Squish Lab Demo Creation

Let me check the current task details. The task is "Wake up and do as you please." The last execution was a success creating the "Lumen Lab" demo (040).

So I should create a new demo. Let me first install shell tools and look at the existing demos folder to see what's there, then create a new advanced Three.js demo.

Let me start.

The task is "Wake up and do as you please." Given my purpose, I should create a new cool Three.js demo. Let me check the current state of the demos folder first.

I have 40 demos already. The last one was "Lumen Lab" (ray optics workbench). I need to create demo 041 with something new and impressive.

Let me look at what themes have been covered:

1. Spinning cube
2. Nebula galaxy
3. Mandelbulb explorer
4. Fluid dynamics
5. Gravity sandbox
6. Event horizon (black hole)
7. Sonic reactor
8. Void runner (game)
9. Azure odyssey
10. Abyssal bloom
11. Physarum garden
12. Powder alchemy (falling sand)
13. Clockwork nebula
14. Neon rain
15. Aurora borealis
16. Skitter (creatures)
17. Coral resonance
18. Threshold
19. Attractorium (strange attractors)
20. Murmuration (flocking)
21. Terraforge (terrain)
22. Cloudsea (clouds)
23. Frostline (frost)
24. Koi pond
25. Ferrofluid
26. Synchrony (coupled oscillators?)
27. Cymatics
28. Teslasphere
29. Pyrosymphony
30. Morphogenesis
31. Galton cascade
32. Wavefront
33. Gossamer
34. Magnetosphere
35. Cloud chamber
36. Wavefunction
37. Epicycle
38. Sandpile
39. Kinetic theory
40. Lumen lab (ray optics)

Ideas for demo 041:

• Diffraction/interference optics — double slit, wave interference... kind of covered by wavefront?
• Elasticity/soft-body physics — a jiggly soft-body simulation with XPBD, deformable objects, can poke and drag. That would be fun and interactive. Soft body jelly cube!
• Cloth simulation — maybe overdone.
• Voronoi fracture / shatter — breakable glass objects. Cool: throw balls at glass panes that shatter with procedural voronoi fracture.
• Double pendulum chaos — covered by epicycle/synchrony-ish? Not exactly, but attractorium covers chaos.
• Turing patterns / reaction-diffusion 3D — morphogenesis covered it probably.
• Magnetic field line visualizer — magnetosphere covered.
• Circuit simulator — eh.
• Spring lattice / elastic membrane — soft body.
• Verlet ragdoll playground — fun but maybe grim.
• Buoyancy / water — koi pond covered water.
• Knots / topology — trefoil knots, knot energy minimization... "Knot garden"? Meh.
• Origami / paper folding — hard.
• Slinky / spring physics — softbody.
• Gyroscope / rigid body playground — rigid body physics with contacts: stack boxes, dominoes, wrecking ball... A "Domino / Rube Goldberg" machine would be fun: build contraptions and knock them over with custom physics.
• Ballistics / artillery game.
• Planetary gearbox / mechanical watch — clockwork nebula covered gears.
• Spirograph harmonograph — epicycle covered.
• Electromagnetic wave propagation / antenna.
• Quantum tunneling — wavefunction covered quantum.
• Plasma globe — teslasphere covered.
• Double-slit interference with wave packets.
• Chladni plate 3D — cymatics covered.
• Elastic collision / billiards.
• Soft-body jelly: XPBD soft body tetrahedralized meshes — squishy bunny/blobs you can poke, stretch, throw; they collide with the floor and obstacles. Very interactive and visually fun with subsurface scattering-ish shading and bloom. I think that's a strong pick: "Jelly Lab" or "Squish".

Actually, a really nice one: "Tensegrity" — floating compression structures. Hmm, niche.

Another idea: "Prism Break / Shatter" — procedural Voronoi glass shattering with slow-mo, physics shards. Very satisfying: click to shoot a steel ball at glass sculptures (pane, wine glass-ish lathe shapes), they shatter into convex shards with per-shard physics (custom simple impulse physics or simple verlet with floor collision), refraction-ish material (transmission might be heavy but MeshPhysicalMaterial with transmission on could work), slow motion toggle, sound. Glass + bloom = pretty.

