A linear gradient across a straight line runs along its length, on screen and in PDF alike

Review fix from aiongg (72d7ea2e, with the line tests of 6db40bdb), ported
to this branch: Fill::linear_axis gives screen and PDF one gradient axis,
and a box with no extent along the gradient's angle takes its longer side.

Co-authored-by: aiongg <aiongtaigi@gmail.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/item.rs

@@ -32,6 +32,21 @@ impl Default for Fill {
 }
 
 impl Fill {
+    /// A linear gradient's axis on an object whose bounds (in its own space) are `b`: the
+    /// Gradient Swatch tool's vector, or the box's extent along the gradient's angle through its
+    /// centre. A box with no extent along that angle (a straight line, across it) takes its longer
+    /// side.
+    pub fn linear_axis(&self, b: Rect) -> (Point, Point) {
+        if let Some([x0, y0, x1, y1]) = self.gradient_vector {
+            return (Point::new(x0, y0), Point::new(x1, y1));
+        }
+        let a = self.gradient_angle.unwrap_or(0.0).to_radians();
+        let half = (b.width() * a.cos().abs() + b.height() * a.sin().abs()) / 2.0;
+        let half = if half > 1e-9 { half } else { b.width().max(b.height()).max(1e-3) / 2.0 };
+        let d = designcraft_geom::Vec2::new(a.cos(), -a.sin()) * half;
+        (b.center() - d, b.center() + d)
+    }
+
     pub fn none() -> Self {
         Fill { swatch: designcraft_color::swatch::NONE.into(), tint: 1.0, gradient_angle: None, gradient_vector: None, overprint: false }
     }

crates/engine/src/cmd/style.rs

@@ -1900,6 +1900,51 @@ mod gradient_stroke_tests {
         assert!(a > 215 && b < 40, "dragged: {a} {b}");
         assert!(s.execute("object.gradient", &json!({"ids": [id], "target": "gap"})).is_err());
     }
+
+    /// A 20 pt horizontal line with a gradient on its stroke, drawn on screen and from its PDF.
+    fn line_with_stroke_gradient(g: serde_json::Value) -> (designcraft_render::Rendered, designcraft_render::Rendered) {
+        let mut s = crate::Session::new();
+        s.execute("file.new", &json!({"width": 200, "height": 120, "facingPages": false})).unwrap();
+        let id = s.execute("line.create", &json!({"a": [20, 60], "b": [180, 60]})).unwrap()["id"].as_u64().unwrap();
+        s.execute("object.stroke", &json!({"ids": [id], "weight": 20})).unwrap();
+        let mut g = g;
+        g["ids"] = json!([id]);
+        g["target"] = json!("stroke");
+        s.execute("object.gradient", &g).unwrap();
+        let d = s.doc().unwrap().doc.clone();
+        let mut rr = designcraft_render::Renderer::new();
+        let screen = rr.render_page(&d, &s.cache, 0, 1.0, false, &Default::default()).unwrap();
+        let pdf = designcraft_pdf::export_pdf(&d, &s.cache, &Default::default()).unwrap();
+        let mut p = crate::Session::new();
+        p.execute("file.new", &json!({"width": 200, "height": 120, "facingPages": false})).unwrap();
+        p.execute("file.place", &json!({"base64": crate::cmd::file::base64_encode(&pdf), "name": "line.pdf", "x": 0, "y": 0, "width": 200})).unwrap();
+        let pd = p.doc().unwrap().doc.clone();
+        let placed = rr.render_page(&pd, &p.cache, 0, 1.0, false, &Default::default()).unwrap();
+        (screen, placed)
+    }
+
+    #[test]
+    fn a_radial_gradient_on_a_straight_lines_stroke_shows_all_its_stops() {
+        // The line's box has no height: the gradient is a circle across the line's length.
+        let (screen, pdf) = line_with_stroke_gradient(
+            json!({"kind": "radial", "stops": [{"location": 0, "color": "#000000"}, {"location": 100, "color": "#ffffff"}]}),
+        );
+        for (what, img) in [("screen", &screen), ("pdf", &pdf)] {
+            let (a, m, b) = along(img);
+            assert!(m < 40 && a > 180 && b > 180, "{what}: {a} {m} {b}");
+        }
+    }
+
+    #[test]
+    fn a_linear_gradient_across_a_straight_line_paints_its_stroke_alike_on_screen_and_in_pdf() {
+        let (screen, pdf) =
+            line_with_stroke_gradient(json!({"angle": 90, "stops": [{"location": 0, "color": "#ff0000"}, {"location": 100, "color": "#0000ff"}]}));
+        let (s, p) = (screen.pixel(100, 60), pdf.pixel(100, 60));
+        // The line's box has no height: the gradient runs across the line's length, so the stroke
+        // shows its middle.
+        assert!(s[1] < 60 && s[0] > 80 && s[2] > 80, "screen: the stroke is painted, {s:?}");
+        assert!((0..3).all(|i| s[i].abs_diff(p[i]) < 40), "screen {s:?}, pdf {p:?}");
+    }
 }
 
