Apply max_volume to the default phases in 2D (#118)

Without phases, TriMesh.from_polymesh uses one default phase per phase
number, and those carried 'max_volume': inf. The 2D meshers read the
area limit of each region as phase.get('max_volume', max_volume), so
the infinite default always won. The global limit is no longer passed
to Triangle under the regional ones (the periodic mesher needs
different limits for the cells and their copies), so the max_volume
argument was ignored: a square meshed with max_volume=0.4 kept two
triangles of area 8. On master the global limit applied underneath.

The default phases no longer have a max_volume, so the argument applies
to their regions, in the bounded and the periodic 2D meshers and in
_facet_sizes. An explicit infinite max_volume of a phase still means no
limit for that phase, as the changelog now says.

The new test meshes a single cell without phases, bounded and periodic,
and checks the largest triangle against the limit. The example digest
of the documentation is updated for the change of the library.

Reported by Kip Hart in the review of PR #114.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: rimoli

CHANGELOG.rst

@@ -97,7 +97,10 @@ Fixed
 - 3D ``mesh_max_volume`` and per-phase ``max_volume`` are now honored by
   TetGen. In 2D, a per-phase ``max_volume`` larger than the global value is
   no longer capped, and an infinite ``mesh_max_volume`` is no longer passed
-  to Triangle as the (mis-parsed) switch ``ainf``.
+  to Triangle as the (mis-parsed) switch ``ainf``. The ``max_volume`` of a
+  phase replaces the global value for that phase, an infinite one included,
+  and phases without one, such as the default phases when none are given,
+  use the global value.
 - ``Ellipsoid.approximate`` mapped the axes incorrectly for the ordering
   c >= a >= b, so those grains were tessellated with the wrong orientation.
   The b >= c >= a ordering is now sorted explicitly as well.

docs/source/sphinx_gallery/plot_demos.py

@@ -24,7 +24,7 @@
 # them is edited, and the build would serve stale figures.
 #
 # Managed by docs/example_digest.py. Do not edit by hand.
-EXAMPLES_DIGEST = '2d39355b71ddd257'
+EXAMPLES_DIGEST = '22bb4ed7ea2ddd9f'
 
 example_dir = '../../../src/microstructpy/examples'
 

src/microstructpy/meshing/trimesh.py

@@ -217,8 +217,8 @@ def from_polymesh(cls, polymesh, phases=None, mesher='Triangle/Tetgen',
             polymesh (PolyMesh): A polygon/polyhedron mesh.
             phases (list): *(optional)* A list of dictionaries containing
                 options for each phase.
-                Default is
-                ``{'material_type': 'solid', 'max_volume': float('inf')}``.
+                Default is ``{'material_type': 'solid'}`` for each phase,
+                meshed with the ``max_volume`` argument.
             mesher (str): {'Triangle/TetGen' | 'Triangle'  | 'TetGen' | 'gmsh'}
                 specify the mesh generator. Default is 'Triangle/TetGen'.
             min_angle (float): The minimum interior angle, in degrees, of an
@@ -844,8 +844,7 @@ def from_polymesh(cls, polymesh, mesh_size, phases=None):
             mesh_size (float): The side length of each pixel/voxel.
             phases (list): *(optional)* A list of dictionaries containing
                 options for each phase.
-                Default is
-                ``{'material_type': 'solid', 'max_volume': float('inf')}``.
+                Default is ``{'material_type': 'solid'}`` for each phase.
 
         """
         if phases is None:
@@ -1965,10 +1964,13 @@ def _amorphous_seed_numbers(pmesh, phases):
 
 
 def _default_phases(polymesh):
-    """Default phases: one solid phase per phase number of the polymesh."""
+    """Default phases: one solid phase per phase number of the polymesh.
+
+    They have no ``max_volume``, so the ``max_volume`` argument of the
+    mesher applies to their regions.
+    """
     n_phases = int(np.max(polymesh.phase_numbers)) + 1
-    return [{'material_type': 'solid', 'max_volume': float('inf')}
-            for _ in range(n_phases)]
+    return [{'material_type': 'solid'} for _ in range(n_phases)]
 
 
 def _pt3d(pt):

tests/meshing/test_trimesh_fixes.py

@@ -342,6 +342,19 @@ def test_triangle_per_phase_max_volume_exceeds_global(case_2d):
     assert np.all(areas[elem_phases != 0] <= 1e-3 + 1e-9)
 
 
+@pytest.mark.parametrize('periodic', [False, True])
+def test_triangle_max_volume_without_phases(periodic):
+    # Without phases, the max_volume argument limits the area of the
+    # triangles. The default phases must not carry a limit of their own,
+    # which would replace it (an infinite one left it unapplied).
+    domain = msp.geometry.Square(side_length=4, corner=(0, 0))
+    seeds = msp.seeding.SeedList([msp.seeding.Seed.factory(
+        'circle', r=1.5, position=(2.2, 1.9))])
+    pmesh = msp.meshing.PolyMesh.from_seeds(seeds, domain, periodic=periodic)
+    mesh = TriMesh.from_polymesh(pmesh, min_angle=20, max_volume=0.4)
+    assert np.max(simplex_volumes(mesh)) <= 0.4 + 1e-9
+
+
 def test_triangle_infinite_max_volume_not_passed(case_2d, monkeypatch):
     pmesh, phases, _ = case_2d
     captured = {}