But a full convex-shard physics engine is work; simple approach: shards get linear + angular velocity from impact point, floor bounce with restitution, no shard-shard collision. That's very doable and looks great.

Alternatively "Elastic Jelly / Soft-body" — XPBD with tetrahedral constraints is also very doable: generate tet mesh from a shape (sphere/blob), distance constraints on edges + volume preservation (or shape matching). Simplest robust: shape matching soft body (Müller style) — particles + goal positions from best-fit rigid transform of rest shape; extremely stable, looks convincingly jiggly. Collision with floor + draggable. Render smooth mesh by skinning a higher-res mesh to particles... that's more complex. Alternative: render the particle lattice as metaballs? Marching cubes is heavy.

Simpler render approach for soft body: use the tet mesh surface as render mesh directly, positions = particle positions, recompute normals. With enough subdivisions (icosphere subdivided 3-4 times, tetrahedralized via Delaunay... no scipy in browser). Hmm, tetrahedralization in JS by hand is a pain.

Alternative soft-body approach: treat a subdivided icosphere mesh itself as a spring lattice: particles on vertices, constraints along mesh edges + bending constraints + pressure (volume) constraint. This is classic "pressure soft body" (like the famous "Jelly" demos). Pressure model: P * V = nRT → force along normals proportional to pressure. Very stable, looks great, no tetrahedralization needed. Sphere/blob shapes work great. Can have multiple blobs, stickiness, grab with mouse raycast, obstacles.

That's a solid plan: "Squish — pressure soft-body playground":

• Soft bodies: closed triangle meshes (icosphere subdiv 4-5, torus?, capsule?) with edge springs (verlet + constraint projection, XPBD), pressure constraint preserving volume.
• Interactions: grab/pull vertices with mouse (raycast to find nearest particle, drag on a plane), poke with impulse, "inflate/deflate" slider, gravity control, spawn shapes.
• Collisions: floor plane, a few static spheres/box obstacles, walls of an invisible box; sphere-sphere approx between soft bodies? Could skip inter-body collision or do cheap particle-particle repulsion between bodies.
• Rendering: MeshPhysicalMaterial with transmission? Maybe glossy translucent material + environment. Flat shading off, recompute vertex normals each frame. Add subtle subsurface color. Bloom post-processing.
• HUD: FPS, particle count, volume, pressure; buttons: spawn blob, reset; sliders: stiffness, pressure, gravity; slow-mo; audio squish sounds on impact (WebAudio).
• Camera: orbit controls.

Also can add "stick to walls" mode? Keep scope manageable.

Performance: subdiv 4 icosphere = 2562 verts, edges ~7680. Verlet with edge constraints iterated 10-20 times per frame in JS: 7680*20 = 150k constraint ops per body per frame — fine. Multiple bodies: maybe 2-3 bodies at subdiv 4. Compute normals each frame — fine.

Pressure model details (based on Maciej Matyka's soft body pressure model):

• For each face: normal n_i, area a_i. Volume V = sum over faces of (p · n) * a / 3 (divergence theorem).
• Pressure force on face: F = n * a * (P_current - P_rest?) Actually F = n * a * k_pressure * (V_rest / V_current - 1)? Matyka uses ideal gas: P = nRT/V, force proportional to pressure difference with external. Simplify: F_face = n_i * a_i * k * (V_rest - V)/V_rest... that deflates when compressed? Wait if compressed, V < V_rest, pressure inside increases, pushing out: F ∝ (nRT/V - P_ext) * a. Let me use F = n_i * a_i * kp * (n_gas / V - n_gas / V_rest) = n_i * a_i * kp' * (1/V - 1/V0) * n_gas. Tune kp so stable. Distribute face force /3 to each vertex.

Springs: XPBD distance constraints on each unique edge with compliance, plus maybe long-range "bend" springs connecting vertex to second-ring neighbors or to center particle to resist folding. Adding one center particle connected to all surface verts with stiff springs helps stability a lot and keeps shape from collapsing.

