Merge pull request #2900 from nrq6993-a11y/stream/vc-issue-2723-reshape-falloff

Fix #2723: Reshape tool falloff follows the path, so nodes stretch progressively

crates/engine/src/cmd/path.rs

@@ -100,7 +100,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Reshape",
             [],
             None,
-            "{id, x, y, dx, dy, tol=3, radius?} Reshape tool: grab the path at (x, y) (adding an anchor there unless one is within tol) and drag it by (dx, dy); nearby anchors follow with a smooth falloff over radius (default: half the subpath's size)",
+            "{id, x, y, dx, dy, tol=3, radius?} Reshape tool: grab the path at (x, y) (adding an anchor there unless one is within tol) and drag it by (dx, dy); nearby anchors follow with a smooth falloff over radius, measured along the path (default: half the subpath's size)",
             has_doc,
             reshape
         ),
@@ -680,18 +680,27 @@ fn reshape(s: &mut Session, p: &Value) -> Result<Value> {
             Some((i, dist)) if dist <= tol => i,
             _ => sp.insert_anchor(seg, t),
         };
-        let origin = sp.anchors[grabbed].p;
+        // Each anchor's distance along the path from the grabbed one, the shorter way round a
+        // closed path: the lengths are measured once, as a path can have many anchors.
+        let pos = sp.anchor_arc_positions();
+        let total = pos.last().copied().unwrap_or(0.0);
+        // The default reach is the subpath's size, as before, now measured along it.
         let r = radius.unwrap_or_else(|| {
             let mut bp = vectorcraft_geom::BezPath::new();
             sp.to_bezpath_into(&mut bp);
             let b = vectorcraft_geom::Shape::bounding_box(&bp);
             (b.width().hypot(b.height()) / 2.0).max(1.0)
         });
+        let from = pos.get(grabbed).copied().unwrap_or(0.0);
+        let closed = sp.closed;
         for (i, a) in sp.anchors.iter_mut().enumerate() {
             let w = if i == grabbed {
                 1.0
             } else {
-                let q = (a.p.distance(origin) / r).min(1.0);
+                let d = (pos.get(i).copied().unwrap_or(f64::INFINITY) - from).abs();
+                let d = if closed { d.min(total - d) } else { d };
+                // A distance or radius that isn't finite gives NaN here, which `min` reads as 1: no move.
+                let q = (d / r).min(1.0);
                 (1.0 - q * q).powi(2)
             };
             let v = delta * w;

crates/engine/src/tests.rs

@@ -2343,6 +2343,29 @@ fn reshape_moves_the_grabbed_point_and_its_neighbourhood() {
     assert!(pts.iter().filter(|p| (p.y - 100.0).abs() < 1e-9).count() == 2);
 }
 
+#[test]
+fn reshape_falloff_follows_arc_length_on_open_paths() {
+    let mut s = session();
+    let id = NodeId(
+        s.execute(
+            "path.create",
+            &json!({
+                "anchors": [{"x": 0, "y": 0}, {"x": 100, "y": 0}, {"x": 200, "y": 0}, {"x": 300, "y": 0}],
+                "closed": false
+            }),
+        )
+        .unwrap()["id"]
+            .as_u64()
+            .unwrap(),
+    );
+    s.execute("path.reshape", &json!({"id": id.0, "x": 300.0, "y": 0.0, "dx": 0.0, "dy": 100.0})).unwrap();
+    let pts: Vec<_> = s.doc().unwrap().doc.node(id).unwrap().path_data().unwrap().subpaths[0].anchors.iter().map(|a| a.p).collect();
+    assert!((pts[3].y - 100.0).abs() < 1e-6, "grabbed end moves full delta: {pts:?}");
+    assert!(pts[0].y.abs() < 1e-6, "far end stays put: {pts:?}");
+    assert!(pts[2].y > 1.0 && pts[2].y < 100.0, "mid-anchor moves partially: {pts:?}");
+    assert!(pts[1].y < pts[2].y, "nearer anchors move more: {pts:?}");
+}
+
 #[test]
 fn graph_tool_drag_creates_a_graph_and_asks_for_data() {
     let mut s = session();

crates/geom/src/path.rs

@@ -221,6 +221,19 @@ impl SubPath {
         let b = &self.anchors[(i + 1) % n];
         !a.has_out() && !b.has_in()
     }
+    /// Arc-length at each anchor from the start of this subpath (one entry per anchor).
+    pub fn anchor_arc_positions(&self) -> Vec<f64> {
+        let n = self.anchors.len();
+        if n == 0 {
+            return vec![];
+        }
+        let mut pos = vec![0.0];
+        for i in 0..self.segment_count() {
+            let len = kurbo::ParamCurveArclen::arclen(&self.segment(i), 1e-3);
+            pos.push(pos.last().copied().unwrap_or(0.0) + len);
+        }
+        pos
+    }
     pub fn to_bezpath_into(&self, out: &mut BezPath) {
         let Some(first) = self.anchors.first() else { return };
         out.move_to(first.p);