Fallback boolean operations to PyVista when trimesh has no backend

Extend the boolean helper to first attempt trimesh-based union/intersection/difference (with engine and then default engine), and on any failure (e.g., no backends available) fall back to PyVista's VTK-backed boolean_* methods on the original PolyData meshes.

This resolves ValueError('No backends available for boolean operations!') on environments such as macOS 3.9 where trimesh boolean backends are not installed, while still preferring trimesh when available.

Author: zasexton

svv/domain/routines/boolean.py

@@ -52,15 +52,32 @@ def _apply(op, eng):
         else:
             raise ValueError("Unsupported boolean operation.")
 
+    result_tm = None
     try:
-        result = _apply(operation, engine)
-    except KeyError:
-        # If the requested engine (e.g., 'manifold') is not registered
-        # in trimesh.boolean._engines, fall back to trimesh's default
-        # engine selection by omitting the explicit engine argument.
-        result = _apply(operation, eng=None)
+        try:
+            result_tm = _apply(operation, engine)
+        except KeyError:
+            # If the requested engine (e.g., 'manifold') is not registered
+            # in trimesh.boolean._engines, fall back to trimesh's default
+            # engine selection by omitting the explicit engine argument.
+            result_tm = _apply(operation, eng=None)
+    except Exception:
+        result_tm = None
+
+    if result_tm is not None:
+        result = convert_to_pyvista(result_tm)
+    else:
+        # Fallback to PyVista/VTK boolean operations when no trimesh
+        # boolean backend is available on the current platform.
+        if operation == 'union':
+            result = pyvista_object_1.boolean_union(pyvista_object_2)
+        elif operation == 'intersection':
+            result = pyvista_object_1.boolean_intersection(pyvista_object_2)
+        elif operation == 'difference':
+            result = pyvista_object_1.boolean_difference(pyvista_object_2)
+        else:
+            raise ValueError("Unsupported boolean operation.")
 
-    result = convert_to_pyvista(result)
     if not result.is_manifold and fix_mesh:
         fix = pymeshfix.MeshFix(result)
         fix.repair(verbose=False)