Effects: a Transform effect on a group paints each copy with its own gradient (#2451)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/effects/src/group.rs

@@ -491,4 +491,32 @@ mod tests {
         assert!((after - before - 30.0).abs() < 1e-6, "the type's gradient starts at {before}, then {after}");
         assert_eq!(start(&members[0]), 0.0, "the square's gradient stays where it was placed");
     }
+
+    #[test]
+    fn a_transform_on_a_group_paints_each_copy_with_its_own_gradient() {
+        use vectorcraft_color::{Gradient, GradientGeom, GradientKind, GradientPaint, GradientStop};
+        let stops = vec![GradientStop::new(0.0, Color::rgb(1.0, 0.0, 0.0)), GradientStop::new(1.0, Color::rgb(0.0, 0.0, 1.0))];
+        let ramp = GradientPaint::new(Gradient { kind: GradientKind::Linear, stops });
+        let geom = GradientGeom { start: (0.0, 5.0).into(), end: (10.0, 5.0).into(), aspect: 1.0, focal: None };
+        let paints = [Paint::Gradient(Box::new(ramp.clone())), Paint::Gradient(Box::new(GradientPaint { geom: Some(geom), ..ramp }))];
+        for (k, paint) in paints.into_iter().enumerate() {
+            let square =
+                Arc::new(Node::path(NodeId(1), shapes::rectangle(Rect::new(0.0, 0.0, 10.0, 10.0)), Appearance::basic(paint, Paint::None, 0.0)));
+            let mut g = Node::group(NodeId(10), vec![square]);
+            let params = serde_json::json!({"moveH": 30, "copies": 2});
+            g.appearance.effects.push(Effect { id: "distort.transform".into(), params, visible: true });
+            let art = evaluate_container(&g).unwrap();
+            let copies = art.children().unwrap()[0].children().expect("the object and its copies are objects of their own");
+            assert_eq!(copies.len(), 3);
+            for (i, c) in copies.iter().enumerate() {
+                // Each copy's own box: an unplaced gradient fits it, a placed one moved with it.
+                assert!((c.geometric_bounds().unwrap().x0 - 30.0 * i as f64).abs() < 1e-6);
+                match c.appearance.fill_paint() {
+                    Paint::Gradient(g) if k == 0 => assert!(g.geom.is_none(), "copy {i} fits its own box"),
+                    Paint::Gradient(g) => assert!((g.geom.unwrap().start.x - 30.0 * i as f64).abs() < 1e-6, "copy {i}"),
+                    p => panic!("a gradient: {p:?}"),
+                }
+            }
+        }
+    }
 }

crates/effects/src/reshape.rs

@@ -268,13 +268,21 @@ pub(crate) fn reshape_leaves(art: &mut Node, fx: &[&Effect]) {
     for e in fx {
         let Some(bounds) = art.geometric_bounds() else { break };
         let params = merged_params(&e.id, &e.params);
+        let transform = (e.id == "distort.transform").then(|| crate::distort::transform_map(bounds, &params, seed));
         for_each_leaf(art, &mut |leaf| {
             let Some((path, rule)) = leaf_geometry(leaf) else { return };
+            // A gradient paints each copy by itself, not the one piece the copies make together.
+            if let Some((m, copies)) = transform
+                && copies > 0
+                && (leaf.appearance.has_unplaced_gradient() || leaf.appearance.has_placed_gradient())
+            {
+                copy_with_gradients(leaf, &path, m, copies);
+                return;
+            }
             let out = apply_one(&e.id, &params, &path, bounds, &GeomContext { seed, ..GeomContext::of(leaf) });
             set_geometry(leaf, out, rule);
         });
-        if e.id == "distort.transform" {
-            let (m, copies) = crate::distort::transform_map(bounds, &params, seed);
+        if let Some((m, copies)) = transform {
             transform_placed(art, m, copies);
         }
     }
@@ -283,6 +291,34 @@ pub(crate) fn reshape_leaves(art: &mut Node, fx: &[&Effect]) {
     }
 }
 
+/// Replace `leaf` (a path or compound path with geometry `path`) by a group of it and `copies`
+/// more, each mapped once more by `m` than the last, as a Transform effect copies an object. Each
+/// copy is an object of its own, so its gradients fit it (a placed gradient moves with it) rather
+/// than spanning every copy. The copies stop where [`crate::distort::transform_within`]'s would.
+fn copy_with_gradients(leaf: &mut Node, path: &PathData, m: Affine, copies: usize) {
+    let each = path.anchor_count();
+    let mut anchors = each;
+    let mut placements: Vec<Arc<Node>> = vec![Arc::new(leaf.clone())];
+    let mut xf = Affine::IDENTITY;
+    for _ in 0..copies {
+        anchors = anchors.saturating_add(each);
+        if anchors > crate::distort::MAX_COPY_ANCHORS {
+            break;
+        }
+        xf *= m;
+        let mut copy = leaf.clone();
+        copy.transform(xf, vectorcraft_doc::Scaling::default());
+        if !copy.geometric_bounds().is_some_and(crate::distort::within_reach) {
+            break;
+        }
+        placements.push(Arc::new(copy));
+    }
+    // The copies become nodes of their own under a group with the object's id and name.
+    let mut g = Node::group(leaf.id, placements);
+    g.name = leaf.name.clone();
+    *leaf = g;
+}
+
 /// The bounds of each group of type's outlines under `n` ([`outline_text`]), `n` included, in
 /// order. A group inside one of them is part of it.
 fn type_outline_bounds(n: &Node, out: &mut Vec<Option<Rect>>) {