Grab interaction: pointer ray → closest vertex within threshold; then while dragging, project mouse onto camera-parallel plane at grab depth; set grabbed particle position via strong spring toward target.

Squish audio: on impact velocity, play filtered noise "splat" with pitch by size. Generative ambient pad maybe.

Also nice: "Jelly color picker", trail/ribbon? Keep it focused.

Post-processing: UnrealBloomPass via three/addons. Environment: RoomEnvironment for nice reflections.

Let me name it 041-squish-lab or 041-jello... "Squish Lab"? "041-jelly-physics"? Let me go with 041-squish title "Squish Lab — soft-body pressure playground". Hmm, maybe "Jello-O-Matic"? I'll go with "Squish Lab".

Actually let me reconsider the demo choice. Previous demos already include powder (particle physics), kinetic theory (gas), galton cascade. Soft-body is genuinely new. Good.

Let me check what an existing recent demo looks like for conventions (import map version, bloom usage). Let me peek at 040-lumen-lab/index.html briefly to match style/structure and confirm three version.

Three r170 with unpkg import maps is the convention. Now let me design and write demo 041: "Squish Lab" — an interactive soft-body pressure playground.

Design: 041-squish-lab

Core concept: XPBD soft-body blobs (inflated balloon-style pressure soft bodies) that the user can grab, stretch, throw, pop(?), inflate, deflate. Multiple bodies, obstacles, floor collision, bloom, generative audio.

Physics (per soft body):

• Particles on a closed triangle mesh (icosphere subdivision; also capsule-ish and torus shapes built procedurally).
• Verlet integration (position based), gravity, damping.
• XPBD distance constraints on unique edges (compliance parameter → stiffness slider).
• Center particle + spoke constraints (stiff) to resist collapse/inversion.
• Pressure: V via divergence theorem; per-face force F = n * area * kp * (nGas/V − nGas/V0) applied each substep as impulse/velocity change. Simpler stable approach used commonly: pressure force applied as acceleration each frame before integration.
• Self-collision: cheap spatial-hash particle-particle repulsion within a body (optional, maybe skip for perf; can enable for thick shapes). Skip self-collision to keep it fast; center + pressure + springs is stable enough.
• Collisions: ground plane (y=0) with friction, invisible box walls (x,z within ±R), static obstacles: spheres and boxes (SDF-based push-out). Particle vs obstacle sphere/box.
• Inter-body: particle-particle repulsion between different bodies using a spatial hash on all particles (radius ~ avg edge length). Doable: total particles ~ 3 bodies × 642 (subdiv 3) ≈ 2k particles, hash grid, O(n·k). With substeps maybe only once per frame, fine.

Actually let me use subdiv 3 icosphere = 642 verts, 1920 faces, 1280 edges. Subdiv 4 = 2562 verts, 7680 edges. For 60fps with ~15 constraint iterations... 2562 verts is heavy with normals recompute but still OK on modern machines. I'll default subdiv 4 for the hero blob (one big blob) and subdiv 3 for spawned ones, with a quality toggle. Let me make iteration count adaptive: substeps = 8, iterations = 2 per substep (XPBD style: small substeps, few iterations — that's the XPBD "small steps" recommendation).

Rendering:

• MeshPhysicalMaterial: translucent-ish, roughness ~0.15, clearcoat, sheen? Use transmission: 0 for perf maybe; instead use glossy colored material with envMap from RoomEnvironment. Or light transmission 0.6 with thickness for a jelly look — transmission is expensive (needs transmission render pass) but one or two bodies OK? Transmission pass renders scene behind — with 3 bodies it could be heavy but likely still fine on desktop. Risky for weaker machines. Alternative: fake jelly — semi-transparent (opacity 0.85), high clearcoat, emissive slight. I'll offer "Jelly (transmission)" material toggle, default off? Transmission actually looks AMAZING for jelly. Let me default transmission ON with thickness, and provide quality toggle to disable. Hmm — multiple transmissive objects in three.js: transmissive objects don't see each other through the transmission pass (they're rendered in the transmission pass as opaque... actually they are excluded). They'd appear as opaque silhouettes through each other. Acceptable.

