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Clean-up flex assumptions.

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Do not allow a mix of `elastic2d != none` with `dof = trilinear` since the latter assumes 3d elasticity.

Also, do not assume that `flex_interp > 0` in the engine. This will enable to use, e.g., `flex_interp = -1` to mean a linear surface finite element instead of a 3d finite element which is currently identified with `flex_interp = 1`.

PiperOrigin-RevId: 903852035
Change-Id: Ia6290b4a05e9e510ffb7f36d141cd525b40d3110
This commit is contained in:
Alessio Quaglino
2026-04-22 08:00:42 -07:00
committed by Copybara-Service
parent 863a084d7f
commit a891782553
14 changed files with 84 additions and 10 deletions
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doc/XMLreference.rst
+1 -1
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@@ -4328,7 +4328,7 @@ stress-strain relationship. See also :ref:`deformable <CDeformable>` objects and
:at:`elastic2d`: :at-val:`[none, bend, stretch, both], "none"`
Elastic contribution to passive forces of 2D flexes. "none": none, "bend": bending only, "stretch": stretching only,
"both": bending and stretching.
"both": bending and stretching. Not yet supported by :ref:`dof<body-flexcomp-dof>` **trilinear** and **quadratic**.
.. _flex-contact:
model/flex/bunny.xml
+1 -1
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@@ -31,7 +31,7 @@
</body>
<flexcomp type="mesh" file="bunny.obj" pos="0 0 .1" dim="2" euler="90 0 0"
radius=".001" rgba="0 .7 .7 1" mass=".05" name="softbody" dof="trilinear">
<elasticity young="1e3" poisson="0.1" damping="0.001" elastic2d="stretch"/>
<elasticity young="1e3" poisson="0.1" damping="0.001" elastic2d="none"/>
<contact selfcollide="none" internal="false"/>
</flexcomp>
</worldbody>
model/flex/bunny_multicell.xml
+1 -1
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@@ -31,7 +31,7 @@
</body>
<flexcomp type="mesh" file="bunny.obj" pos="0 0 0" dim="2" euler="90 0 0" cellcount="3 3 3"
radius=".001" rgba="0 .7 .7 1" mass=".05" name="softbody" dof="trilinear">
<elasticity young="1e3" poisson="0.1" damping="0.01" elastic2d="stretch"/>
<elasticity young="1e3" poisson="0.1" damping="0.01" elastic2d="none"/>
<contact selfcollide="none" internal="false"/>
</flexcomp>
</worldbody>
model/flex/bunny_quadratic.xml
+1 -1
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@@ -31,7 +31,7 @@
</body>
<flexcomp type="mesh" file="bunny.obj" pos="0 0 -.01" dim="2" euler="90 0 0"
radius=".002" rgba="0 .7 .7 1" mass=".05" name="softbody" dof="quadratic">
<elasticity young="1e3" poisson="0.1" damping="0.0001" elastic2d="stretch"/>
<elasticity young="1e3" poisson="0.1" damping="0.0001" elastic2d="none"/>
<contact selfcollide="none" internal="false"/>
</flexcomp>
</worldbody>
model/flex/bunny_with_uv.xml
+1 -1
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@@ -37,7 +37,7 @@
</body>
<flexcomp type="mesh" file="bunny_with_uv.obj" pos="0 0 .1" dim="2" euler="90 0 0"
radius=".001" material="matsponge" mass=".05" name="softbody" dof="trilinear">
<elasticity young="1e3" poisson="0.1" damping="0.001" elastic2d="stretch"/>
<elasticity young="1e3" poisson="0.1" damping="0.001" elastic2d="none"/>
<contact selfcollide="none" internal="false"/>
</flexcomp>
</worldbody>
src/engine/engine_core_constraint.c
+4
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@@ -275,6 +275,7 @@ static int mj_vertBodyWeight(const mjModel* m, const mjData* d, int f, int* v,
