Chapter 4: Collision, Cloth Sim & Rigging Considerations

Created by Sarah Choi (prompt writer using ChatGPT)

Collision, Cloth Sim & Rigging Considerations

Great carry and interface design doesn’t survive contact with animation unless you plan for physics from the sketch. Belts, packs, holsters, and on‑body UI introduce rigid masses, dangling elements, and soft–hard interfaces that can break silhouettes, interpenetrate, and jitter unless you provide a clear technical blueprint. This article gives character concept artists a production‑ready framework for collision proxies, cloth‑sim setup, and rigging decisions so designs move cleanly and read at gameplay distance.

Think in Volumes First, Details Last

Before adding stitches and buckles, block the costume as a stack of volumes that match how simulation and rigging will treat them: pelvis‑locked belt torus, semi‑rigid chest yoke, rigid holster shells, soft pouch bodies, cable arcs with limited slack, and cape or skirt sheets. Draw these as simple primitives in orthos and side views. Note the standoff—the air gap between layers—because shadow lanes and collision thickness come from that gap. If a design has no room for a collider, it will clip when the character runs.

Collision Proxy Strategy for Carry Elements

Every rigid or semi‑rigid item needs a proxy shape simple enough for stable collisions. Holsters and hard pouches become beveled boxes or capsules; belt buckles collapse to a single slab; battery slabs on packs use a tapered box with rounded edges; UIs become thin plates. Indicate proxy shapes on callouts with measurements and orientation. Where two rigid parts meet cloth, add a shallow collision shelf—a lip or binding—that keeps fabric from popping over edges under wind or sprint. For thigh platforms, extend the proxy slightly under the plate to prevent cloth suction during crouch.

Soft–Hard Interface Grammar

Cloth will balloon around hard parts unless anchored. Show stitch lines, bar tacks, and binding tapes that justify pin groups in simulation. Float plates and holsters on webbing ladders or elastic bridges so the solver can allow limited slide rather than hard welds; this prevents buzz and sells load. Under big plates, specify spacer mesh pads that act as collision thickness and airflow buffers. Depict witness tents—tiny dimples between anchor points—to communicate tension and provide AO that strengthens the read at distance.

Belts: Semi‑Rigid Splines, Not Loose Ropes

Belts should not behave as noodles. Treat the main belt as a semi‑rigid spline constrained to the pelvis with a small number of bones or a kinematic curve. Show where the belt should articulate—over hip crests and at a split front—and where it should stay stiff—over lumbar pads and buckle slab. Any drop legs or thigh straps should hinge from a pelvic yoke, not from cloth skirts, with a defined swing range and a stop so they don’t whip unrealistically.

Packs and Frames: Load Transfer Over Decoration

Backpacks need a frame‑to‑belt relationship or they will swim. Explain that relationship with visible stays, exo rails, or a hidden frame line in callouts. Shoulder straps are stabilizers; they should collide with the torso but also have load lifter constraints near the collar. Place pack UI (battery meters, beacons) on stable edges so they don’t shear under strap stretch. For capes and hoods, show a cape cutout or split around pack collars to avoid interpenetration and create clean wind response.

Holsters and Sheaths: Draw Cones, Then Proxies

Provide a dashed draw cone that defines the motion path of the hand and the tool. Holster throats need reinforced collision lips and a little internal taper to prevent the weapon sinking through. Angle and cant should match seated and prone poses, and the proxy must clear thigh and tasset volumes at maximum hip flex. For cross‑draw sheaths, include a swivel mount in the design so rigs can align the scabbard to the draw arc during run cycles without clipping coats or capes.

Flaps, Lids, and Pull Tabs: Controlled Freedom

Flaps should be simulated with stiff roots and soft tips; design binding tapes and stitch bars at the root to justify higher bend stiffness. Add keeper elastics or tuck loops to tame strap tails and reduce solver jitter. Pull tabs need asymmetric geometry and material contrast but must not exceed the local collision depth; oversized tabs become snag factories. For stealth kits, design covered snaps and peached textiles so there’s less collision noise in rapid motion.

