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@@ -1023,6 +1023,161 @@ implementation cost:
 
 % Future sessions land below this line as new \subsection entries.
 
+% -----------------------------------------------------------------
+\subsection{2026-04-25 --- Session 3: Milestone 0.3 (I3 fix,
+minimum-change deformation, SplitSegment preserve)}
+\label{sec:session-3}
+
+\paragraph{Goal of the session.}
+Three things came out of looking at session-2's figures:
+
+\begin{enumerate}[noitemsep]
+  \item The Y-intersection figure had visible parcel overlaps. A
+  programmatic test confirmed a real I3 violation — parcels from
+  adjacent block edges were converging into the same interior near
+  the acute outer corners.
+  \item The road-edit figure was \emph{too} eager to deform parcels.
+  When a road's bottom-right corner moved outward, both the bottom
+  road's parcels (whose road just got longer along the same line)
+  and the right road's parcels (whose road actually rotated) showed
+  up as deformed. The user's preference: only deform parcels on a
+  road whose direction \emph{actually changed}.
+  \item Pushing on milestone 0.3 work — \texttt{SplitSegment}
+  preserve, and as much of the acute-corner / curve story as we can
+  fit.
+\end{enumerate}
+
+\paragraph{Decisions locked in this session.}
+
+\begin{decision}[D14, 2026-04-25 -- Minimum-change deformation
+(``no-op preserve'')]
+When a road edit doesn't change a road's \emph{line} — only its
+endpoints shift along the same line, e.g.\ when one node moves
+parallel to the road — a parcel whose frontage is still entirely on
+the new segment is reported as ``Untouched''. It stays at its
+absolute coordinates and isn't added to any
+\texttt{EditOutcome} bucket. Trade-off: strict
+\emph{vertex-by-vertex} inverse-restore is no longer guaranteed
+(corner parcels that got displaced by an earlier edit aren't pulled
+back to their original spot when the edit is reversed); the
+\texttt{road\_edit\_inverse\_restores} test now checks centroid
+drift bounded by the edit delta instead.
+\end{decision}
+
+\begin{decision}[D15, 2026-04-25 -- Bisector-clip at acute corners,
+not obtuse]
+Acute corners (interior angle $< 60^\circ$) get no corner parcel —
+the rectangle/parallelogram construction would extend past the
+wedge boundary. Instead, regular parcels along the two edges meeting
+at an acute corner get bisector-clipped at that corner, so their
+territories stay separated. Obtuse corners ($\geq 60^\circ$) keep
+the milestone-0.2 corner parcel and need no bisector clip.
+\end{decision}
+
+\begin{decision}[D16, 2026-04-25 -- \texttt{SplitSegment} preserve
+on 4-vertex parcels]
+When a road is split, parcels whose frontage entirely on one side of
+the split point have their \texttt{frontage\_road} rebound (no
+geometric change, reported as Deformed). Parcels whose frontage
+spans the split point are cut into two parcels along a perpendicular
+through the split — only for the simple 4-vertex (rectangle) case;
+more complex polygon shapes fall back to Condemn. Buildings stay
+with the larger of the two halves.
+\end{decision}
+
+\paragraph{What landed.}
+
+\begin{itemize}[leftmargin=*]
+  \item \texttt{tests/degenerate.rs::y\_intersection\_no\_overlaps} — a
+  programmatic centroid-in-other-polygon check. Caught the real
+  Y-intersection I3 violation; passes after the bisector-clip fix.
+  \item \texttt{subdivide.rs}: re-introduced
+  \texttt{corner\_bisector} and \texttt{clip\_with\_bisector}, called
+  conditionally for parcels at acute corners only.
+  \item \texttt{deform.rs}: new \texttt{DeformResult::Untouched}
+  branch and the line-unchanged check that returns it. Parcels in
+  \texttt{Untouched} state are left alone.
+  \item \texttt{deform.rs}: \texttt{split\_segment\_path} +
+  \texttt{rebind\_frontage\_road} helpers. \texttt{road\_split\_preserves}
+  is now active and passing.
+  \item Two acute-intersection tests
+  (\texttt{acute\_intersection\_15deg/5deg}) are now active and pass
+  the I1–I3 invariant check; they don't yet exercise full
+  sliver-merge but no longer trigger panics or overlaps.
+\end{itemize}
+
+\paragraph{Deviations from spec.}
+
+\subsubsection*{2026-04-25 --- Inverse-restore is centroid-bounded,
+not vertex-exact}
+
+What changed: \cref{sec:edit-handling}'s I7 (``Applying an edit and
+then its inverse restores the original parcel set within
+$\varepsilon_{\text{geom}}$ for all preserved parcels'') is satisfied
+in spirit but not literally. With the minimum-change deformation
+path of D14, parcels whose frontage line didn't change keep their
+absolute coordinates; an earlier edit might have left a corner
+parcel at, say, $(0.5, 0)$ instead of its original $(0, 0)$, and the
+inverse edit will not pull it back. The
+\texttt{road\_edit\_inverse\_restores} test instead asserts that
+each surviving parcel's \emph{centroid} drifts no more than the
+magnitude of the edit delta itself.
+
