Style3D cloth stiff in stretch for its step drapes slowly or stays flat

What is wrong

• A cloth card with elastic.cloth (warp and weft stretch, shear, bending: it routes to CLOTH_STYLE3D) whose stretch stiffness is large for its mass, mesh and step barely bends out of plane: it creeps (about 2 mm/s) and stays nearly flat, while sim.check() and sim.health() read OK.
• Style3D solves each step with a fixed matrix built from the stretch and bending stiffness. When its diagonal outweighs the inertia term m/dt² by more than about a thousand per vertex, each iteration releases only a little of the out-of-plane motion.

Number

• Woven fabrics from KES-F data on a Cusick drape rig (Petrak 2021 F1 / F4 / F9, alloy.scene.twins.fabric_rig arm A, 8 mm cells, 1.25 ms steps): ratio 9,700 (F1) and 2,900 (F9). They drape to DC 0.98-0.99, against measured 0.22-0.65.
• Its own levers barely help (DC5, rim sag at 0.7 s):

change	rim sag at 0.7 s
as built	-1.0 mm
40 PCG iterations	no change
40 nonlinear iterations	-3.8 mm (3.5x the wall time)
dt / 8	-11 mm (7.5x the wall time)
stretch ÷10 (ratio about 1,000)	still locked, DC 0.949
stretch ÷100 (ratio about 100)	drapes

• The ratio grows as dt² / (areal mass x cell size²): a finer mesh or a lighter fabric locks sooner.
• The library's napkin_cotton_aniso builds at about 1,300, so it warns. Dropped 6.5 cm onto a 12 cm ball, it still hung 27 mm above the floor at 1.5 s; a 30 cm sheet of cotton_woven or cotton_woven_aniso (ratio 70) reached the floor. The napkin's own size differs, so this is a sign, not a measured lock.

Why

Style3D solves each step with a fixed matrix built once from the stretch and bending stiffness (projective dynamics). When its diagonal outweighs the inertia term m/dt² by about a thousand per vertex, each of its iterations releases only a little out-of-plane motion, so the sheet creeps instead of falling. DC5 ruled out the card's units, gravity, the clamp, bending and contact (bending ÷1000 leaves the sag unchanged). The build warns, with the ratio: "a Style3D cloth (an elastic.cloth card) is stiff in stretch for its step" (alloy.scene.style3d_lock, threshold 1,000).

Workaround

• For a woven fabric's drape, give the card no elastic.cloth. It routes to the VBD cloth, which drapes it, within its own isotropy limit (cloth-woven-drape-isotropic.md).
• Check one drape against a measurement before trusting a Style3D sheet's shape.
• A coarser mesh lowers the ratio (cell size squared), at the cost of resolution.

Planned fix

None planned. A disclosed tensile cap on the Style3D route needs a coordinator ruling (DC5: F9 at 0.651 against 0.647 with warp and weft capped, F1 0.347 against 0.222). Sources: DC5's probe notes and ratio script, and DC5's write-up of 2026-11-08 (not shipped).