
-            curve.line(c.end_outer());
         }
     }
 
@@ -1079,7 +1095,7 @@ fn fill_segment(
         if c.arc_inner() {
             curve.arc_line(c.mid_inner(), c.center_inner(), c.start_inner());
         } else {
-            curve.line(c.center_inner());
+            c.end_cap(&mut curve, end_cap);
         }
     }
 
@@ -1134,6 +1150,16 @@ struct ControlPoints {
 }
 
 impl ControlPoints {
+    /// Rotate point around the origin, relative to the top-left.
+    fn rotate_centered(&self, point: Point) -> Point {
+        match self.corner {
+            Corner::TopLeft => point,
+            Corner::TopRight => Point { x: -point.y, y: point.x },
+            Corner::BottomRight => Point { x: -point.x, y: -point.y },
+            Corner::BottomLeft => Point { x: point.y, y: -point.x },
+        }
+    }
+
     /// Move and rotate the point from top-left to the required corner.
     fn rotate(&self, point: Point) -> Point {
         match self.corner {
@@ -1280,6 +1306,77 @@ impl ControlPoints {
             y: self.stroke_after,
         })
     }
+
+    /// Draw the cap at the beginning of the segment.
+    ///
+    /// If this corner has a stroke before it,
+    /// a default "butt" cap is used.
+    ///
+    /// NOTE: doesn't support the case where the corner has a radius.
+    pub fn start_cap(&self, curve: &mut Curve, cap_type: LineCap) {
+        if self.stroke_before != Abs::zero()
+            || self.radius != Abs::zero()
+            || cap_type == LineCap::Butt
+        {
+            // Just the default cap.
+            curve.line(self.outer());
+        } else if cap_type == LineCap::Square {
+            // Extend by the stroke width.
+            let offset =
+                self.rotate_centered(Point { x: -self.stroke_after, y: Abs::zero() });
+            curve.line(self.end_inner() + offset);
+            curve.line(self.outer() + offset);
+        } else if cap_type == LineCap::Round {
+            // We push the center by a little bit to ensure the correct
+            // half of the circle gets drawn. If it is perfectly centered
+            // the `arc` function just degenerates into a line, which we
+            // do not want in this case.
+            curve.arc(
+                self.end_inner(),
+                (self.end_inner()
+                    + self.rotate_centered(Point { x: Abs::raw(1.0), y: Abs::zero() })
+                    + self.outer())
+                    / 2.,
+                self.outer(),
+            );
+        }
+        curve.line(self.end_outer());
+    }
+
+    /// Draw the cap at the end of the segment.
+    ///
+    /// If this corner has a stroke before it,
+    /// a default "butt" cap is used.
+    ///
+    /// NOTE: doesn't support the case where the corner has a radius.
+    pub fn end_cap(&self, curve: &mut Curve, cap_type: LineCap) {
+        if self.stroke_after != Abs::zero()
+            || self.radius != Abs::zero()
+            || cap_type == LineCap::Butt
+        {
+            // Just the default cap.
+            curve.line(self.center_inner());
+        } else if cap_type == LineCap::Square {
+            // Extend by the stroke width.
+            let offset =
+                self.rotate_centered(Point { x: Abs::zero(), y: -self.stroke_before });
+            curve.line(self.outer() + offset);
+            curve.line(self.center_inner() + offset);
+        } else if cap_type == LineCap::Round {
+            // We push the center by a little bit to ensure the correct
+            // half of the circle gets drawn. If it is perfectly centered
+            // the `arc` function just degenerates into a line, which we
+            // do not want in this case.
+            curve.arc(
+                self.outer(),
+                (self.outer()
+                    + self.rotate_centered(Point { x: Abs::zero(), y: Abs::raw(1.0) })
+                    + self.center_inner())
+                    / 2.,
+                self.center_inner(),
+            );
+        }
+    }
 }
 
 /// Helper to draw arcs with Bézier curves.

tests/suite/visualize/rect.typ

@@ -54,6 +54,22 @@
 #v(3pt)
 #rect(width: 20pt, height: 20pt, stroke: (thickness: 5pt, join: "round"))
 
+--- rect-stroke-caps ---
+// Separated segments
+#rect(width: 20pt, height: 20pt, stroke: (
+  left: (cap: "round", thickness: 5pt),
+  right: (cap: "square", thickness: 7pt),
+))
+// Joined segment with different caps.
+#rect(width: 20pt, height: 20pt, stroke: (
+  left: (cap: "round", thickness: 5pt),
+  top: (cap: "square", thickness: 7pt),
+))
+// No caps when there is a radius for that corner.
+#rect(width: 20pt, height: 20pt, radius: (top: 3pt), stroke: (
+  left: (cap: "round", thickness: 5pt),
+  top: (cap: "square", thickness: 7pt),
+))
 --- red-stroke-bad-type ---
 // Error: 15-21 expected length, color, gradient, tiling, dictionary, stroke, none, or auto, found array
 #rect(stroke: (1, 2))