Let me default to a nice opaque-ish glossy "gummy" material (no transmission) with slight emissive rim... I'll test only via syntax, so choose safe: MeshPhysicalMaterial with clearcoat 1, roughness 0.25, opacity 0.92 transparent, envMapIntensity. Plus a soft inner glow: second mesh slightly smaller with emissive? Skip.

• Ground: large reflective-ish disc with grid shader or subtle radial gradient; contact shadow via blob shadows? Use real shadows: directional light with shadow map, bodies cast shadows. Transparent materials cast weird shadows; set castShadow true, material.shadowSide. Gummy opaque-enough fine.
• Bloom: UnrealBloomPass with low strength (0.35), threshold ~0.8 so only highlights/emissive bloom.
• Background: gradient sky via large sphere shader or scene.background color + fog. RoomEnvironment for IBL.

Interactions:

• OrbitControls (rotate/zoom/pan) — but left-drag orbits; grab uses pointer down near a blob: raycast against blob meshes; if hit → grab (disable orbit). Else orbit. This is the standard approach.
• Grab: find nearest particle to hit point; drag moves target on plane parallel to camera through grab depth (or fixed-distance plane). Strong positional spring on grabbed particle (move particle toward target with limited max displacement per step → stretchy feel).
• Right-drag or shift-drag: "poke" impulse? Simpler: quick flick while grabbing throws it (natural from velocity).
• Scroll UI panel: sliders — Gravity, Pressure (inflate), Stiffness (compliance), Damping; buttons — Spawn Blob (random pastel color), Pop (remove), Reset; toggles — Slow-mo, Sound, Wireframe(?), Jelly material.
• Click "Pop!": fun — burst into particles? Popping: remove body, spawn particle burst (Points with velocities) + pop sound. That's a nice touch and cheap. Implement burst as simple points system with life.
• Hover highlight: emissive boost on hovered body.

Audio (WebAudio, generative, starts on first interaction):

• Squish impact: noise burst through lowpass + sine thump; intensity from impact velocity; rate-limited.
• Ambient pad: two detuned sines + slow LFO, very quiet.
• Pop: quick pitch sweep up + noise.

HUD: overlay divs — title, stats (bodies, particles, FPS), controls list, sliders. Keep the same visual language as previous demos (dark glassy panels).

Presets? Maybe a couple: "Bouncy Castle" (3 blobs high pressure), "Zero-G Float" (gravity 0, gentle drift), "Heavy" (high gravity). Simple preset buttons applying slider values.

Let me also add obstacles: a few rounded static shapes (spheres, a torus?) in the scene that blobs collide with — SDF collision against analytic spheres/boxes. Keep 2-3 obstacles: a big sphere and a spinning bar? A rotating paddle that smacks blobs = fun. Add a slowly rotating "spinner" (capsule bar) obstacle with collision that imparts velocity from surface motion. That adds life. Collision vs moving obstacle: compute point velocity of obstacle at contact, blend.

Hmm, rotating bar collision needs OBB SDF + contact velocity. Doable: transform particle into bar local frame (inverse rotation about Y), clamp to box, push out along local normal, contact velocity = ω × r. Fine.

Performance budget: default 1 hero blob (subdiv 4, 2562 particles, 7680 edges) + substeps 8, iterations 2 → 16 × 7680 ≈ 123k constraint solves/frame. JS can do ~10-50M simple ops/frame-budget... 123k × ~20 ops = 2.5M ops — fine. Spawned blobs subdiv 2 (162 verts) or 3 (642). Cap total particles ~6000.

Wait — XPBD with 8 substeps and pressure computed per substep: volume via 1920-7680 faces × 8 substeps = 60k face ops — fine.

Also mesh normal recompute each frame for 2562-vert geometry: computeVertexNormals in three is OK-ish; write custom fast normal accumulation using flat arrays for speed. I'll implement custom: zero normals, accumulate face normals, normalize. ~8k faces → fine.

Inter-body repulsion via spatial hash once per frame (not per substep) — fine.

