Fix #374: Shape Builder cuts regions with open paths and lines

The Shape Builder only used the filled areas of the selected paths, so a
stroke-only line or open path across a shape did nothing. Like the
reference app's Shape Builder, open paths now act as cutting edges:

- pathops::shape_builder: with an open path in the selection, the
  regions are the Live Paint planar map's faces (every path an edge), so a
  line across a shape splits it and lines can enclose a region of their
  own; it also returns each open path cut where other paths meet it. Closed
  shapes alone, more than SHAPE_BUILDER_MAX_SEGMENTS segments, or a map
  that can't be built fall back to the filled-area arrangement.
- Filled open paths count as closed (an invisible closing edge) where that
  encloses an area; unfilled open paths and straight lines stay edges.
- shapeBuilder.merge: merging or deleting regions on both sides of a line
  piece removes that piece (the line is split, its other pieces kept); an
  area only lines enclose takes the current fill and the front-most
  object's look. Alt (erase) within `tolerance` of a line piece deletes it.
  Returns `lines` (pieces deleted).
- shapeBuilder.regions also lists the line pieces.
- The tool shares BuilderMap/Touched with the command, highlights the line
  piece under the pointer while Alt is held and passes its tolerance.

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

crates/engine/src/cmd/buildcmds.rs

@@ -16,10 +16,11 @@ use serde_json::{Value, json};
 use vectorcraft_color::{Color, Paint};
 use vectorcraft_doc::appearance::{AppearanceItem, FillLayer, StrokeLayer};
 use vectorcraft_doc::{Appearance, Document, Node, NodeId, NodeKind, Selection};
-use vectorcraft_geom::{FillRule, Point, Rect, Shape as _};
+use vectorcraft_geom::{FillRule, PathData, Point, Rect};
 use vectorcraft_pathops as po;
 use vectorcraft_tools::builder::{
-    self as b, EDGE_NAME, FACE_NAME, LIVE_PAINT_NAME, SOURCES_NAME, edge_near, face_at, is_live_paint, sample_polyline, shapes_for, sorted_roots,
+    self as b, BuilderMap, EDGE_NAME, FACE_NAME, LIVE_PAINT_NAME, SOURCES_NAME, Touched, edge_near, face_at, is_live_paint, sample_polyline,
+    sorted_roots,
 };
 use vectorcraft_trace as tr;
 
