Render: soft edges fade to the shape's outline (#289)

The soft-edge mask was the bounding box shrunk and blurred, so ovals and
other non-rectangular shapes kept hard edges where the box was wider, and
a rectangle's fade began outside it. The mask is now the outline itself,
shrunk by 0.85 r (a stroke punched out of the fill) and blurred by r/3:
transparent at the outline, half opaque 0.85 r inside, as measured on
PowerPoint's export of a 12 pt soft edge.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/render/src/lib.rs

@@ -765,18 +765,23 @@ impl Renderer {
             }
         }
         if let Some(r) = soft {
-            // Feather: fade the shape's alpha towards the outline.
+            // Feather: the shape (fill and line) fades to nothing at its outline. Measured on
+            // PowerPoint's export: the mask is the outline shrunk by 0.85 r, blurred by a third of
+            // r, so it's half opaque 0.85 r inside the edge and transparent at the edge itself.
             ctx.set_transform(m);
             ctx.push_layer(
                 None,
                 Some(peniko::BlendMode::new(peniko::Mix::Normal, peniko::Compose::DestIn)),
                 None,
                 None,
-                Some(blur(r * 0.5 * scale_of(m))),
+                Some(blur(r / 3.0 * scale_of(m))),
             );
             ctx.set_paint(peniko::Color::from_rgba8(0, 0, 0, 255));
-            let inset = Rect::new(0.0, 0.0, w, h).inset(-r * 0.6);
-            ctx.fill_rect(&inset);
+            ctx.fill_path(&outline);
+            ctx.push_blend_layer(peniko::BlendMode::new(peniko::Mix::Normal, peniko::Compose::DestOut));
+            ctx.set_stroke(kurbo::Stroke::new(r * 1.7).with_join(kurbo::Join::Round));
+            ctx.stroke_path(&outline);
+            ctx.pop_layer();
             ctx.pop_layer();
             ctx.pop_layer();
         }

crates/render/src/tests.rs

@@ -846,3 +846,29 @@ fn rect_path_gradients_draw_x_shaped_contours() {
     // Along the diagonal to a corner the colour follows the vertical share.
     assert!((v(168, 120) - 51.0).abs() < 6.0, "{}", v(168, 120));
 }
+
+#[test]
+fn soft_edges_fade_to_the_outline() {
+    // An ellipse with a 12 pt soft edge: transparent at its outline, half opaque about 10 pt
+    // inside, opaque further in, as in PowerPoint's export (the fade follows the curve, not the
+    // bounding box).
+    let sh = Shape {
+        id: ShapeId(780),
+        xfrm: Some(Xfrm::new(100.0, 100.0, 300.0, 200.0)),
+        geom: Geom::Preset { name: "ellipse".into(), adj: vec![] },
+        fill: Some(deckcraft_model::style::Fill::solid(ColorRef::rgb(deckcraft_color::Rgba::rgb(0, 0, 0)))),
+        line: Some(deckcraft_model::style::Line { fill: Some(deckcraft_model::style::Fill::None), ..Default::default() }),
+        effects: Some(deckcraft_model::Effects { soft_edge: Some(12.0), ..Default::default() }),
+        ..Default::default()
+    };
+    let p = deck_with(vec![sh]);
+    let img = render_slide(&p, 0, &RenderOpts { scale: 1.0, ..Default::default() });
+    let alpha = |x: u32, y: u32| 1.0 - img.pixel(x, y)[0] as f64 / 255.0;
+    // Top of the ellipse is at y = 100, x = 250.
+    assert!(alpha(250, 101) < 0.08, "edge {}", alpha(250, 101));
+    let mid = alpha(250, 110);
+    assert!((0.3..0.7).contains(&mid), "0.85 r inside: {mid}");
+    assert!(alpha(250, 130) > 0.97, "inside {}", alpha(250, 130));
+    // Near the ends of the bounding box's top edge, outside the ellipse, nothing is drawn.
+    assert!(alpha(110, 105) < 0.01);
+}