Structure of index.html: single file, import map (three@0.170.0), module script. Sections:

1. HTML/CSS UI (panel, buttons, sliders, stats).
2. Imports.
3. Audio module.
4. SoftBody class (build from geometry, step, render mesh sync).
5. World (bodies, obstacles, gravity, substeps loop).
6. Interaction (pointer grab, raycast, hover).
7. UI wiring, presets.
8. Main loop, resize.

Then test: extract physics into a node test (like last time): volume computation on unit sphere ≈ 4/3π, constraint keeps edge length, pressure equilibrium (inflated sphere volume > rest volume with pressure>0... hmm pressure model), no NaN after 600 steps, energy settles (KE decreases with damping), floor collision keeps particles y>=0. node --check full module.

Shapes to build:

• Icosphere: new THREE.IcosahedronGeometry(r, detail) — non-indexed? IcosahedronGeometry is indexed? PolyhedronGeometry produces non-indexed? In three, PolyhedronGeometry generates non-indexed (position attribute with duplicated verts? Actually PolyhedronGeometry builds with setAttribute positions per-face, non-indexed). I need welded vertices with unique edges and index buffer. I'll weld by position hash. Write weldGeometry(geo) returning {positions: Float32Array unique, indices, map old→new}. Since polyhedron non-indexed: positions length = 3*faces; weld by rounding coords.
• Capsule: CapsuleGeometry(r, len, capSegs, radSegs) — indexed already? CapsuleGeometry extends LatheGeometry? CapsuleGeometry in three is based on... it has an index. Either way weld handles both.
• Torus: TorusGeometry — indexed; weld fine. Pressure model requires closed surface — torus is closed, fine. Volume via divergence theorem works for torus (genus 1 still fine).

Body-building: given geometry → weld → particles (pos, prevPos, invMass), edges from index pairs (unique via Set), center particle? Adding center particle complicates inter-body hashing slightly but fine. Actually center spoke constraints: with pressure + edge springs + substeps, likely stable without center. Center helps prevent collapse/inversion under hard pokes. I'll include optional center particle with medium-stiff spokes; exclude center from rendering & from inter-body repulsion.

Volume computation must include faces; V0 = rest volume. Pressure force: commonly F_i = n_i * a_i * k_press * (nRT/V). Implement per substep as velocity delta: for each face, f = narea(pressureDiff); apply v += fdtinvMass/3 each vertex. pressureDiff = kp * (q/V - q/V0) where q is "gas amount" (inflate slider scales q, default q=1). If V < V0 → positive → pushes out. If user inflates, q up → bigger.

Also "over-pressure safety": clamp pressureDiff to avoid explosion.

Stability: edge constraints XPBD with compliance α. dt per substep h = dt/substeps. XPBD: Δλ = (-C - α̃λ)/(ΣinvMass·|∇C|² + α̃), α̃ = α/h². Stiffness slider maps compliance: α from ~1e-7 (very stiff) to 5e-6 (soft)? XPBD compliance units: for distance constraint α in m/N... practical values: 0 = rigid. I'll use α range [0, 2e-6] mapped from slider 1..0. With 8 substeps, α=0 is fine (PBD).

Damping: v = (1 - dampingdt). Plus XPBD damping optional.

Grab: store grabbed particle index & body; target point; each substep: p.pos += clamp(target - pos, maxStep) * grabK. Grabbed particle invMass temporarily reduced (×0.1) so it drags body.

Throw: on release, particle keeps velocity — good.

Hover raycast: against meshes each pointermove (throttled).

Floor: y=0 plane; particle radius r_p (avg edge*0.55) → y >= r_p; friction: tangential velocity *= 0.8 on contact; also prevPos adjust. Simpler PBD: if y < r: y = r; apply friction to prevPos xz.

Walls: invisible cylinder radius R=14 keep blobs in view: if |xz| > R - r: push in.

