Fix [0,0,0] "bullet hole" artefact in spherical barycentric resampling

barycentric_spherical_map() could leave a few target vertices unmapped,
collapsing their interpolated value to exactly [0, 0, 0]. Two causes:

1. Search neighbourhood too small. Only the `neighborhood` (=10) nearest
   source-triangle centroids were searched per target point. Registration
   (e.g. mris_register) can distort triangle areas by >200x on the sphere,
   pushing a target point's true containing triangle past the nearest 10
   centroids (observed rank 11-12), so it was never found. Default bumped to
   30 (negligible cost) so the correct triangle is found and exact barycentric
   weights are used.

2. Fallback guard tested the wrong mask. The "assign unmatched points to the
   nearest face" fallback was guarded by `np.count_nonzero(remaining)`, where
   `remaining` is the leftover mask from the final loop iteration, not "any
   points still unassigned". When the last iteration matched nothing, the
   fallback was skipped and those points kept face == -1 with zero weights.
   Downstream, `source.faces[-1]` negative-indexes the last face and the zero
   weights yield a sample of exactly [0, 0, 0]. Now guard on the actual
   `intersecting_faces == -1` mask so the fallback always runs.

Effect: a vertex teleported to the origin, with its 1-ring neighbours stretched
up to ~32 mm. Reproduced deterministically on dHCP white-matter surfaces
resampled to a 245k-vertex canonical topology; 56/1121 subjects had >=1 such
hole in the left white surface alone. With the fix, output is unchanged for all
correctly-mapped vertices and the holes disappear.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: inhuszar

surfa/mesh/sphere.py

@@ -113,7 +113,7 @@ def spherical_to_cartesian(points):
     return np.stack([x, y, z], axis=1)
 
 
-def barycentric_spherical_map(source, target, neighborhood=10):
+def barycentric_spherical_map(source, target, neighborhood=30):
     """
     Map the points of a target sphere to the barycentric coordinates of the nearest
     triangle on a source sphere.
@@ -125,7 +125,7 @@ def barycentric_spherical_map(source, target, neighborhood=10):
     target : Mesh
         Target sphere mesh.
     neighborhood : int, optional
-        Max number of nearest triangles to consider for each target point.
+        Max number of nearest triangles to consider for each target point. 
 
     Returns
     -------
@@ -214,12 +214,12 @@ def barycentric_spherical_map(source, target, neighborhood=10):
         intersecting_faces[remaining] = faces[hits]
         intersecting_barycenters[remaining] = barycentric[hits]
 
-    # if any target points were not assigned a face, just assign them the
-    # center of the nearest face
-    if np.count_nonzero(remaining) != 0:
-        missing = intersecting_faces == -1
+    # If any target points were not assigned a face, 
+    # fall back to the nearest face (barycentric centroid).
+    missing = intersecting_faces == -1
+    if np.count_nonzero(missing) != 0:
         intersecting_faces[missing] = nearest[0][missing]
-        intersecting_barycenters[missing] = 0.33333333
+        intersecting_barycenters[missing] = 1.0 / 3.0
 
     return intersecting_faces, intersecting_barycenters