HRLW Bodies POP

Replaces The Bullet Solver COMP with its Actor COMPs. In TouchDesigner you build rigid-body scenes from COMPs; here one POP takes a POP of points, one per body and simulates them on the GPU. It adds things the Bullet Solver COMP workflow doesn't give you directly: joints that break, glued prefractured objects, rubber that squashes, metal that dents, plastic that softens in heat, and bodies that float in the Liquid POP.

Operator type Hrlwbodies, label HRLW Bodies.

What it does#

Simulates rigid spheres, boxes, capsules and meshes with Augmented Vertex Block Descent (AVBD; Giles, Diaz and Yuksel, SIGGRAPH 2025): each body moves by one small Newton step on its own inertia plus every contact and joint touching it, bodies that don't touch move in parallel (graph colouring), and hard contacts and joints come from stiffnesses that ramp up and forces carried over from the last substep, so stacks stand still with few iterations. Contacts have friction (sliding, sticking, rolling). Joints can be ball, weld or spring, and any joint can break.

Quick start (beginner)#

1. Make a POP of points, one per body (for example a Grid POP lifted in y). Optional attributes: shape, pscale, mass, Cd.
2. Add an HRLW Bodies POP, wire the points into input 1.
3. Wire the Bodies POP into an HRLW Geometry POP and give it a material (for example Wood or Rubber).
4. Watch them fall onto the Floor (on by default, at Floor Height 0).
5. Add collider geometry: any other HRLW Geometry POP in the network is a static collider (Collision HRLW Geometry POPs is *). Its extents must lie inside the Collider Box (Colliders page); turn on Fit Box to Colliders to have the box follow them.
6. Press Reset (Bodies page) after changing the number of bodies or any input geometry.

Heavier stacks and long chains want more Substeps per Frame (default 2) or Iterations (default 8). The Info CHOP's max_penetration rising means it needs more of either.

Inputs#

Attributes#

Body points (input 1)#

Joints (input 2)#

Jointed bodies don't collide with each other. With mesh bodies from one POP, joints name bodies by their place in meshid order (lowest first).

Output attributes#

Parameters#

Meshes#

• Instanced meshes (third input): the meshes split by meshid; a point's meshid picks its mesh, placed by P and orient, scaled by its size (Size defaults to 1: the mesh as modelled).
• One POP of triangles/quads (first input): split into bodies by meshid. Each mesh starts where it is. Moving the input doesn't move the bodies (press Reset); changing its triangles or meshids starts over.
• Meshes collide as their convex hulls (a mug is a closed cup to others; see Concave Meshes below) and render as themselves with their own N, Tex and Color. Mass comes from the hull's volume × density; they turn about their real centre of mass.

Geometry POPs as bodies#

Name HRLW Geometry POPs in Body HRLW Geometry POPs (Interact page; names and patterns like the collider field) and each becomes one mesh body, left out of the static colliders. Kinematic (default): the body follows its POP every frame (the rigid motion that best fits its points, worked out on the GPU) and pushes the other bodies about without being pushed back, so animating the POP's transform drives a paddle, a hand or a hook a chain hangs from. The POP draws itself, so the body is left out of this POP's mesh output. Dynamic: the body starts where the POP is and falls like any other, drawn in this POP's output, so take the POP itself out of the Path Tracer's scene. Its mass, friction and bounce are the Input page's. A POP's mesh is read when it first appears and whenever its point or triangle count changes, a frame or so later, and the simulation starts over then (a POP that changes its topology every frame keeps restarting it). Concave Meshes applies to them too. At most 64.

Grabbing bodies with the mouse#

Make a Panel CHOP on the Container COMP (or window) that shows the Path Tracer TOP and name it in Mouse CHOP: its u, v (0 to 1 across the image) and select (or lselect) are read; without those names the first three channels are. Camera From names the HRLW Path Tracer TOP you look through (empty: the only one there is). Pressing the button picks the body under the cursor (kinematic bodies can't be held); holding it, a damped spring pulls the point you grabbed toward the cursor at the depth you grabbed it, so the body swings and collides as you drag it. Grab Strength is how fast it follows (Hz, for every body's own mass), Grab Damping how quickly it settles (1: no overshoot). The spring gives way under a hard pull instead of stiffening, so dragging into a wall slows the body down rather than blowing it up. The Panel must show the whole image, not cropped or letterboxed, for the cursor to line up.

Emitting bodies#

Name a POP of points in Emitter POP. Every Period seconds (or a period attribute) each point emits Bodies per Emission, with a random Velocity Spread, Spin Spread and Position Jitter. Each lives Life seconds (life attribute; 0 forever). The emitter's P, orient, v and w are read at each emission, so an animated emitter sprays as it moves. Changing a point's shape, size, mesh, material, the number of points or Max Alive takes a Reset.

Soft bodies, dents and heat#

The soft, dent and heat values come from the Deformation page (picking a Material loads them there) or the body's materialid. A rubber ball squashes against what it hits, sags and wobbles; a metal body dents where it is hit hard enough; plastic and wax go soft in heat and keep the shape they were bent into; a charred soft body loses pressure and slumps. Spheres, boxes and capsules stay rigid (give them a mesh to bend); dents show on meshes, spheres and capsules (a box's own mesh has only corners). A glued prefractured object dents as one piece: a hit near a seam presses in the pieces on both sides, sized by the whole object, and the hidden cut faces never dent.

Advanced#

Info CHOP. bodies, joints, broken_joints, contacts, pairs, colors (groups for parallel solving), max_speed, dropped_pairs (pairs that found no room; there is more after a reset), mesh_points, mesh_triangles, collider_tris, gpu_mb, colliders_ms, sim_ms, output_ms, soft_bodies, soft_nodes, emitted_alive, emitter_spares. For debugging: serial_bodies (a few is fine, many slows the step), color_rounds, static_asleep, loose_pieces, glued_joints, max_penetration, max_joint_error, max_contact_g (compare with Break Strength and Wake Force).

Concave meshes (Bodies page, Concave Meshes (convex parts), off). At Reset each mesh is split into up to Max Convex Parts (16) convex parts, measured on a grid Concave Detail (32) cells across it. Cups hold what drops in, chain links hang through each other, a torus has its hole, and liquids and gases fill an unbroken cup (thin parts are thickened for them as thin bodies are). Approximate: a part's inner side is flat where the mesh curves; more parts follow curves closer. Chains hold: a 30-link chain swung hard stays linked at the default iterations. Concave pairs cost more (each nearby pair of parts is checked); convex scenes cost the same as before. Takes effect at Reset.

Liquids and fractured glass. For a Liquid POP or Volume Solver, bodies thinner than its Collider Thickness are thickened to it, so liquid can't seep through a fractured glass's thin walls. Meshes are convex hulls to the solvers unless Concave Meshes is on, so fracture a glass (or turn it on) to fill it; its glue tears when the liquid's push passes its strength (lower gluestrength for an earlier burst; the liquid's push on a body is capped at 4 g of its weight).

Rendering. An HRLW Geometry POP wired straight to the Bodies POP reads its output on the GPU, as with a liquid. Because the mesh keeps its points and triangles in the same order until a reset, that HRLW Geometry POP refits its acceleration structure instead of rebuilding, and its motion vectors pair points by number. Points output is for instancing in TouchDesigner.

Not yet. Moving HRLW Geometry colliders act as if still. No rolling resistance.

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