}
int order = m->flex_interp[f];
order = order < 0 ? -order : order;
int npc = (order+1)*(order+1)*(order+1); // number of nodes per cell
// cell lookup: get local coords and node indices
@@ -709,6 +710,7 @@ void mj_instantiateEquality(const mjModel* m, mjData* d) {
int f = id[0];
int nodenum = m->flex_nodenum[f];
int order = m->flex_interp[f];
order = order < 0 ? -order : order;
// skip if not interpolated (order == 0 or no nodes)
if (!order || !nodenum) {
@@ -1676,6 +1678,7 @@ void mj_diagApprox(const mjModel* m, mjData* d) {
int flex_id = m->eq_obj1id[id];
int nstart = m->flex_nodeadr[flex_id];
int order = m->flex_interp[flex_id];
order = order < 0 ? -order : order;
int npc = (order+1)*(order+1)*(order+1);
// per-cell constraint count
@@ -2296,6 +2299,7 @@ static int mj_ne(const mjModel* m, mjData* d, int* nnz) {
// per-cell strain constraints: each equality is one cell
int f = id[0];
int order = m->flex_interp[f];
order = order < 0 ? -order : order;
if (!order || !m->flex_nodenum[f]) {
break;
}
src/engine/engine_core_smooth.c
+4 -2
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@@ -595,7 +595,8 @@ void mj_flex(const mjModel* m, mjData* d) {
}
}
int order = m->flex_interp[f];
int interp = m->flex_interp[f];
int order = interp < 0 ? -interp : interp;
int cx = m->flex_cellnum[3*f+0];
int cy = m->flex_cellnum[3*f+1];
int cz = m->flex_cellnum[3*f+2];
@@ -2624,7 +2625,8 @@ void mj_rnePostConstraint(const mjModel* m, mjData* d) {
case mjEQ_FLEXSTRAIN: {
// increment: trilinear uses 2 center (I1,J-1) + 3*ngauss shear, quadratic uses 6*ngauss
k = m->eq_obj1id[id];
int order = m->flex_interp[k];
int interp_k = m->flex_interp[k];
int order = interp_k < 0 ? -interp_k : interp_k;
int nodenum = m->flex_nodenum[k];
if (order && nodenum) {
int nquad = order + 1;
src/engine/engine_derivative.c
+2
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@@ -887,6 +887,7 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
if (!m->flex_interp[f]) continue;
if (m->flex_rigid[f]) continue;
int order = m->flex_interp[f];
order = order < 0 ? -order : order;
int npc = (order+1)*(order+1)*(order+1);
if (npc > max_npc) max_npc = npc;
if (m->flex_nodenum[f] > max_nodenum) max_nodenum = m->flex_nodenum[f];
@@ -966,6 +967,7 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
}
int order = m->flex_interp[f];
order = order < 0 ? -order : order;
int npc = (order+1)*(order+1)*(order+1);
int cx = m->flex_cellnum[3*f+0];
int cy = m->flex_cellnum[3*f+1];
src/engine/engine_passive.c
+1
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@@ -236,6 +236,7 @@ static void mj_springdamper(const mjModel* m, mjData* d) {
if (m->flex_interp[f]) {
int order = m->flex_interp[f];
order = order < 0 ? -order : order;
int npc = (order+1)*(order+1)*(order+1); // nodes per cell
int cx = m->flex_cellnum[3*f+0];
int cy = m->flex_cellnum[3*f+1];
src/engine/engine_setconst.c
+1
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@@ -714,6 +714,7 @@ static void makeFlexBandwidth(mjModel* m, mjData* d) {
for (int f = 0; f < m->nflex; f++) {
if (!m->flex_interp[f]) continue;
int order = m->flex_interp[f];
order = order < 0 ? -order : order;
int nodeadr = m->flex_nodeadr[f];
int nodenum = m->flex_nodenum[f];
int cx = m->flex_cellnum[3*f+0];
src/engine/engine_vis_interact.c
+1
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@@ -864,6 +864,7 @@ int mjv_select(const mjModel* m, const mjData* d, const mjvOption* vopt,
