The implicit effective metric assembles the stretch stiffness from
world-space edge vectors, but a flex vertex body's slide dofs are
expressed in its parent body's frame. When that frame is rotated the
assembled operator is therefore not the Jacobian of the passive stretch
force, which mj_flexPassiveStretch already maps into the dof frame with
xmat^T. The metric is then inconsistent with the force it linearizes:
implicit integration loses its stability guarantee, and models that the
same flex handles comfortably in an unrotated frame diverge.
Apply the matching change of basis in both places that build or apply the
stretch stiffness: mjd_flexStretch_mul rotates the input dof vector into
world and the scattered result back, and mjd_flexStiff_assemble sandwiches
each 3x3 block as R_bi^T * blk * R_bj. Both are no-ops when the parent is
unrotated. Bending needs no change: its blocks are isotropic, and
R^T (q I) R = q I.
This completes the fix in fe9dc584, which covered the passive force paths
and the interp (trilinear) derivative but not the standard stretch one.
On a mesh flex inside a body with a 90-degree rotation, the metric's
directional agreement with the force Jacobian goes from cos = 0.57 to
cos = 1.0, and a hanging sheet that previously reached 176% strain
settles at 0.87%.
Fast path in mj_flexPassiveInterp, mj_flexPassiveBendInterp, and
mj_flexPassiveStretch assumed body slide joints are world-aligned
(J = I). When parent body has a non-identity quaternion, joint axes
are rotated (J = R_body), causing wrong force mapping and instability
(NaN/Inf in QACC).
Fix: project world-frame forces onto body local frame via
mju_mulMatTVec3(R_body^T, force) before adding to qfrc_spring/damper.
Also fix the derivative paths:
- mjd_flexInterp_kernel fast path: R^T * K_rot * R * vec
- mjd_flexStiff_assemble (CSR): R_bi^T * K_rot_block * R_bj
The CSR-assembled stiffness matrix is the actual path used by the
implicit CG solver (mj_flexCG gate). The test uses solver="CG" to
activate this path; without it, flex stiffness is integrated
explicitly and no derivative fix can help.
Ported from GitHub PR https://github.com/google-deepmind/mujoco/pull/3379
Original author: Devansh (https://github.com/devansh0703)
Fixes https://github.com/google-deepmind/mujoco/issues/3364
PiperOrigin-RevId: 952789284
Change-Id: If924f7160dd16c0cc88170d80e6e605da0fe2e04
The flexcomp compiler previously rejected pins on dim-2 flexes with
bending (elastic2d bend/both). Allow them: mj_flexPassiveBend treats a
pinned vertex (body without 3 free slide dofs) as static -- zero
velocity, and no bending force applied to it (the reaction is carried
by the pin) -- while its position still enters every neighbor's
bending force, which is exactly what the pin constrains.
PiperOrigin-RevId: 944410969
Change-Id: Ib0a69c8d5fb6f64d3e2a76af3b6b2c7be1898191
Also add MJTOL_SCALE to fixture to allow tests to be run with zero tolerance. This is useful when assesing the impact of code changes (A/B comparison of failure values)
PiperOrigin-RevId: 924219083
Change-Id: Ifdd09ac850904ca8dd79179930ce738a4b37d284
The normal jump residual in flex elasticity calculations now rotates the rest-frame normal jump into the current frame using the face's corotational quaternion before subtracting the jump from the current normal difference.
PiperOrigin-RevId: 913242127
Change-Id: Ia62b28ecccd59e79225737d8225ef4131b333c96
This change adds a new passive force computation for flexes with elastic2d="bend" and dof="trilinear". The bending energy is based on the squared difference of normals between adjacent face elements at their shared edge midpoint. The edge data is precomputed during model compilation and stored in flex_bending.
PiperOrigin-RevId: 910772638
Change-Id: I3b12c7b7f1ba6ac1875df495d89e8cfec921ca80
When elastic2d="stretch" is set on an interpolated flexcomp, treat the bounding box boundary as membrane elements rather than volumetric cells. This computes plane-stress stiffness over the boundary faces and updates the runtime force/derivative kernels accordingly.
Interior vertex tracking (moving vertices that follow the deforming shell) is not yet implemented so all mesh vertices need to be on the bounding box surface or the background grid should have no interior nodes (i.e. cellcount should be 1 on at least one axis).
PiperOrigin-RevId: 907654080
Change-Id: I51b90e2f6a1d1b036f9604e42de20e377dc5d3f9
Alessio Quaglino