+Why: ``minimal cost'' deformation is what the user explicitly asked
+for. Strict vertex-exact inverse-restore is incompatible with that
+goal — it forces every parcel touching an incident road to be
+re-projected on every edit, even when the parcel didn't really need
+to move. Bounded drift is the right trade-off.
+
+Affected sections: \cref{sec:edit-handling} (I7 reading clarified).
+
+\subsubsection*{2026-04-25 --- \texttt{Untouched} is not a public
+\texttt{EditOutcome} bucket}
+
+What changed: Internally the deformation pipeline distinguishes four
+results — Deformed, Untouched, Regenerate, Condemned — but the
+public \texttt{EditOutcome} struct only exposes the three buckets
+that carry \texttt{ParcelId}s of parcels that materially changed.
+Untouched parcels simply don't appear in the result. Callers can
+still infer them: any parcel-id that existed before the edit and
+isn't in any of the four buckets after is implicitly Untouched.
+
+Why: surfacing an explicit ``Untouched'' bucket would mean every
+edit on a 10\,000-parcel city walks 10\,000 ids back to the caller,
+defeating the point of minimum-change. We let absence carry meaning.
+
+Affected sections: \cref{sec:edit-handling} (§4.2 outcome bucket
+list now reads as ``parcels that materially changed go into one of
+these four buckets''; absence implies no change).
+
+\paragraph{Test status.}
+24 unit tests, 20 integration tests (was 16 in session 2), 1 doc
+test. Two named tests still \texttt{\#[ignore]}-d:
+\texttt{cul\_de\_sac} and \texttt{curved\_road\_high\_curv} — both
+need real curved-road handling that's the milestone-0.4 headline.
+
+\paragraph{What's next --- milestone 0.4 queue.}
+
+\begin{enumerate}[noitemsep]
+  \item True sliver-merge for acute corners: instead of
+  bisector-clipping into thin trapezoids, merge the would-be sliver
+  with its longer-frontage neighbor. Removes the visual mess at
+  acute corners without changing the I3 invariant.
+  \item Curved-road support: discretize curves into polylines with
+  variable depth caps based on local radius. Unlocks
+  \texttt{curved\_road\_high\_curv}.
+  \item Pie-slice parcels for cul-de-sac bulbs.
+  \item OBB regularization (\cref{sec:regularization}).
+  \item Spatial index (\texttt{rstar}) for affected-parcel lookup
+  (\cref{sec:open}'s Q2) — once the linear scan starts to bite at
+  scale.
+  \item Q4: regeneration biased to preserve building footprints.
+  \item ``Fill-the-corner-after-edit'' regenerate: when a corner
+  parcel ends up disconnected from its road after a node move (e.g.,
+  the gap visible at the bottom-right of \cref{fig:edit-after}),
+  regenerate just that corner to close the gap.
+\end{enumerate}
+
 % -----------------------------------------------------------------
 \subsection{2026-04-25 --- Session 2: Milestone 0.2 (corner parcels,
 sticky back edges, preserve-on-deform)}
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+version = "1.0.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8848ee67ecc8aedbaf3e4122217aff892639231befc6a1b58d29fff4c2cabaa"
diff --git a/road_parceling/src/parcel/deform.rs b/road_parceling/src/parcel/deform.rs
index 99d06c4..b74adf9 100644
--- a/road_parceling/src/parcel/deform.rs
+++ b/road_parceling/src/parcel/deform.rs
@@ -106,9 +106,20 @@ pub fn apply_road_edit(
         RoadEdit::DeleteSegment { road } => {
             delete_segment_path(parcels, road, graph, params, &mut outcome)?;
         }
-        RoadEdit::SplitSegment { .. } | RoadEdit::InsertSegment { .. } => {
-            // Split/Insert preservation is milestone-0.3 work — fall
-            // back to a regenerate of every affected face.
+        RoadEdit::SplitSegment { road, at } => {
+            split_segment_path(
+                parcels,
+                &graph_before,
+                graph,
+                road,
+                at,
+                params,
+                &mut outcome,
+            )?;
+        }
+        RoadEdit::InsertSegment { .. } => {
+            // Insert preservation is milestone-0.3+ work — for now,
+            // regenerate every face touched by the new segment.
             let affected = roads_affected_by_old_id(&graph_before, edit);
             regenerate_path(parcels, graph, params, &affected, &mut outcome)?;
         }
@@ -270,8 +281,7 @@ fn deform_parcel_after_road_move(
         let len_after = road_vec_after.length();
         if len_after > EPS_GEOM {
             let dir_after = road_vec_after / len_after;
-            let parallel =
-                (dir_before.x * dir_after.y - dir_before.y * dir_after.x).abs() < 1e-6;
+            let parallel = (dir_before.x * dir_after.y - dir_before.y * dir_after.x).abs() < 1e-6;
             if parallel {
                 let new_normal = DVec2::new(-dir_after.y, dir_after.x);
                 let perp_dist = (p_a - pa_after).dot(new_normal).abs();
@@ -379,6 +389,185 @@ fn find_frontage_index(polygon: &Polygon, road_a: DVec2, road_b: DVec2) -> Optio
     best.map(|(i, _)| i)
 }
 
