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Move elasticity computation to mjCFlex.

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PiperOrigin-RevId: 675949380
Change-Id: Ia48f4fd6ae1ede206ccacd1205914e7e4b3c7aec
This commit is contained in:
Alessio Quaglino
2024-09-18 05:22:05 -07:00
committed by Copybara-Service
parent 0657e3e871
commit ddbc083810
7 changed files with 250 additions and 212 deletions
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plugin/elasticity/membrane.cc
+1 -74
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@@ -27,54 +27,6 @@
namespace mujoco::plugin::elasticity {
namespace {
// local tetrahedron numbering
constexpr int kNumEdges = Stencil2D::kNumEdges;
constexpr int kNumVerts = Stencil2D::kNumVerts;
// area of a triangle
mjtNum ComputeVolume(const mjtNum* x, const int v[kNumVerts]) {
mjtNum normal[3];
mjtNum edge1[3];
mjtNum edge2[3];
mju_sub3(edge1, x+3*v[1], x+3*v[0]);
mju_sub3(edge2, x+3*v[2], x+3*v[0]);
mju_cross(normal, edge1, edge2);
return mju_norm3(normal) / 2;
}
// compute local basis
void ComputeBasis(mjtNum basis[9], const mjtNum* x, const int v[kNumVerts],
const int faceL[2], const int faceR[2], mjtNum area) {
mjtNum basisL[3], basisR[3];
mjtNum edgesL[3], edgesR[3];
mjtNum normal[3];
mju_sub3(edgesL, x+3*v[faceL[0]], x+3*v[faceL[1]]);
mju_sub3(edgesR, x+3*v[faceR[1]], x+3*v[faceR[0]]);
mju_cross(normal, edgesR, edgesL);
mju_normalize3(normal);
mju_cross(basisL, normal, edgesL);
mju_cross(basisR, edgesR, normal);
// we use as basis the symmetrized tensor products of the edge normals of the
// other two edges; this is shown in Weischedel "A discrete geometric view on
// shear-deformable shell models" in the remark at the end of section 4.1;
// equivalent to linear finite elements but in a coordinate-free formulation.
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
basis[3*i+j] = ( basisL[i]*basisR[j] +
basisR[i]*basisL[j] ) / (8*area*area);
}
}
}
} // namespace
// factory function
std::optional<Membrane> Membrane::Create(const mjModel* m, mjData* d,
@@ -118,41 +70,16 @@ Membrane::Membrane(const mjModel* m, mjData* d, int instance, mjtNum nu,
}
}
// vertex positions
mjtNum* body_pos = m->flex_xvert0 + 3*m->flex_vertadr[f0];
// loop over all triangles
const int* elem = m->flex_elem + m->flex_elemdataadr[f0];
for (int t = 0; t < m->flex_elemnum[f0]; t++) {
const int* v = elem + (m->flex_dim[f0]+1) * t;
for (int i = 0; i < kNumVerts; i++) {
for (int i = 0; i < Stencil2D::kNumVerts; i++) {
int bi = m->flex_vertbodyid[m->flex_vertadr[f0]+v[i]];
if (bi && m->body_plugin[bi] != instance) {
mju_error("Body %d does not have plugin instance %d", bi, instance);
}
}
// triangles area
mjtNum volume = ComputeVolume(body_pos, v);
// material parameters
mjtNum mu = E / (2*(1+nu)) * mju_abs(volume) / 4 * thickness;
mjtNum la = E*nu / ((1+nu)*(1-2*nu)) * mju_abs(volume) / 4 * thickness;
// local geometric quantities
mjtNum basis[kNumEdges][9] = {{0}, {0}, {0}};
// compute edge basis
for (int e = 0; e < kNumEdges; e++) {
ComputeBasis(basis[e], body_pos, v,
Stencil2D::edge[Stencil2D::edge[e][0]],
Stencil2D::edge[Stencil2D::edge[e][1]], volume);
}
// compute metric tensor
// TODO: do not write in a const mjModel
MetricTensor<Stencil2D>(m->flex_stiffness + 21 * m->flex_elemadr[f0], t, mu,
la, basis);
}
// allocate array