Render: pictures cast their shadow from their pixels, as in PowerPoint (#338)

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

crates/render/src/lib.rs

@@ -785,7 +785,12 @@ 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 picture (or picture fill) casts its shadow from its pixels, not its frame.
+            let content = match (&s.kind, &fill) {
+                (ShapeKind::Picture { fill: pf }, _) | (_, Some(Fill::Picture(pf))) => Some(paint::resolved(rctx, pf, None).into_owned()),
+                _ => None,
+            };
+            self.effects_below(ctx, rctx, e, fx_ph, &geo, &fill, content.as_ref(), m, w, h);
         }
         let soft = effects.as_ref().and_then(|e| e.soft_edge).filter(|r| *r > 0.0);
         if let Some(r) = soft {
@@ -959,6 +964,7 @@ impl Renderer {
         ph: Option<Rgba>,
         geo: &Geometry,
         fill: &Option<Fill>,
+        content: Option<&deckcraft_model::style::PictureFill>,
         m: Affine,
         w: f64,
         h: f64,
@@ -986,8 +992,24 @@ impl Renderer {
             // Blurred by a third of the blur radius, as PowerPoint's export shows.
             ctx.push_layer(None, None, Some((c.a as f32) / 255.0), None, Some(blur(sh.blur / 3.0 * k)));
             ctx.set_transform(sm);
-            ctx.set_paint(color(c.with_alpha(255), 1.0));
-            ctx.fill_path(&silhouette);
+            match content {
+                // As PowerPoint draws it: the shadow is the picture's own coverage (a logo on a
+                // transparent background casts the logo's shadow): the picture, clipped to the
+                // shape, recoloured to the shadow colour.
+                Some(pf) => {
+                    ctx.push_clip_layer(&silhouette);
+                    paint::picture(ctx, rctx.pres, pf, Rect::new(0.0, 0.0, w, h), sm, None);
+                    ctx.push_layer(None, Some(peniko::BlendMode::new(peniko::Mix::Normal, peniko::Compose::SrcIn)), None, None, None);
+                    ctx.set_paint(color(c.with_alpha(255), 1.0));
+                    ctx.fill_rect(&Rect::new(0.0, 0.0, w, h).inflate(1.0, 1.0));
+                    ctx.pop_layer();
+                    ctx.pop_layer();
+                }
+                None => {
+                    ctx.set_paint(color(c.with_alpha(255), 1.0));
+                    ctx.fill_path(&silhouette);
+                }
+            }
             ctx.pop_layer();
         }
         if let Some(g) = &e.glow {

crates/render/src/tests.rs

@@ -1506,3 +1506,50 @@ fn group_child_scale_does_not_scale_line_widths() {
         assert!(d * 20 < drawn, "{name}: {d} differing values against {drawn} inked pixels");
     }
 }
+
+#[test]
+fn a_picture_casts_its_shadow_from_its_pixels() {
+    // PowerPoint shades only where a picture paints: a logo on a transparent background casts the
+    // logo's shadow, not its frame's. Here the left half of the image is transparent.
+    let png = {
+        let img = image::RgbaImage::from_fn(8, 4, |x, _| if x < 4 { image::Rgba([0, 0, 0, 0]) } else { image::Rgba([20, 160, 20, 255]) });
+        let mut out = std::io::Cursor::new(vec![]);
+        image::DynamicImage::ImageRgba8(img).write_to(&mut out, image::ImageFormat::Png).unwrap();
+        out.into_inner()
+    };
+    let shadow = deckcraft_model::style::Shadow {
+        color: ColorRef::rgb(deckcraft_color::Rgba::BLACK),
+        blur: 0.0,
+        dist: 20.0,
+        dir: 90.0,
+        inner: false,
+        sx: 1.0,
+        sy: 1.0,
+        kx: 0.0,
+        ky: 0.0,
+        align: String::new(),
+        rotate_with_shape: false,
+    };
+    for as_fill in [false, true] {
+        let mut p = Presentation::default();
+        let media = p.add_media("logo.png", "image/png", png.clone());
+        let pf = deckcraft_model::style::PictureFill { media, ..Default::default() };
+        let mut sh = Shape { id: ShapeId(600), xfrm: Some(Xfrm::new(100.0, 100.0, 400.0, 200.0)), ..Default::default() };
+        if as_fill {
+            sh.fill = Some(Fill::Picture(pf));
+            sh.line = Some(deckcraft_model::style::Line { fill: Some(Fill::None), ..Default::default() });
+        } else {
+            sh.kind = ShapeKind::Picture { fill: pf };
+        }
+        sh.effects = Some(deckcraft_model::Effects { outer_shadow: Some(shadow.clone()), ..Default::default() });
+        std::sync::Arc::make_mut(&mut p.slides[0]).shapes = vec![sh];
+        let img = render_slide(&p, 0, &RenderOpts { scale: 1.0, ..Default::default() });
+        // Below the picture, 10 pt down: shadow under the opaque right half only.
+        let left = img.pixel(200, 310);
+        let right = img.pixel(400, 310);
+        assert!(left[0] > 240, "no shadow under the transparent half ({as_fill}): {left:?}");
+        assert!(right[0] < 40, "shadow under the opaque half ({as_fill}): {right:?}");
+        // The transparent half shows the slide, not a shadow behind it.
+        assert!(img.pixel(200, 200)[0] > 240, "{as_fill}");
+    }
+}