+// -----------------------------------------------------------------
+// Split-segment preserve path
+// -----------------------------------------------------------------
+
+fn split_segment_path(
+    parcels: &mut ParcelSet,
+    graph_before: &RoadGraph,
+    graph_after: &RoadGraph,
+    old_road: RoadId,
+    split_point: DVec2,
+    _params: &SubdivisionParams,
+    outcome: &mut EditOutcome,
+) -> Result<(), ParcelError> {
+    let Some((a, b)) = graph_before.road_nodes(old_road) else {
+        return Err(ParcelError::UnknownEntity);
+    };
+    let pos_a = graph_before
+        .node_position(a)
+        .ok_or(ParcelError::UnknownEntity)?;
+    let pos_b = graph_before
+        .node_position(b)
+        .ok_or(ParcelError::UnknownEntity)?;
+    // Find the new node m in graph_after at split_point and the two
+    // new roads it splits the old road into.
+    let mut new_node: Option<NodeId> = None;
+    for (nid, node) in &graph_after.nodes {
+        if (node.pos - split_point).length() < 0.01 {
+            new_node = Some(nid);
+            break;
+        }
+    }
+    let m = new_node.ok_or(ParcelError::InconsistentEdit("split node not found".into()))?;
+    let r1 = graph_after
+        .find_road_between(a, m)
+        .ok_or(ParcelError::InconsistentEdit(
+            "split road r1 missing".into(),
+        ))?;
+    let r2 = graph_after
+        .find_road_between(m, b)
+        .ok_or(ParcelError::InconsistentEdit(
+            "split road r2 missing".into(),
+        ))?;
+
+    let road_vec = pos_b - pos_a;
+    let road_len_sq = road_vec.length_squared();
+    if road_len_sq < EPS_GEOM * EPS_GEOM {
+        return Err(ParcelError::InconsistentEdit("zero-length old road".into()));
+    }
+    let t_split = (split_point - pos_a).dot(road_vec) / road_len_sq;
+
+    let parcel_ids: Vec<ParcelId> = parcels.parcels_on_road(old_road).collect();
+    for pid in parcel_ids {
+        let Some(parcel) = parcels.parcels.get(pid) else {
+            continue;
+        };
+        let n = parcel.polygon.len();
+        let fi = parcel.frontage_edge_index;
+        let v = parcel.polygon.vertices();
+        let p_a = v[fi];
+        let p_b = v[(fi + 1) % n];
+        let t_a = (p_a - pos_a).dot(road_vec) / road_len_sq;
+        let t_b = (p_b - pos_a).dot(road_vec) / road_len_sq;
+        let (t_lo, t_hi) = if t_a < t_b { (t_a, t_b) } else { (t_b, t_a) };
+
+        if t_hi <= t_split + EPS_GEOM {
+            // Frontage entirely on the a-side → r1.
+            // No geometric change; just rebind frontage_road.
+            rebind_frontage_road(parcels, pid, r1);
+            outcome.deformed.push(pid);
+        } else if t_lo >= t_split - EPS_GEOM {
+            // Frontage entirely on the b-side → r2.
+            rebind_frontage_road(parcels, pid, r2);
+            outcome.deformed.push(pid);
+        } else {
+            // Frontage spans the split → cut into two.
+            // Only handle the simple 4-vertex case; fall through to
+            // condemn for higher-vertex parcels.
+            if n != 4 {
+                drop_parcel(parcels, pid);
+                outcome.condemned.push(pid);
+                continue;
+            }
+            let q_b = v[(fi + 2) % n];
+            let q_a = v[(fi + 3) % n];
+            let frontage_vec = p_b - p_a;
+            let len_sq = frontage_vec.length_squared();
+            if len_sq < EPS_GEOM * EPS_GEOM {
+                drop_parcel(parcels, pid);
+                outcome.condemned.push(pid);
+                continue;
+            }
+            let t_local = (split_point - p_a).dot(frontage_vec) / len_sq;
+            if t_local <= EPS_GEOM || t_local >= 1.0 - EPS_GEOM {
+                // Split coincident with an endpoint — just rebind.
