Rope System

The Rope System is a container in the Particle System mold: the nodes you place inside it are never part of the global execution plan — the container schedules them itself every frame, collects their configuration outputs, and feeds a single GPU simulation. One Rope System equals one physics world; multiple Rope Systems each own an independent simulation with their own viewport gizmos.

The solver is XPBD (Extended Position-Based Dynamics) running entirely on the GPU: Verlet integration with distance and bend constraints, one compute threadgroup per rope. It follows the small-steps principle (Macklin et al. 2019) — many small substeps with few iterations converge far better than few substeps with many — using a fixed 1/480 s substep. Because the timestep never varies, the simulation is deterministic to the bit: equal inputs replay the identical simulation, every run. Seeded drape jitter breaks perfect symmetry, and the same seed drapes the same way forever.

Taut cables get special engineering: long-range attachment constraints deliver pin tension to the whole cable instantly, so a tight cable never develops mid-rope vibration; pin motion is interpolated across substeps; rendering uses a temporal blend between the last two solver states so uneven substep counts never read as shimmer; and an axial velocity filter kills longitudinal ringing while transverse swing stays free.

Forces stack. Every enabled force node inside the container contributes an acceleration that is summed per particle, per substep, on the GPU — up to 16 force nodes. Colliders are collected the same way, up to 64 collision primitives per system (a multi-point polyline collider contributes one primitive per segment). Cables are capped at 1024 per system. If several Find Nearest Endpoint nodes exist, the first one wins — one matcher per system.

Anything can drive the inside. The container boundary has a wildcard + input: connect any cable from outside (an LFO, audio energy, a Vision landmark cloud) and a correctly named Port node appears inside, ready to wire to any rope parameter.

The Points output closes the loop. All simulated cable particles are exported through exactly the same transform the render applies (Scale, then Rotation X/Y/Z, then Position offset, then perspective), so downstream nodes — Plexus, Points Visualize, particle emitters, a Replicator — track the rendered strands with pixel accuracy. The export is decimated to at most 8192 points on very heavy systems.

Outputs: Image (the composited cable texture, full viewport), Width / Height (viewport-fit dimensions for the Canvas), Rope Count (number of enabled cables this frame), Points (structure — every simulated cable particle in screen-accurate coordinates), and Torn (a one-frame Bool pulse whenever any cable tears).

• Fingertip cables — Camera Input → Vision (Hands, Hand Tips Only) → Landmarks to Points → into the Rope System boundary → Rope Start Points: one cable hangs from every fingertip and follows the hand in real time.
• Audio-reactive curtain — a Rope with a Start Points array along the top edge, Audio FFT band energy → boundary + → Rope Wind Strength: the curtain gusts with the music.
• Plexus riding the strands — Rope System Points → Plexus: a web of connection lines grows between the physical cables; add Point Animation in between for extra drift.
• Maypole — Start Points from a circle of points (Geometry to Points), a single static End X/Y on the Rope: every cable converges on one point; rotate the start circle and the ribbons wind.
• Tear-triggered events — Torn → Counter / Do Once → flash a Rectangle through a Canvas, or fire a MIDI Sender note the instant any cable snaps.
• Slow-motion replay — drive External Time from a Timer node through a Math multiply: scrub the same deterministic simulation backward and forward like a video.

• The simulation is deterministic per seed and per input stream: identical Seeds, forces and pin motion always replay the same drape and swing. Change a Rope's Seed to get a different but equally repeatable variation.
• With no force nodes inside, cables keep momentum only — "space cables". Add a Rope Gravity node for normal weight; it is not built in.
• Quality is the first knob when taut cables look soft or heavily loaded systems sag through colliders — tier 2 nearly doubles solver work over tier 0.
• The Points output is already in final screen space (perspective applied, Z flattened to 0) — feed it straight to 2D consumers without re-projecting.
• Live topology changes are slot-stable: changing the number of points in a Start Points array rebuilds only the affected cables, and surviving cables keep their physics state — nothing pops or restarts.
• Gizmos (collider outlines, attractor rings, endpoint markers) are drawn per system, so several Rope Systems can coexist with their own overlays.