[QUESTION] Deformed geometry and fem.normal #614
Description
Hi,
I'm running into an error when computing vertex normal fields using fem.integrate on a deformed surface mesh.
This works fine for computing normals in the rest configuration but fails in the deformed configuration when using make_deformed_geometry() with a triangle mesh embedded in 3D.
Step Summary
I’m trying to compute the current normal field for bending forces. To do this, I use:
geo = self.u_field.make_deformed_geometry()
domain = fem.Cells(geo)
self.normals = NormalFieldSpace(geo, domain) # see class belowWhere NormalFieldSpace looks like this:
@fem.integrand
def normal_integrand(s: fem.Sample, domain: fem.Domain, v: fem.Field):
n = fem.normal(domain, s)
return wp.dot(v(s), n)
@wp.kernel
def normalise_vectors_kernel(v: wp.array(dtype=wp.vec3)):
i = wp.tid()
v[i] = wp.normalize(v[i])
def normalise_vectors(v: wp.array(dtype=wp.vec3)):
wp.launch(normalise_vectors_kernel, dim=v.shape[0], inputs=[v])
class NormalFieldSpace:
def __init__(self, geo, domain, degree=1):
self.space = fem.make_polynomial_space(
geo, degree=degree, dtype=wp.vec3,
element_basis=fem.ElementBasis.LAGRANGE
)
self.test = fem.make_test(space=self.space, domain=domain)
self.field = self.space.make_field()
self.init_field(domain)
def init_field(self, domain):
fem.integrate(
normal_integrand,
domain=domain,
fields={'v': self.test},
output=self.field.dof_values,
assembly='nodal',
)
normalise_vectors(self.field.dof_values)Error
I get runtime errors like the following when computing the current normal field using a deformed geometry as shown above:
RuntimeError: Error while parsing function "cell_deformation_gradient" at warp\warp\fem\geometry\deformed_geometry.py:119:
return self.field.eval_reference_grad_inner(cell_arg, s) + self.base.cell_deformation_gradient(
;Error while parsing function "eval_grad_inner_ref_space" at warp\warp\fem\field\nodal_field.py:128:
return eval_grad_inner(args, s, grad_transform)
;Error while parsing function "eval_grad_inner" at warp\warp\fem\field\nodal_field.py:121:
grad_transform,
;Input types must be the same, got ["matrix(shape=(3, 3), dtype=<class 'warp.types.float32'>)", "matrix(shape=(3, 2), dtype=<class 'warp.types.float32'>)"]
RuntimeError: Error while parsing function "integrate_kernel_fn" at warp\warp\fem\integrate.py:730:
vol = domain.element_measure(domain_arg, sample)
;Error while parsing function "cell_measure" at warp\warp\fem\geometry\geometry.py:263:
F = self.cell_deformation_gradient(args, s)
;Error while parsing function "cell_deformation_gradient" at warp\warp\fem\geometry\deformed_geometry.py:119:
return self.field.eval_reference_grad_inner(cell_arg, s) + self.base.cell_deformation_gradient(
;Error while parsing function "eval_grad_inner_ref_space" at warp\warp\fem\field\nodal_field.py:128:
return eval_grad_inner(args, s, grad_transform)
;Error while parsing function "eval_grad_inner" at warp\warp\fem\field\nodal_field.py:121:
grad_transform,
;Input types must be the same, got ["matrix(shape=(3, 3), dtype=<class 'warp.types.float32'>)", "matrix(shape=(3, 2), dtype=<class 'warp.types.float32'>)"]
Diagnosis
It seems that when using a deformed geometry built with make_deformed_geometry() over a 3D triangle mesh, the internal computations (e.g., within fem.integrate and fem.normal) try to combine a (3×3) gradient with a (3×2) one — likely due to a mismatch between the deformed and reference spaces.
Questions
- Is this a known issue when using
make_deformed_geometry()with deformed surface meshes in 3D? - Is there a better way to compute current normals over a deformed surface mesh? Keep in mind I must also compute normal gradients (thus the normal field).
Happy to share more details if needed.
Thanks,
Chris