@@ -38,7 +39,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Shape Builder",
             [],
             None,
-            "{ids?: [id…] (default: selection), points: [[x,y]…] (drag path; one point = click), erase?: bool (Alt-drag deletes the regions), fill?: \"#rrggbb\" | {color|swatch|gradient|none} (default: the fill of the object under the first point)} → {ids, merged, regions}",
+            "{ids?: [id…] (default: selection), points: [[x,y]…] (drag path; one point = click), erase?: bool (Alt-drag deletes the regions, and the pieces of unfilled open paths within tolerance), tolerance?: pt (2, erase only), fill?: \"#rrggbb\" | {color|swatch|gradient|none} (default: the fill of the object under the first point; the current fill for an area only lines enclose)} → {ids, merged, regions, lines}",
             has_selection_or_ids,
             sb_merge
         ),
@@ -47,7 +48,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Shape Builder Regions",
             [],
             None,
-            "{ids?} → {regions: [{bounds: [x0,y0,x1,y1], area, sources: [id…]}]} faces of the planar arrangement",
+            "{ids?} → {regions: [{bounds: [x0,y0,x1,y1], area, sources: [id…]}], lines: [{bounds, source: id}]} faces of the planar arrangement (unfilled open paths cut them; sources is empty for an area only they enclose) and the pieces of those open paths",
             has_selection_or_ids,
             sb_regions
         ),
@@ -224,37 +225,51 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
         return Err(bad(C, "points must be a non-empty list of finite [x,y]"));
     }
     let erase = bool_or(p, "erase", false);
+    // Line pieces are only deleted (Alt), never merged.
+    let line_tol = erase.then(|| f64_or(p, "tolerance", 2.0).clamp(0.0, 1000.0));
     let fill = fill_param(s, p, C)?;
+    let current_fill = s.paint.fill.clone();
     let roots = root_ids(s, p)?;
     let label = if erase { "Shape Builder Delete" } else { "Shape Builder" };
-    let (ids, merged, count) = s.edit(label, |d, sel| {
-        let (shapes, leaves) = shapes_for(d, &roots);
+    let (ids, merged, regions, lines) = s.edit(label, |d, sel| {
+        let (map, leaves) = BuilderMap::new(d, &roots);
         let leaves: Vec<Node> = leaves.into_iter().cloned().collect();
-        if shapes.is_empty() {
+        if map.shapes.is_empty() {
             return Err(EngineError::Other("Shape Builder: select paths or compound paths".into()));
         }
-        let regs = po::regions(&shapes);
-        let shapes_r: Vec<(kurbo::BezPath, Rect)> = regs.iter().map(|r| (r.path.to_bezpath(), r.path.bounds().unwrap_or(Rect::ZERO))).collect();
         let step = (d.bounds_of(&roots, false).map(|r| r.width().max(r.height())).unwrap_or(100.0) / 2000.0).max(0.25);
-        let mut touched: Vec<usize> = vec![];
+        let mut touched = Touched::default();
         for q in sample_polyline(&pts, step) {
-            if let Some(i) = shapes_r.iter().position(|(bp, bb)| bb.contains(q) && bp.winding(q) != 0)
-                && !touched.contains(&i)
-            {
-                touched.push(i);
-            }
+            touched.add(map.hit(q, line_tol));
         }
         if touched.is_empty() {
             return Err(EngineError::Other("Shape Builder: no region under the pointer".into()));
         }
-        let picked: Vec<&po::Region> = touched.iter().map(|&i| &regs[i]).collect();
-        let union = po::merge_regions(&picked);
-        let affected: Vec<bool> = (0..shapes.len()).map(|i| picked.iter().any(|r| r.sources.contains(&i))).collect();
-        let style_src = regs[touched[0]].top();
+        let picked: Vec<&po::Region> = touched.regions.iter().filter_map(|&i| map.regions.get(i).map(|r| &r.0)).collect();
+        let union = if picked.is_empty() { PathData::default() } else { po::merge_regions(&picked) };
+        // The merged shape takes the look of the front-most object over the first region, or (for an
+        // area only lines enclose) of the front-most object and the current fill.
+        let covered = picked.iter().find_map(|r| r.top());
+        let style_src = covered.or(map.shapes.len().checked_sub(1));
+        let merged_fill = fill.or_else(|| covered.is_none().then_some(current_fill));
         let mut new_nodes: Vec<Node> = vec![];
         let mut merged_id = None;
-        for (i, (sh, leaf)) in shapes.iter().zip(&leaves).enumerate() {
-            if !affected[i] {
+        for (i, (sh, leaf)) in map.shapes.iter().zip(&leaves).enumerate() {
+            let pieces: Vec<usize> = map.lines.iter().enumerate().filter(|(_, (l, _))| l.key == i as u64).map(|(k, _)| k).collect();
+            if !pieces.is_empty() {
+                // An open path loses the pieces deleted and those between merged (or deleted) regions.
+                let keep: Vec<usize> =
+                    pieces.iter().copied().filter(|k| !touched.lines.contains(k) && !map.line_between(*k, &touched.regions)).collect();
+                if keep.len() == pieces.len() {
+                    new_nodes.push(d.reid(leaf));
+                } else {
+                    for k in keep {
+                        if let Some((l, _)) = map.lines.get(k) {
+                            new_nodes.push(shape_node(d, l.path.clone(), Some(leaf)));
+                        }
+                    }
+                }
+            } else if !picked.iter().any(|r| r.sources.contains(&i)) {
                 new_nodes.push(d.reid(leaf));
             } else {
                 let rest = po::boolean(&sh.path, sh.rule, &union, FillRule::NonZero, po::BoolOp::Difference);
@@ -262,9 +277,9 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
                     new_nodes.push(shape_node(d, rest, Some(leaf)));
                 }
             }
-            if !erase && Some(i) == style_src {
+            if !erase && !union.is_empty() && Some(i) == style_src {
                 let mut m = shape_node(d, union.clone(), Some(leaf));
-                if let Some(f) = &fill {
+                if let Some(f) = &merged_fill {
                     m.appearance.set_fill(f.clone());
                 }
                 merged_id = Some(m.id);
@@ -273,9 +288,9 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
         }
         let new_ids = replace_roots(d, &roots, new_nodes)?;
         sel.set(new_ids.iter().copied());
-        Ok((new_ids, merged_id, touched.len()))
+        Ok((new_ids, merged_id, touched.regions.len(), touched.lines.len()))
     })?;
-    Ok(json!({ "ids": ids_json(&ids), "merged": merged.map(|m| m.0), "regions": count }))
+    Ok(json!({ "ids": ids_json(&ids), "merged": merged.map(|m| m.0), "regions": regions, "lines": lines }))
 }
 