Cables, Hoses, and Tethers: Limited Slack Systems

Route cables along strap corridors and specify retainer loop spacing. Provide explicit slack lengths per run and breakaway couplers near neck and hips. In your callouts, indicate snag points and travel limits for reels or coil housings. If a hose crosses a joint, propose a serpentine path with clamps that act as bone constraints; never allow cables to cross draw cones or hinge corridors without a standoff.

On‑Body UI: Rigid Islands and Touch Targets

Buttons and displays belong on rigid islands with collision plates beneath; soft‑mounted screens wobble and look fake. Mark glove tolerances and clearance cones for finger taps. Place small bezels or guard rails around critical buttons to deflect cloth. For emissive rings on belts and helmets, use continuous loops so LODs can maintain a single collision strip rather than many tiny LEDs that snag.

Cloth Types, Solver Settings, and “Behavior Notes”

Your material choice implies solver parameters. Woven shells get higher bend resistance and low stretch on straight grain, with bias allowed; knits allow higher stretch and damping for fast recovery; laminates and leather want higher bend stiffness and minimal shear; spacer meshes behave like soft springs. Add a behavior box to each material swatch: bend stiffness, stretch limit, damping, and plasticity notes tied to camera distance. Suggest plastic crease memory for leather bellows and kink radii for coated shells to prevent unrealistic flutter.

Standoff, Shadow Lanes, and Readability

Mobility reads depend on shadow lanes under overlaps. Reserve at least a finger of standoff under pauldron lips, abdominal lames, and holster edges. If a design closes those gaps, the solver has no room to breathe and the read flattens. Use hem weights and hidden cord channels to keep capes from sticking to thighs; in fantasy, coin weights at corners do the same job. Call out standoff distances numerically so rigging can set collision thickness accordingly.

Pose Library and Stress Tests

Deliver a mini pose library with kneel, sprint, climb, mount, roll, and seated poses. In each, sketch where collisions are most at risk and note any parts that should switch to kinematic during extreme actions (e.g., lock a holster during a roll). Provide a snag audit diagram showing any forward‑facing hooks, strap tails, or tabs that might catch while vaulting.

Wind, Rain, and VFX Interaction

Design shapes that help the solver sell environment. Long, narrow panels flutter and slap; broad, laminated shells should drum with lower frequency. Add drip paths that route water away from closures; define wet state mass gain for textiles so capes hang heavier in rain. Indicate where frost or dust should increase friction and reduce flap motion; this gives VFX and sim a shared target.

LOD & Streaming: Simplify Without Losing Function

At distance, merge tiny hardware into large, clean shapes but preserve the functional silhouette: belt slab, holster block, pack spine, UI ring. Replace pouch flaps with single planes and bake AO at roots; collapse webbing ladders into broad bands. Keep at least one emissive loop and one anchor cluster per region so access and load paths remain legible. Include a collision LOD plan that swaps complex proxies for simpler ones as distance increases.

Failure Modes & States in Motion

Design for states: cinched, neutral, loosened; latched vs unlatched flaps; safe vs armed holsters. Show how straps slip into keepers in neutral and how they hang in loosened states. Define failure language that the solver can emulate: strap pull frees a flap, elastic rebound returns it; broken snap converts to a dangling state with a limited swing range and a stop to avoid infinite spin.

Packaging: What to Hand to Tech Art

Your final package should include: orthos with collision proxy overlays; standoff and thickness callouts; draw‑cone diagrams; strap routing with hinge points and keepers; cable runs with slack lengths and breakaways; cloth behavior notes per material; pose library with stress annotations; LOD collision plan; and a table of constraint types (pin, limited‑slide, hinge) per attachment. Provide reference GIFs or quick thumbnails of expected motion rhythm for capes, flaps, and bells so animation can match the vibe.

Closing Thought

When you design for collision and rigging from the first thumbnail, you turn problems into features: crisp shadow lanes, satisfying flap snaps, believable belt sway, and tools that draw without clipping. Your costume stops being a pile of props and becomes a machine that moves with the character—and reads cleanly at any camera distance.