if (m->flex_interp[i]) {
mjtNum* coord = m->flex_vert0 + 3*(m->flex_vertadr[i] + vertid);
int order = m->flex_interp[i];
order = order < 0 ? -order : order;
int npc = (order+1)*(order+1)*(order+1);
// cell lookup: get local coords and node indices
src/engine/engine_vis_visualize.c
+1
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@@ -1452,6 +1452,7 @@ static void addFlexBvhGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
int cy = m->flex_cellnum[3*f+1];
int cz = m->flex_cellnum[3*f+2];
int order = m->flex_interp[f];
order = order < 0 ? -order : order;
int NX = cx * order + 1;
int NY = cy * order + 1;
int NZ = cz * order + 1;
src/user/user_mesh.cc
+13 -3
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@@ -4100,6 +4100,19 @@ void mjCFlex::Compile(const mjVFS* vfs) {
}
nelem = (int)elem_.size()/(dim+1);
// elastic2d checks
if (elastic2d) {
if (thickness <= 0) {
throw mjCError(this, "2d elasticity requires positive thickness");
}
if (interpolated) {
throw mjCError(this, "interpolated flex does not yet support 2d elasticity");
}
if (dim != 2 && !interpolated) {
throw mjCError(this, "2d elasticity requires 2d flex");
}
}
// set nvert, rigid, centered; check size
if (vert_.empty()) {
centered = true;
@@ -4342,9 +4355,6 @@ void mjCFlex::Compile(const mjVFS* vfs) {
// bending stiffness (2D only)
if (dim == 2 && (elastic2d == 1 || elastic2d == 3)) {
if (thickness < 0) {
throw mjCError(this, "thickness must be positive for bending stiffness");
}
bending.assign(nedge*17, 0);
for (unsigned int e = 0; e < nedge; e++) {
test/user/user_mesh_test.cc
+52
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@@ -819,6 +819,58 @@ TEST_F(MjCMeshTest, VolumeSmallAllowedShell) {
mj_deleteModel(model);
}
TEST_F(MjCMeshTest, Flex2DElasticityRequiresPositiveThickness) {
static constexpr char xml[] = R"(
<mujoco>
<worldbody>
<flexcomp name="f" type="grid" count="3 3 1" spacing="1 1 1" dim="2" dof="2d">
<elasticity young="1" thickness="0" elastic2d="bend"/>
</flexcomp>
</worldbody>
</mujoco>
)";
std::array<char, 1024> error;
mjModel* model = LoadModelFromString(xml, error.data(), error.size());
EXPECT_THAT(model, testing::IsNull());
EXPECT_THAT(error.data(),
HasSubstr("2d elasticity requires positive thickness"));
}
TEST_F(MjCMeshTest, InterpolatedFlexDoesNotSupport2DElasticity) {
static constexpr char xml[] = R"(
<mujoco>
<worldbody>
<flexcomp name="f" type="grid" count="3 3 1" spacing="1 1 1" dim="2" dof="trilinear">
<contact selfcollide="none"/>
<elasticity young="1" thickness="1" elastic2d="bend"/>
</flexcomp>
</worldbody>
</mujoco>
)";
std::array<char, 1024> error;
mjModel* model = LoadModelFromString(xml, error.data(), error.size());
EXPECT_THAT(model, testing::IsNull());
EXPECT_THAT(
error.data(),
HasSubstr("interpolated flex does not yet support 2d elasticity"));
}
TEST_F(MjCMeshTest, Flex2DElasticityRequires2DFlex) {
static constexpr char xml[] = R"(
<mujoco>
<worldbody>
<flexcomp name="f" type="grid" count="3 3 3" spacing="1 1 1" dim="3" dof="2d">
<elasticity young="1" thickness="1" elastic2d="bend"/>
</flexcomp>
</worldbody>
</mujoco>
)";
std::array<char, 1024> error;
mjModel* model = LoadModelFromString(xml, error.data(), error.size());
EXPECT_THAT(model, testing::IsNull());
EXPECT_THAT(error.data(), HasSubstr("2d elasticity requires 2d flex"));
}
TEST_F(MjCMeshTest, VolumeNegativeThrowsError) {
static constexpr char xml[] = R"(
<mujoco>