+                let target = if t_local <= 0.5 { r1 } else { r2 };
+                rebind_frontage_road(parcels, pid, target);
+                outcome.deformed.push(pid);
+                continue;
+            }
+            let q_star = q_a + (q_b - q_a) * t_local;
+            // The "a side" of the split goes to r1, "b side" to r2.
+            // p_a is closer to a if t_a < t_b. (Polygon CCW means
+            // frontage goes from p_a to p_b along the road, but
+            // direction can be either way relative to road A→B.)
+            let (a_side_road, b_side_road) = if t_a < t_b { (r1, r2) } else { (r2, r1) };
+            // Build new polygons.
+            let poly_a = Polygon::new(vec![p_a, split_point, q_star, q_a]);
+            let poly_b = Polygon::new(vec![split_point, p_b, q_b, q_star]);
+            let (Ok(poly_a), Ok(poly_b)) = (poly_a, poly_b) else {
+                drop_parcel(parcels, pid);
+                outcome.condemned.push(pid);
+                continue;
+            };
+            let block = parcel.block;
+            let area_a = poly_a.area();
+            let area_b = poly_b.area();
+            let parcel_owned = parcels
+                .parcels
+                .remove(pid)
+                .ok_or(ParcelError::UnknownEntity)?;
+            if let Some(v) = parcels.by_block.get_mut(&parcel_owned.block) {
+                v.retain(|&x| x != pid);
+            }
+            if let Some(v) = parcels.by_road.get_mut(&parcel_owned.frontage_road) {
+                v.retain(|&x| x != pid);
+            }
+            outcome.condemned.push(pid);
+            // Building stays with the larger half.
+            let mut building_a: Option<super::BuildingHandle> = None;
+            let mut building_b: Option<super::BuildingHandle> = None;
+            if area_a >= area_b {
+                building_a = parcel_owned.building;
+            } else {
+                building_b = parcel_owned.building;
+            }
+            let edge_kinds = crate::parcel::classify::classify_edges(4, 0);
+            let new_a = Parcel {
+                polygon: poly_a,
+                edge_kinds: edge_kinds.clone(),
+                frontage_road: a_side_road,
+                frontage_edge_index: 0,
+                block,
+                building: building_a,
+            };
+            let new_b = Parcel {
+                polygon: poly_b,
+                edge_kinds,
+                frontage_road: b_side_road,
+                frontage_edge_index: 0,
+                block,
+                building: building_b,
+            };
+            let new_a_id = parcels.insert(new_a);
+            let new_b_id = parcels.insert(new_b);
+            outcome.created.push(new_a_id);
+            outcome.created.push(new_b_id);
+        }
+    }
+    Ok(())
+}
+
+fn rebind_frontage_road(parcels: &mut ParcelSet, pid: ParcelId, new_road: RoadId) {
+    let old_road = if let Some(p) = parcels.parcels.get(pid) {
+        p.frontage_road
+    } else {
+        return;
+    };
+    if old_road == new_road {
+        return;
+    }
+    if let Some(p) = parcels.parcels.get_mut(pid) {
+        p.frontage_road = new_road;
+    }
+    if let Some(v) = parcels.by_road.get_mut(&old_road) {
+        v.retain(|&x| x != pid);
+    }
+    parcels.by_road.entry(new_road).or_default().push(pid);
+}
+
 // -----------------------------------------------------------------
 // Delete-segment path: condemn + regenerate-block
 // -----------------------------------------------------------------
diff --git a/road_parceling/src/parcel/subdivide.rs b/road_parceling/src/parcel/subdivide.rs
index 9698bf9..03a6f23 100644
--- a/road_parceling/src/parcel/subdivide.rs
+++ b/road_parceling/src/parcel/subdivide.rs
@@ -184,8 +184,7 @@ pub(crate) fn subdivide_block(
 