 /// Remove the roots and insert `nodes` where the front-most root was. Returns the new ids.
@@ -300,20 +315,22 @@ fn replace_roots(d: &mut Document, roots: &[NodeId], nodes: Vec<Node>) -> Result
 fn sb_regions(s: &mut Session, p: &Value) -> Result<Value> {
     let roots = root_ids(s, p)?;
     let d = &s.doc()?.doc;
-    let (shapes, leaves) = shapes_for(d, &roots);
-    let regs = po::regions(&shapes);
-    let out: Vec<Value> = regs
+    let (map, leaves) = BuilderMap::new(d, &roots);
+    let id_of = |i: usize| leaves.get(i).map(|n| n.id.0);
+    let regions: Vec<Value> = map
+        .regions
         .iter()
-        .filter_map(|r| {
-            let bb = r.path.bounds()?;
-            Some(json!({
+        .map(|(r, _, bb)| {
+            json!({
                 "bounds": [bb.x0, bb.y0, bb.x1, bb.y1],
                 "area": po::area(&r.path, FillRule::NonZero),
-                "sources": r.sources.iter().map(|&i| leaves[i].id.0).collect::<Vec<_>>(),
-            }))
+                "sources": r.sources.iter().filter_map(|&i| id_of(i)).collect::<Vec<_>>(),
+            })
         })
         .collect();
-    Ok(json!({ "regions": out }))
+    let lines: Vec<Value> =
+        map.lines.iter().map(|(l, bb)| json!({ "bounds": [bb.x0, bb.y0, bb.x1, bb.y1], "source": id_of(l.key as usize) })).collect();
+    Ok(json!({ "regions": regions, "lines": lines }))
 }
 
 // ---------- Live Paint ----------

crates/engine/src/tests_build.rs

@@ -172,6 +172,154 @@ fn shape_builder_tool_drives_the_command() {
     assert_eq!(layer_children(&s), 1);
 }
 