 #[cfg(test)]

crates/pdf/src/export.rs

@@ -978,7 +978,7 @@ impl Exporter<'_> {
 
     /// A swatch fill (solid or gradient) in the item's inner space. `None` for [None]/unknown.
     fn fill_paint(&mut self, fill: &designcraft_doc::Fill, bounds: Rect) -> Option<krilla::paint::Paint> {
-        let (swatch, tint, angle) = (fill.swatch.as_str(), fill.tint, fill.gradient_angle);
+        let (swatch, tint) = (fill.swatch.as_str(), fill.tint);
         if let Some(g) = designcraft_color::swatch::resolve_gradient(&self.doc.swatches, swatch) {
             let kind = g.kind;
             let expanded = g.expanded_stops();
@@ -1036,13 +1036,7 @@ impl Exporter<'_> {
                     .into()
                 }
                 _ => {
-                    let a = angle.unwrap_or(0.0).to_radians();
-                    let half = ((bounds.width() * a.cos().abs() + bounds.height() * a.sin().abs()) / 2.0).max(1e-3);
-                    let d = Vec2::new(a.cos(), -a.sin()) * half;
-                    let (p0, p1) = match v {
-                        Some([x0, y0, x1, y1]) => (designcraft_geom::Point::new(x0, y0), designcraft_geom::Point::new(x1, y1)),
-                        None => (c - d, c + d),
-                    };
+                    let (p0, p1) = fill.linear_axis(bounds);
                     LinearGradient {
                         x1: p0.x as f32,
                         y1: p0.y as f32,

crates/render/src/lib.rs

@@ -889,7 +889,7 @@ pub fn color_of(c: &designcraft_color::Color, alpha: f32) -> peniko::Color {
 
 /// Set a swatch fill (solid or gradient) as the paint. Returns false for [None]/unknown.
 fn set_fill_paint(ctx: &mut RenderContext, doc: &Document, fill: &designcraft_doc::Fill, bounds: Rect) -> bool {
-    let (swatch, tint, angle) = (fill.swatch.as_str(), fill.tint, fill.gradient_angle);
+    let (swatch, tint) = (fill.swatch.as_str(), fill.tint);
     if let Some(g) = designcraft_color::swatch::resolve_gradient(&doc.swatches, swatch) {
         let stops: Vec<peniko::ColorStop> = g.expanded_stops().iter().map(|(o, c, a)| peniko::ColorStop::from((*o, color_of(c, *a)))).collect();
         if stops.is_empty() {
@@ -901,16 +901,13 @@ fn set_fill_paint(ctx: &mut RenderContext, doc: &Document, fill: &designcraft_do
                 let r = Vec2::new(x1 - x0, y1 - y0).hypot().max(1e-3);
                 peniko::Gradient::new_radial((x0, y0), r as f32).with_stops(stops.as_slice())
             }
-            (_, Some([x0, y0, x1, y1])) => peniko::Gradient::new_linear((x0, y0), (x1, y1)).with_stops(stops.as_slice()),
             (designcraft_color::GradientKind::Radial, None) => {
                 let r = bounds.width().max(bounds.height()) / 2.0;
                 peniko::Gradient::new_radial(c, r as f32).with_stops(stops.as_slice())
             }
             _ => {
-                let a = angle.unwrap_or(0.0).to_radians();
-                let half = (bounds.width() * a.cos().abs() + bounds.height() * a.sin().abs()) / 2.0;
-                let d = Vec2::new(a.cos(), -a.sin()) * half;
-                peniko::Gradient::new_linear(c - d, c + d).with_stops(stops.as_slice())
+                let (a, b) = fill.linear_axis(bounds);
+                peniko::Gradient::new_linear(a, b).with_stops(stops.as_slice())
             }
         };
         ctx.set_paint(grad);