Obstacles:

1. Two static spheres ("boulders") positions (±4.5, 1.2, ∓2), radius 1.2.
2. Rotating bar: center (0, 1.0, 0)? That collides with spawn. Place bar center at (0,0.9,4.5), half-length 3, radius 0.35, rotating about Y axis through its center, ω=0.8 rad/s. Collision: transform p to local: dx = p - c; rotate by -θ; in local frame bar axis along X: closest point on segment clamp x∈[-L,L], y=z=0; dist vector; if < r_bar + r_p → push out along dir; add surface velocity v_surf = ω × r_world at contact; set particle velocity partially to v_surf (impart smack). Also the bar should visually rotate (mesh sync).

Spawn positions: hero blob at (0, 3.2, 0). Spawn button adds blob at random position above, random pastel color, random shape among sphere/capsule/torus? Torus with pressure... torus equilibrium fine. Fun: "donut"! Keep shapes: Blob (icosphere), Capsule, Donut.

Popping: button pops the last grabbed / hovered body or oldest spawned? Simplest: "Pop" pops body currently hovered or last touched; if none, pops oldest spawned (keep hero). Or click a blob while in "pop mode"? Simplest robust: each body mesh gets a small ✕? no. I'll do: pressing X or clicking "Pop" button pops hovered/last-grabbed body; spawn burst particles.

Pop burst: Points with ~120 particles from body surface particles, velocities outward + up, gravity, fade 1s, additive blending. Nice.

Stats: FPS (EMA), body count, particle count.

Presets:

• "Bounce House": gravity 1×, pressure 1.1, spawn 3 extra blobs.
• "Zero-G": gravity 0.02.
• "Floppy": stiffness low (compliance up), pressure 0.85.
• "Reset": clear spawned, hero restored.

UI: left panel with sliders & buttons; top-left title; bottom hints ("drag blob to grab · drag space to orbit · scroll to zoom").

Edge case: pointer capture, touch support (single touch grab/orbit handled by OrbitControls + our raycast logic; two-finger orbit default).

Now, one concern: OrbitControls left-drag vs grab: on pointerdown, raycast; if hit blob → grabbing=true, controls.enabled=false; else controls handle. On pointerup: release, controls.enabled=true.

Raycast against soft meshes: three Raycaster with BufferGeometry works (uses index/positions). 7k-face mesh raycast is OK on click.

Normals recompute: custom function.

Transmission material default: I'll default jellyMode ON (transmission 0.85, thickness 1.5, roughness 0.08, ior 1.35) — but multiple transmissive bodies heavy... The transmission pass renders the scene once for all transmissive objects (single transmission render target shared). Cost ~2× scene render. With simple scene fine. When jellyMode off: standard gummy (transparent 0.9). I'll wire toggle.

Hmm, transparent/transmissive + shadows: transmissive objects cast shadow as opaque — fine, looks good.

Also add subtle "sparkle" env: RoomEnvironment via PMREMGenerator.

Bloom with transmission: fine.

Let me also consider ground material: dark, slightly reflective (MeshStandardMaterial metalness 0.1 roughness 0.4) + grid lines shader? Keep simple: big circle with radial gradient texture generated on canvas + faint grid via another canvas texture. I'll generate a canvas texture: radial fade + grid lines.

Sound engine: small class with master gain; functions: thump(strength, pitch), pop(), ambient start/stop. AudioContext resume on first pointer event.

Physics test extraction: I'll write physics core as pure functions/classes not importing three (use plain arrays) — so node test can import the same code? The demo is a single HTML; I'll write the physics section as a template that I can extract between markers (like previous run did). Simpler: write physics in a separate <script type="module">? Still imports nothing — I can copy the section into /tmp test file. I'll put clear markers // === PHYSICS CORE START/END === and extract with sed for the node test. Physics core must not reference THREE — use custom tiny vec helpers on Float32Array or {x,y,z}. Use flat arrays with indices for speed anyway.

Plan the SoftBody core data:

• nP particles: px,py,pz arrays (Float32Array), prev arrays, vx? Verlet: keep pos & prev; velocity = (pos-prev)/h.
• Edges: eA, eB (Uint32Array), rest (Float32Array).
• Faces: fA,fB,fC (Uint32Array) for pressure & normals & rendering index.
• Center particle index optional: nP (extra one), spokes arrays.
• invMass: all 1 except center maybe 1; grabbed particle handled separately.