+/// A red 100 × 100 square crossed by a stroke-only vertical line at x = 50, both selected.
+fn square_and_line(s: &mut Session) -> (NodeId, NodeId) {
+    let a = rect(s, 0.0, 0.0, 100.0, 100.0);
+    set_fill(s, a, Color::rgb(1.0, 0.0, 0.0));
+    let l = line(s, (50.0, -20.0), (50.0, 120.0));
+    s.execute("select.set", &json!({"ids": [a.0, l.0]})).unwrap();
+    (a, l)
+}
+
+/// The open paths among `ids`, as (anchors, bounds).
+fn open_paths(s: &Session, ids: &[NodeId]) -> Vec<(usize, Rect)> {
+    ids.iter()
+        .map(|i| node(s, *i))
+        .filter(|n| !outline(n).0.is_closed())
+        .map(|n| (outline(&n).0.anchor_count(), n.geometric_bounds().unwrap()))
+        .collect()
+}
+
+#[test]
+fn shape_builder_line_splits_a_shape_into_regions() {
+    let mut s = session();
+    let (a, l) = square_and_line(&mut s);
+    let r = s.execute("shapeBuilder.regions", &json!({})).unwrap();
+    let regs = r["regions"].as_array().unwrap();
+    assert_eq!(regs.len(), 2, "{r}");
+    for x in regs {
+        assert!((x["area"].as_f64().unwrap() - 5000.0).abs() < 1e-6);
+        assert_eq!(x["sources"], json!([a.0]));
+    }
+    let lines = r["lines"].as_array().unwrap();
+    assert_eq!(lines.len(), 3, "the line cut at both sides of the square");
+    assert!(lines.iter().all(|x| x["source"] == l.0));
+}
+
+#[test]
+fn shape_builder_click_on_a_side_of_a_line_splits_the_shape() {
+    let mut s = session();
+    square_and_line(&mut s);
+    let r = s.execute("shapeBuilder.merge", &json!({"points": [[25, 50]]})).unwrap();
+    assert_eq!(r["regions"], 1);
+    let out = ids(&r);
+    let merged = NodeId(r["merged"].as_u64().unwrap());
+    assert_eq!(node(&s, merged).geometric_bounds().unwrap(), Rect::new(0.0, 0.0, 50.0, 100.0));
+    assert_eq!(node(&s, merged).appearance.fill_paint(), Paint::solid(Color::rgb(1.0, 0.0, 0.0)));
+    let closed: Vec<f64> = out.iter().filter(|i| outline(&node(&s, **i)).0.is_closed()).map(|i| area(&s, *i)).collect();
+    assert_eq!(closed.len(), 2, "both halves");
+    assert!(closed.iter().all(|a| (a - 5000.0).abs() < 1.0), "{closed:?}");
+    // The line only bounds the region: it stays whole.
+    assert_eq!(open_paths(&s, &out), vec![(2, Rect::new(50.0, -20.0, 50.0, 120.0))]);
+}
+
+#[test]
+fn shape_builder_merge_across_a_line_removes_the_piece_between() {
+    let mut s = session();
+    square_and_line(&mut s);
+    let r = s.execute("shapeBuilder.merge", &json!({"points": [[25, 50], [75, 50]]})).unwrap();
+    assert_eq!(r["regions"], 2);
+    let out = ids(&r);
+    let merged = NodeId(r["merged"].as_u64().unwrap());
+    assert!((area(&s, merged) - 10000.0).abs() < 1.0);
+    assert_eq!(out.len(), 3, "merged square + the line's two loose ends");
+    let mut ends = open_paths(&s, &out);
+    ends.sort_by(|a, b| a.1.y0.total_cmp(&b.1.y0));
+    assert_eq!(ends, vec![(2, Rect::new(50.0, -20.0, 50.0, 0.0)), (2, Rect::new(50.0, 100.0, 50.0, 120.0))]);
+    s.execute("edit.undo", &json!({})).unwrap();
+    assert_eq!(layer_children(&s), 2);
+}
+
+#[test]
+fn shape_builder_alt_deletes_a_region_or_a_line_piece() {
+    let mut s = session();
+    square_and_line(&mut s);
+    let r = s.execute("shapeBuilder.merge", &json!({"points": [[75, 50]], "erase": true})).unwrap();
+    assert!(r["merged"].is_null());
+    let out = ids(&r);
+    let left: Vec<NodeId> = out.iter().copied().filter(|i| outline(&node(&s, *i)).0.is_closed()).collect();
+    assert_eq!(left.len(), 1);
+    assert_eq!(node(&s, left[0]).geometric_bounds().unwrap(), Rect::new(0.0, 0.0, 50.0, 100.0));