     let real_corner: Vec<bool> = (0..n)
         .map(|i| {
-            is_real_corner(graph, block, i, params)
-                && interior_angles[i] > 60.0_f64.to_radians()
+            is_real_corner(graph, block, i, params) && interior_angles[i] > 60.0_f64.to_radians()
         })
         .collect();
     // Acute corners: real corners (degree-3 or sharp degree-2) with
@@ -194,8 +193,7 @@ pub(crate) fn subdivide_block(
     // separated.
     let acute_corner: Vec<bool> = (0..n)
         .map(|i| {
-            is_real_corner(graph, block, i, params)
-                && interior_angles[i] <= 60.0_f64.to_radians()
+            is_real_corner(graph, block, i, params) && interior_angles[i] <= 60.0_f64.to_radians()
         })
         .collect();
 
@@ -405,9 +403,7 @@ pub(crate) fn subdivide_block(
             let mut working = raw_poly;
             if acute_corner[i] {
                 if let Some(b) = corner_bisector(&verts, i) {
-                    if let Some(clipped) =
-                        clip_with_bisector(&working, verts[i], b, frontage_mid)
-                    {
+                    if let Some(clipped) = clip_with_bisector(&working, verts[i], b, frontage_mid) {
                         working = clipped;
                     } else {
                         continue;
diff --git a/road_parceling/tests/degenerate.rs b/road_parceling/tests/degenerate.rs
index 65c6aab..a7cd269 100644
--- a/road_parceling/tests/degenerate.rs
+++ b/road_parceling/tests/degenerate.rs
@@ -55,17 +55,45 @@ fn assert_invariants_i1_i3(parcels: &road_parceling::ParcelSet) {
 // ----------------------------------------------------------------------
 
 #[test]
-#[ignore = "milestone-0.2: sliver-merge logic for acute corners"]
 fn acute_intersection_15deg() {
-    // Two roads sharing a node meet at 15°. Sliver-corner parcels
-    // must merge with neighbors; I1–I3 must hold.
+    // Two roads sharing a node meet at 15°. With the bisector-clip
+    // at acute corners (milestone 0.3), no proper sliver-merge yet,
+    // but I1–I3 must hold.
+    let mut g = RoadGraph::new();
+    let apex = g.add_node(DVec2::new(0.0, 0.0));
+    let p1 = g.add_node(DVec2::new(100.0, 0.0));
+    let angle = 15.0_f64.to_radians();
+    let p2 = g.add_node(DVec2::new(100.0 * angle.cos(), 100.0 * angle.sin()));
+    g.add_road(apex, p1).unwrap();
+    g.add_road(apex, p2).unwrap();
+    g.add_road(p1, p2).unwrap();
+    g.rebuild_topology().unwrap();
+    let params = SubdivisionParams::default();
+    let parcels = subdivide_all(&g, &params).unwrap();
+    // We don't insist parcels exist (a 15° wedge may be too narrow
+    // for any parcel to satisfy `min_area`), but if any do, they
+    // must satisfy I1–I3.
+    assert_invariants_i1_i3(&parcels);
 }
 
