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Compute mesh areas correctly for integer vertices #39

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Compute mesh areas correctly for integer vertices #39

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d3bf8ca
Reproduce integer mesh area overflow and preserve translated coordinates
antonsoo Oct 18, 2026
3df6d8a
Promote integer mesh edges before area products
antonsoo Oct 18, 2026
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9 changes: 9 additions & 0 deletions src/potpourri3d/mesh.py
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Original file line number Diff line number Diff line change
Expand Up @@ -229,9 +229,18 @@ def cotan_laplacian(V, F, denom_eps=0.):
def face_areas(V, F):
validate_mesh(V, F, force_triangular=True)

integer_coordinates = np.issubdtype(V.dtype, np.integer)
if integer_coordinates:
# Keep coordinate differences exact, including unsigned and wide integers.
V = V.astype(np.int64 if V.dtype.itemsize < 8 else object)

vec_ij = V[F[:,1],:] - V[F[:,0],:]
vec_ik = V[F[:,2],:] - V[F[:,0],:]

if integer_coordinates:
vec_ij = vec_ij.astype(float)
vec_ik = vec_ik.astype(float)

areas = 0.5 * np.linalg.norm(np.cross(vec_ij, vec_ik), axis=1)

return areas
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26 changes: 26 additions & 0 deletions test/potpourri3d_test.py
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Expand Up @@ -39,6 +39,32 @@ def is_nonnegative(A, eps=1e-6):
return np.all(A.data > -eps)

class TestCore(unittest.TestCase):

def test_integer_mesh_areas(self):
cases = [(np.int8, 16), (np.uint8, 16),
(np.int16, 300), (np.uint16, 300),
(np.int32, 100000), (np.uint32, 100000),
(np.int64, 300), (np.uint64, 300),
(np.float32, 300), (np.float64, 300)]
for dtype, side in cases:
for winding in [[0, 1, 2], [0, 2, 1]]:
with self.subTest(dtype=dtype, winding=winding):
V = np.array([[0, 0, 0], [side, 0, 0], [0, side, 0]], dtype=dtype)
F = np.array([winding])
original = V.copy()
area = 0.5 * side * side
np.testing.assert_allclose(pp3d.face_areas(V, F), [area])
np.testing.assert_allclose(pp3d.vertex_areas(V, F), [area / 3.] * 3)
np.testing.assert_array_equal(V, original)

def test_integer_mesh_areas_large_offset(self):
for dtype in [np.int64, np.uint64]:
with self.subTest(dtype=dtype):
V = np.array([[0, 0, 0], [3, 0, 0], [0, 3, 0]], dtype=dtype)
V += 2**60
F = np.array([[0, 1, 2]])
np.testing.assert_allclose(pp3d.face_areas(V, F), [4.5])
np.testing.assert_allclose(pp3d.vertex_areas(V, F), [1.5] * 3)

def test_write_read_mesh(self):

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