Fix #2788: a dashed stroke with object opacity folds the opacity into its colour instead of compositing a canvas-wide layer

crates/render/src/lib.rs

@@ -2229,14 +2229,16 @@ fn single_plain_fill(n: &Node) -> bool {
     matches!((painted.next(), painted.next()), (Some(AppearanceItem::Fill(f)), None) if f.blend == vectorcraft_color::BlendMode::Normal && f.opacity >= 1.0 && !matches!(f.paint, vectorcraft_color::Paint::Pattern { .. }))
 }
 
-/// Number of visible painted fill/stroke items (opacity folding is exact only for one).
+/// Number of visible painted fill/stroke items (opacity folding is exact only for one). A dashed
+/// stroke is one paint: its dashes are outlined into one path and filled once, so overlapping
+/// dashes don't darken where they overlap.
 fn painted_items(n: &Node) -> usize {
     n.appearance
         .items
         .iter()
         .filter(|i| match i {
             AppearanceItem::Fill(f) => f.visible && !f.paint.is_none() && matches!(f.paint, vectorcraft_color::Paint::Solid { .. }),
-            AppearanceItem::Stroke(s) => s.visible && !s.paint.is_none() && s.width > 0.0 && matches!(s.paint, vectorcraft_color::Paint::Solid { .. }) && s.dash.is_none(),
+            AppearanceItem::Stroke(s) => s.visible && !s.paint.is_none() && s.width > 0.0 && matches!(s.paint, vectorcraft_color::Paint::Solid { .. }),
         })
         .count()
         .max(if n.appearance.items.iter().any(|i| matches!(i, AppearanceItem::Fill(f) if f.visible && !matches!(f.paint, vectorcraft_color::Paint::Solid { .. } | vectorcraft_color::Paint::None)) || matches!(i, AppearanceItem::Stroke(s) if s.visible && !matches!(s.paint, vectorcraft_color::Paint::Solid { .. } | vectorcraft_color::Paint::None))) { 2 } else { 0 })

crates/render/src/tests.rs

@@ -956,3 +956,34 @@ fn zero_length_subpaths_draw_dots_with_their_cap() {
     let butt = dots(vectorcraft_doc::LineCap::Butt);
     assert_eq!(butt.pixels.iter().filter(|&&p| p != 255).count(), 0, "a butt cap draws no dot");
 }
+
+/// #2788: a dashed solid stroke is one paint, so its object opacity folds into its colour instead
+/// of compositing a viewport-wide layer; the result matches the layered one, overlapping dashes
+/// included.
+#[test]
+fn dashed_stroke_opacity_folds_into_its_colour() {
+    let dashed = |d: &mut Document, object: f32, stroke: f32| {
+        let mut n = rect_node(d, Rect::new(20.0, 20.0, 80.0, 60.0), Paint::None, Paint::solid(Color::rgb(0.2, 0.4, 0.9)), 6.0);
+        let st = n.appearance.stroke_mut().unwrap();
+        st.dash = Some(vectorcraft_doc::Dash { pattern: vec![4.0, 1.0], ..Default::default() });
+        st.cap = vectorcraft_doc::LineCap::Round;
+        st.opacity = stroke;
+        n.opacity = object;
+        n
+    };
+    let doc = |object: f32, stroke: f32| {
+        let mut d = Document::new(100.0, 100.0);
+        let n = dashed(&mut d, object, stroke);
+        let l = d.layers[0].id;
+        d.insert(Some(l), 0, n).unwrap();
+        d
+    };
+    let mut d = Document::new(100.0, 100.0);
+    assert_eq!(painted_items(&dashed(&mut d, 0.6, 1.0)), 1, "a dashed stroke takes the folded fast path");
+    let folded = render(&doc(0.6, 1.0));
+    let layered = render(&doc(1.0, 0.6));
+    assert_ne!(folded.pixel(20, 40), [255, 255, 255, 255], "the stroke is drawn");
+    for (a, b) in folded.pixels.chunks(4).zip(layered.pixels.chunks(4)) {
+        assert!(a.iter().zip(b).all(|(x, y)| (*x as i32 - *y as i32).abs() <= 1), "folded {a:?} vs layered {b:?}");
+    }
+}