Reject negative sigma1 in LuGre bristle model - #3486
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Three diagnostics that clang does not emit, so they were missed until the gcc-14 CI jobs, which build with -Werror: - mji_scl3 takes a restrict-qualified destination, so passing the same array as source and destination is undefined. Two calls did: negating the edge separating axis, and halving the witness midpoint. The first now negates in place, the second forms the midpoint directly. - Adding const to a pointer to array is not a valid implicit conversion before C23. The pointer it qualified was a redundant alias of the clip buffer, and is removed. No functional change: the box-box fuzzer's output hash over 60k configurations is identical before and after.
- The 25-line comment describing the collider's two stages, the reference and incident face convention, and why the manifold is not reduced below the clipped polygon. The last paragraph is load-bearing: reducing the patch to four points is what an earlier draft did, and it costs two to three orders of magnitude of residual motion on stacks of plates. - contact_net.xml's b1 orientation, from euler="5 4 3" back to "5 5 5". The re-posed model does not converge under mjUSESINGLE: SensorTest.ContactNet spends minutes in mj_solPrimal instead of hundredths of a second, so the single-precision suite never finishes. The original orientation passes in both precisions with the current collider, in 0.02s and 0.01s. No functional change to the collider: the box-box fuzzer's output hash over 60k configurations is unchanged.
On mjWARN_BADQACC the warning handler resets mjData, which rewinds data->time to one timestep. A loop conditioned on data->time then cannot terminate: the model diverges, resets, falls, diverges again, indefinitely. Every step is sub-millisecond, so it presents as a compute hang rather than a failure, and a CI job sits in it until the runner is killed. This test hit that under mjUSESINGLE: 100k steps without data->time reaching 0.2. Bounding by step count turns the same event into a prompt failure. 80 further occurrences of the idiom remain across the test suite; they are only latent as long as their models do not diverge.
giusenso
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sigma1 in LuGre bristle modelsigma1 in LuGre bristle model
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Thanks, but I closed #3487 as WAI: MuJoCo is liberal with parameter values throughout and I don't think LuGre should be special. The line we do enforce is structural: a configuration that's meaningless (a gain with no input to act on, or a τ→V map with K=0) is rejected; a value that's merely unphysical is yours to choose. Separately, this branch has picked up unrelated changes (engine_collision_box.c, the sensor test and contact_net.xml), so it isn't mergeable as-is. Closing. |
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Summary
Reject non-finite or negative LuGre sigma1 (see #3487). Negative values correspond to anti-damping, which causes the friction model to inject mechanical energy rather than dissipate it.
Validation covers both
<dcmotor>and rawdyntype="dcmotor"configurations, including inherited defaults. Documentation is updated accordingly.Testing
Added C API and MJCF compilation tests for negative, non-finite, inherited, and raw dynprm values.