This gives a 3x speedup in implicitfast.
Also cleanup old code that was used in the dense factorization of the stiffness matrix before we switched to CG.
PiperOrigin-RevId: 915900315
Change-Id: Id6973c4bfd7d371a43ec6db982703969b23a3550
The flex interp stiffness matrix is Negative Semi-Definite (NSD). When forming the RHS for implicit integration, the term involving the velocity and stiffness should be added, not subtracted. A new test is added to ensure energy stability for flex interp stretch stiffness with the implicitfast integrator.
PiperOrigin-RevId: 914845245
Change-Id: Iaaf0914909128e64e195f17cc5f2f344a8a43bc2
Standard flex (flex_interp=0) with thin-plate bending treated bending forces purely explicitly. This caused contact-induced vertex vibrations and non-physical energy injection for flat resting sheets, because the solver treated each vertex as an independent mass during contact and contact normals are orthogonal to stretch constraints.
Fix: extend the existing preconditioned CG solver to include the constant bending stiffness K_bend in the implicit operator via matrix-free mat-vec.
PiperOrigin-RevId: 914774020
Change-Id: I45e0d6749abb6f873566203bccae956514b2576b
with a preconditioned Conjugate Gradient (CG) solver that operates
on the full system matrix.
The previous approach extracted flex DOFs into a reduced banded system,
factored it separately, and overwrote the global solve. This required
precomputed bandwidth (makeFlexBandwidth), parent-joint detection,
coupling corrections, and a FlexInterpContext struct — and only worked
for standalone flex trees without parent joints.
The new CG solver uses the already-factored global system (M - h*qDeriv)
as a preconditioner and adds the flex stiffness contribution via
matrix-free products (mjd_flexInterp_mulKD/mulK). This handles any
kinematic configuration — including flexes attached to articulated
chains or with parent joints — without sparsity pattern restrictions.
Before (`bunny_multicell`):
```
Simulation time : 50.80 s
Steps per second : 197
Realtime factor : 0.20 x
Time per step : 5080.3 µs
CG iters / step : 3.16
Contacts / step : 31.04
Constraints / step : 124.15
Degrees of freedom : 178
Dynamic memory usage : 0.4% of 100M
```
After:
```
Simulation time : 9.52 s
Steps per second : 1051
Realtime factor : 1.05 x
Time per step : 951.7 µs
CG iters / step : 3.21
Contacts / step : 30.90
Constraints / step : 123.61
Degrees of freedom : 178
Dynamic memory usage : 0.3% of 100M
```
PiperOrigin-RevId: 913758038
Change-Id: If5aa617b2d535c86aec9bd71c9e0003a2b38bdd7
Total testspeed runtime for `2humanoids100.xml` reduced by 27.6% (63.4 -> 49.5s) due to early termination on small islands
PiperOrigin-RevId: 906910915
Change-Id: If55ad468c3680ef44eda7000455a77f8003b3122
The flex interpolation stiffness matrix within the implicit/implicitfast solvers is now built and factorized in a banded format instead of a dense one. This involves:
- Calculating the bandwidth based on the sparsity of the mass/damping matrix and the connectivity within flex cells.
- Allocating and populating a banded matrix `H`.
- Using `mju_cholFactorBand` and `mju_cholSolveBand` for factorization and solving.
This change improves performance for flexes with many DOFs but local coupling.
PiperOrigin-RevId: 901297952
Change-Id: I3efe06353d1903ea65ab30dc49685cede228bb68
The reduced dense factorization for flex interpolation now considers all DOFs in the kinematic chain of the body containing the flex, using mj_bodyChain, instead of only the DOFs directly associated with that body. This is necessary for correctly handling pinned flexes when their parent body is part of a larger kinematic structure.
PiperOrigin-RevId: 872854468
Change-Id: Idbe9fb459084dde9e8eb1076c70dbb685c1b0bdb
The derivative calculation for actuator velocity in implicit integrators now correctly accounts for the `actearly` flag, using the next activation value when `actearly` is true.
PiperOrigin-RevId: 868598722
Change-Id: Ia180afb15b31a718170aeaf9d4ac514bb9e6073b
The implicit integrator was not correctly accounting for the off-diagonal coupling terms between flex and non-flex (parent) degrees of freedom in the mass matrix. This change extracts these coupling terms during the factorization step and applies a correction to the flex forces before solving for the flex accelerations, ensuring that the parent accelerations influence the flex dynamics. A new test verifies that the implicit integrator now matches Euler for small timesteps in a model with flex-parent coupling.
PiperOrigin-RevId: 868098126
Change-Id: Ia8cccd7dd428cd0c0898e1663a7e41017a2311b2
The functions and associated tests related to managing time-stamped data buffers have been renamed from `mju_delay*` to `mju_history*` to better reflect their general purpose beyond just handling delays.
PiperOrigin-RevId: 866978339
Change-Id: I8655e91bce783287ad2adc412d13a1c39aa2c322
Alessio Quaglino