 #[test]
-#[ignore = "milestone-0.2: sliver-merge or typed error for knife-edge angles"]
 fn acute_intersection_5deg() {
-    // Knife-edge angle. Library must not panic; either typed error
-    // or valid output is acceptable.
+    // Knife-edge 5° angle. Library must not panic.
+    let mut g = RoadGraph::new();
+    let apex = g.add_node(DVec2::new(0.0, 0.0));
+    let p1 = g.add_node(DVec2::new(100.0, 0.0));
+    let angle = 5.0_f64.to_radians();
+    let p2 = g.add_node(DVec2::new(100.0 * angle.cos(), 100.0 * angle.sin()));
+    g.add_road(apex, p1).unwrap();
+    g.add_road(apex, p2).unwrap();
+    g.add_road(p1, p2).unwrap();
+    g.rebuild_topology().unwrap();
+    let params = SubdivisionParams::default();
+    // Either succeeds with valid parcels or returns a typed error;
+    // both are acceptable, but it must not panic.
+    if let Ok(parcels) = subdivide_all(&g, &params) {
+        assert_invariants_i1_i3(&parcels);
+    }
 }
 
 #[test]
@@ -356,9 +384,53 @@ fn road_delete_condemns() {
 }
 
 #[test]
-#[ignore = "milestone-0.2: split-segment must preserve, not regenerate"]
 fn road_split_preserves() {
-    // Splitting a segment should preserve the parcels on it.
+    // Splitting a segment should preserve the parcels on it. Each
+    // pre-split parcel either stays (frontage rebinds to one of the
+    // two new roads) or is split into two parcels along the
+    // perpendicular through the split point.
+    let mut g = rectangle_graph(200.0, 100.0);
+    let params = SubdivisionParams::default();
+    let mut parcels = subdivide_all(&g, &params).unwrap();
+    let pre_count = parcels.len();
+
+    // Pick the bottom road (longest, has many parcels) and split at
+    // x = 100, y = 0.
+    let bottom_road = g
+        .road_endpoints()
+        .find(|&(_, _, b)| {
+            let pos = g.node_position(b).unwrap();
+            pos.y.abs() < 1e-6 && pos.x > 100.0
+        })
+        .map(|(rid, _, _)| rid)
+        .unwrap();
+
+    let outcome = apply_road_edit(
+        &mut parcels,
+        &mut g,
+        RoadEdit::SplitSegment {
+            road: bottom_road,
+            at: DVec2::new(100.0, 0.0),
+        },
+        &params,
+    )
+    .unwrap();
+    // Most parcels should be deformed (frontage_road rebound, no
+    // geometric change). At most one is condemned (the one whose
+    // frontage spans the split point — and it gets two replacements).
+    assert!(
+        outcome.regenerated.is_empty(),
+        "split path must not trigger block regenerate; got {} regenerated",
+        outcome.regenerated.len()
+    );
+    assert!(
+        !outcome.deformed.is_empty(),
+        "split path should preserve at least some parcels"
+    );
+    // Net parcel count should be ≥ pre_count (a split can ADD
+    // parcels but never lose them outright).
+    assert!(parcels.len() >= pre_count - 1);
+    assert_invariants_i1_i3(&parcels);
 }
 
 #[test]
@@ -507,17 +579,19 @@ fn y_intersection_no_overlaps() {
     let parcels = subdivide_all(&g, &params).unwrap();
 
     let parcels_vec: Vec<_> = parcels.iter().collect();
-    for i in 0..parcels_vec.len() {
-        let centroid_i = parcels_vec[i].1.polygon().centroid();
-        for j in 0..parcels_vec.len() {
+    for (i, (_, pi)) in parcels_vec.iter().enumerate() {
+        let centroid_i = pi.polygon().centroid();
+        for (j, (_, pj_pair)) in parcels_vec.iter().enumerate() {
             if i == j {
                 continue;
             }
-            let pj = parcels_vec[j].1.polygon();
+            let pj = pj_pair.polygon();
             assert!(
                 !pj.contains(centroid_i),
                 "I3 violation: centroid of parcel {} ({:?}) is inside parcel {}",
-                i, centroid_i, j
+                i,
+                centroid_i,
+                j
             );
         }
     }