A thin elastic rod can fall through a soft body between its vertices

What is wrong

• An elastic rod (add_rod, route rod_vbd) dropped on a soft body (a soft material such as silicone_40a, route soft_vbd) can sink through it to whatever is below. The soft body does not move.
• It happens when the rod is thinner than the gaps between the soft body's vertices. A rod that lands between two rows of vertices falls straight through. One that lands on a row rests there for a moment, rolls off sideways, then falls through.
• Nothing flags it. sim.check()["ok"] is True and sim.health() shows no penetration, because the rod and the soft body never touch. The build does not warn.

Number

• Scene. A 0.2 x 0.2 x 0.04 m silicone_40a pad lies on the ground. A copper wire (copper_c110, radius 1.5 mm, 0.16 m long, along y) is dropped from 30 mm above the pad's top. CPU, one world, dt 0.01 s, a development probe script (not shipped).
• The pad's mesh. Its vertex rows are 12.5 mm apart (16 x 16 x 3 cells, 1,156 vertices). Each vertex touches what comes within 2.5 mm of it. So the wire meets a vertex only when its centre line passes within 4 mm of it, and the gap between two rows that it meets nothing in is 12.5 - 2 x 4 = 4.5 mm wide.

Pad bottom 4.5 mm above the ground, pad top 42.4 mm once settled. z is the wire's centre line, so 1.5 mm is the wire on the ground. In both runs sim.check()["ok"] is True with no problems listed.

wire starts over	what happens	lowest z at the end
a vertex row (x = 0)	rests 4 mm above the top from 0.10 s, slides 3.4 mm aside by 0.40 s, falls	1.5 mm at 0.50 s
a gap between rows (x = 6.25 mm)	never touches the pad, falls straight through	1.5 mm at 0.15 s

• The audit's start. The audit's pad started with its bottom 1 mm above the ground, inside the vertices' 2.5 mm reach. The ground pushed it up at the first steps: its top rose from 41 mm to 77.6 mm by 0.10 s before it fell back. sim.check() then lists ENERGY_JUMP, and ok stays True. The wire still ended on the ground the same way (1.5 mm at 0.50 s from over a row, at 0.15 s from a gap), so the jump is not the cause.

Why

• A soft body touches rigid shapes and elastic rods at its vertices only. Each vertex is a small sphere, here of radius 2.5 mm, a fifth of the cell. There is no contact between a rod and the faces of the soft body between the vertices.
• A rod meets a vertex only when its centre line passes within the rod's radius plus the vertex radius of it. When half the vertex spacing is larger than that (6.25 mm against 1.5 + 2.5 mm here), the rod fits between two rows and meets nothing.
• A round rod resting on a single row of vertices is not stable. Any sideways offset tilts the vertices' push away from the row, so the rod rolls off into a gap.
• Cloth has the same limit for small rigid bodies: cloth-small-body-passes-through.md.

Workaround

• If the softness of the body does not matter to the task, make it rigid: add_box(..., rigid=True). The same wire on the same pad made rigid rested on its top (centre line 1.50 mm above it, the wire's radius, through 0.60 s; sim.check()["ok"] True, no problems). An elastic rod beside rigid bodies is verified against a law.
• The chain route is not a way out here. A cable of the same size on it (cable_usb_composite, sim.routes() says chain), dropped on the same pad, failed its health check as it landed: PEN_FAIL between 0.05 and 0.10 s, the world frozen with the cable still 9 mm above the pad, sim.check()["ok"] False.
• Size check. A soft box of volume V gets cells about (6 V / 5,000)^(1/3) on a side (12.4 mm for this pad), with vertices of a fifth of a cell. A rod whose radius plus that vertex radius is under half a cell can fall between the rows.
• Read the rod's lowest z and the soft body's top after a short run before you trust a rod on a soft body.

Planned fix

• Contact between rod segments and the faces of a soft body, so that a rod between vertices still meets the surface. Until then, a build warning when a rod is thinner than the soft body's gaps. After the 0.1.0 release (WP9).