Render: an unfilled shape's glow follows its line

A glow always filled the shape's whole silhouette, so a frame with no fill
(a rounded-rectangle border with a soft glow, common in corporate decks)
came out as a solid tinted panel over the slide. PowerPoint glows such a
shape along its outline only and leaves the inside clear; a filled shape's
glow is unchanged.

Checked against PowerPoint's own PDF export of 31 real decks (597 slides):
the affected slides' mean pixel difference drops from ~29 to ~4 (of 255).

crates/render/src/lib.rs

@@ -668,7 +668,9 @@ impl Renderer {
         let fill = if let Some(t) = st.tint { Some(Fill::solid(deckcraft_model::ColorRef::rgb(t))) } else { fill };
         // Effects below the shape.
         if let Some(e) = &effects {
-            self.effects_below(ctx, rctx, e, fx_ph, &geo, &fill, m, w, h);
+            // A shape without a fill (an outline, a frame) glows along its line only.
+            let filled = is_pic || fill.as_ref().is_some_and(|f| !matches!(f, Fill::None));
+            self.effects_below(ctx, rctx, e, fx_ph, &geo, &fill, filled, m, w, h);
         }
         let soft = effects.as_ref().and_then(|e| e.soft_edge).filter(|r| *r > 0.0);
         if let Some(r) = soft {
@@ -801,6 +803,7 @@ impl Renderer {
         ph: Option<Rgba>,
         geo: &Geometry,
         fill: &Option<Fill>,
+        filled: bool,
         m: Affine,
         w: f64,
         h: f64,
@@ -837,9 +840,15 @@ impl Renderer {
             ctx.push_layer(None, None, Some((c.a as f32) / 255.0), None, Some(blur(g.radius * 0.35 * k)));
             ctx.set_transform(m);
             ctx.set_paint(color(c.with_alpha(255), 1.0));
-            ctx.set_stroke(kurbo::Stroke::new(g.radius.max(0.0) * 1.2).with_join(kurbo::Join::Round));
-            ctx.stroke_path(&silhouette);
-            ctx.fill_path(&silhouette);
+            if filled {
+                ctx.set_stroke(kurbo::Stroke::new(g.radius.max(0.0) * 1.2).with_join(kurbo::Join::Round));
+                ctx.stroke_path(&silhouette);
+                ctx.fill_path(&silhouette);
+            } else {
+                // The glow spreads its radius to both sides of the line; the inside stays clear.
+                ctx.set_stroke(kurbo::Stroke::new(g.radius.max(0.0) * 2.0).with_join(kurbo::Join::Round));
+                ctx.stroke_path(&silhouette);
+            }
             ctx.pop_layer();
         }
         if let Some(rf) = &e.reflection {

crates/render/src/tests.rs

@@ -124,6 +124,34 @@ fn effects_tables_charts_render() {
     assert!(px[0] < 250, "{px:?}");
 }
 
+#[test]
+fn glow_of_an_unfilled_shape_follows_its_line() {
+    // A frame (no fill, thin line) with a strong glow: the glow lines its edge, the inside stays
+    // the slide's white, as PowerPoint draws it.
+    let glow = |fill: Option<Fill>| {
+        let mut sh = Shape {
+            id: ShapeId(610),
+            xfrm: Some(Xfrm::new(100.0, 100.0, 400.0, 300.0)),
+            fill,
+            line: Some(deckcraft_model::style::Line::solid(ColorRef::rgb(deckcraft_color::Rgba::rgb(0, 176, 240)), 0.25)),
+            ..Default::default()
+        };
+        sh.effects = Some(deckcraft_model::Effects {
+            glow: Some(deckcraft_model::style::Glow { color: ColorRef::rgb(deckcraft_color::Rgba::rgb(0, 0, 255)), radius: 8.0 }),
+            ..Default::default()
+        });
+        render_slide(&deck_with(vec![sh]), 0, &RenderOpts { scale: 1.0, ..Default::default() })
+    };
+    let img = glow(Some(Fill::None));
+    let inside = img.pixel(300, 250);
+    assert!(inside[0] > 245 && inside[1] > 245, "{inside:?}");
+    let edge = img.pixel(300, 97);
+    assert!(edge[0] < 230, "{edge:?}");
+    // A filled shape's glow is still the whole silhouette's (the fill covers its inside).
+    let img = glow(Some(Fill::solid(ColorRef::rgb(deckcraft_color::Rgba::rgb(255, 255, 255)))));
+    assert!(img.pixel(300, 94)[0] < 240);
+}
+
 #[test]
 fn placeholder_prompts_only_in_edit_view() {
     let p = Presentation::default();