+    assert_eq!(open_paths(&s, &out).len(), 1, "the line stays whole");
+
+    // Alt on the line inside the square deletes that piece only.
+    let mut s = session();
+    square_and_line(&mut s);
+    let r = s.execute("shapeBuilder.merge", &json!({"points": [[51, 50]], "erase": true, "tolerance": 2})).unwrap();
+    assert_eq!((r["regions"].as_u64(), r["lines"].as_u64()), (Some(0), Some(1)));
+    let out = ids(&r);
+    assert_eq!(out.len(), 3);
+    let sq: Vec<NodeId> = out.iter().copied().filter(|i| outline(&node(&s, *i)).0.is_closed()).collect();
+    assert_eq!(sq.len(), 1);
+    assert!((area(&s, sq[0]) - 10000.0).abs() < 1.0, "the square is untouched");
+    assert_eq!(open_paths(&s, &out).len(), 2);
+}
+
+#[test]
+fn shape_builder_fills_an_area_lines_enclose() {
+    let mut s = session();
+    let l1 = line(&mut s, (0.0, 0.0), (100.0, 100.0));
+    let l2 = line(&mut s, (100.0, 0.0), (0.0, 100.0));
+    let l3 = line(&mut s, (-10.0, 80.0), (110.0, 80.0));
+    s.execute("select.set", &json!({"ids": [l1.0, l2.0, l3.0]})).unwrap();
+    let r = s.execute("shapeBuilder.regions", &json!({})).unwrap();
+    assert_eq!(r["regions"].as_array().unwrap().len(), 1);
+    assert_eq!(r["regions"][0]["sources"], json!([]));
+    let r = s.execute("shapeBuilder.merge", &json!({"points": [[50, 70]]})).unwrap();
+    let merged = NodeId(r["merged"].as_u64().unwrap());
+    assert!((area(&s, merged) - 900.0).abs() < 1e-6, "{}", area(&s, merged));
+    assert_eq!(node(&s, merged).appearance.fill_paint(), s.paint.fill, "the current fill");
+    assert!(!node(&s, merged).appearance.stroke_paint().is_none(), "the lines' stroke");
+    assert_eq!(ids(&r).len(), 4, "the triangle + the three lines, which only bound it");
+}
+
+#[test]
+fn shape_builder_closes_filled_open_paths() {
+    let mut s = session();
+    let r = s.execute("path.create", &json!({"anchors": [{"x": 0, "y": 0}, {"x": 100, "y": 0}, {"x": 100, "y": 100}]})).unwrap();
+    let p = NodeId(r["id"].as_u64().unwrap());
+    set_fill(&mut s, p, Color::rgb(0.0, 1.0, 0.0));
+    s.execute("select.set", &json!({"ids": [p.0]})).unwrap();
+    let r = s.execute("shapeBuilder.regions", &json!({})).unwrap();
+    assert_eq!(r["regions"].as_array().unwrap().len(), 1, "the fill's area, closed by an invisible edge");
+    assert!((r["regions"][0]["area"].as_f64().unwrap() - 5000.0).abs() < 1e-6);
+    assert_eq!(r["lines"], json!([]));
+    // Unfilled, it is only an edge.
+    s.edit("t", |d, _| {
+        d.node_mut(p).unwrap().appearance.set_fill(Paint::None);
+        Ok(())
+    })
+    .unwrap();
+    let r = s.execute("shapeBuilder.regions", &json!({})).unwrap();
+    assert_eq!(r["regions"], json!([]));
+    assert_eq!(r["lines"].as_array().unwrap().len(), 1);
+}
+
+#[test]
+fn shape_builder_tool_alt_deletes_line_pieces() {
+    let mut s = session();
+    square_and_line(&mut s);
+    let v = ViewInfo::default();
+    s.select_tool("shapeBuilder", v).unwrap();
+    let alt = Mods { alt: true, ..Default::default() };
+    s.pointer(&PointerEvent::new(PointerKind::Move, 51.0, 50.0).with_mods(alt), v).unwrap();
+    assert_eq!(s.overlays(v).len(), 1, "the line piece is highlighted");
+    s.pointer(&PointerEvent::new(PointerKind::Down, 51.0, 50.0).with_mods(alt), v).unwrap();
+    s.pointer(&PointerEvent::new(PointerKind::Up, 51.0, 50.0).with_mods(alt), v).unwrap();
+    assert_eq!(s.journal.last().unwrap().0, "shapeBuilder.merge");
+    assert_eq!(layer_children(&s), 3, "square + the line's two loose ends");
+}
+
 // ---------- Live Paint ----------
 
 fn live(s: &mut Session) -> NodeId {