Step(world params, h):

1. For each substep: a. prev = pos; integrate: pos += (pos-prev)(1-damph) + gh² (gravity accel). Verlet with damping: v = (pos-prev)(1-damph); pos += v + ah². b. Pressure: compute volume V (sum over faces cross/dot). For each face compute n (unnormalized cross), area=0.5|n|; V += dot(a,n)/6 (divergence theorem: V = Σ (p0 · (p1×p2))/6). pressureForce per vertex accumulate: F = n_face * (kp*(q/V - q/V0)) * (1/3) each vertex (n_face not normalized — includes area weighting!). Nice: force on face = P * area * n̂ = P * (n/2) with n=cross unnormalized. So per-face vertex force = P * n / 6. Apply as velocity change: Δv = F * h * invMass → apply to prev? In PBD apply to pos: pos += Fh²invMass. Implement as acceleration before integration. So compute pressure acceleration first, add into gravity accel per particle: ax,ay,az arrays zeroed, add gravity, add pressure/node. Then integrate once. c. Solve edge constraints (iters × edges) XPBD. d. Solve spoke constraints. e. Collisions: floor, walls, obstacles (spheres, bar), grabbed particle constraint. f. Inter-body repulsion: once per frame (outside substeps) as positional push.
2. After substeps: sync mesh positions, recompute normals.

Inter-body + self thickness: skip self.

Inter-body repulsion: hash grid cell size c = 2*r_p_max. For each particle pair across different bodies closer than rA+rB → push apart 50/50 along delta. Once per frame, one iteration — soft but fine.

Bar obstacle contact velocity: after pushing out position, also adjust prev so velocity gains surface velocity component: v += (v_surf - v)·n̂ * n̂ * 1.2 (bounce) — implement: compute v = (pos-prev)/h; vn = v·n̂; if vn < v_surf·n̂: set vn' = v_surf·n̂*1.1 + small restitution... Keep simpler: add impulse along n̂: pos += n̂ * push; prev -= n̂ * extra. I'll implement: relative normal velocity toward obstacle compensated with restitution 0.6.

Floor bounce: restitution small (0.2) via prev adjust? Verlet: if y<r: y=r; vy kill: prevY = posY + (posY-prevY)rest? Let prevY = posY(1+rest) - restprevY... Actually reflect: v = pos-prev; v.y = -v.yrest → prev = pos - v. Apply friction on xz: v.xz *= friction → prev.xz = pos.xz - v.xz.

OK. Numbers: hero blob radius 1.6, detail 4 (IcosahedronGeometry(1.6, 4)) → verts 2562, faces 5120? Icosahedron detail 4: 204^4=5120 faces, 2562 verts, 7680 edges. Hmm detail param in three: IcosahedronGeometry(radius, detail) subdivides detail times → faces = 204^detail. detail=4 → 5120 faces, verts = 10*4^4+2=2562. Yes.

Substeps 8, iterations 2: edges 7680*16 = 123k solves — each solve ~15 flops → 2M flops, fine. Pressure per substep: 5120 faces × 8 = 41k face computations — fine. Normals once per frame: 5120.

Spawned: capsule (CapsuleGeometry(0.9, 1.2, 6, 12) → verts (capSegsradSegs...) ~ 612*? let me not compute; weld handles; roughly a few hundred) — good. Donut: TorusGeometry(1.0, 0.45, 12, 28) → 13*29=377 verts. Good.

Cap: total particles ~ 2562 + 8 spawned*(~400) ≈ 5.8k — fine.

Blob shadows: use real shadow map from directional light. Transparent material shadows OK.

Alright — also the "inflate" slider changes q (gas amount) of hero or all? Global pressure slider scales q for all bodies; each body also gets slight random q. "Pop" needs a target — track activeBody = last hovered/grabbed.

Also hovering shows a tooltip? Skip.

Wireframe toggle: sets material.wireframe — fun to see the lattice. Include.

Slow-mo toggle: timeScale 0.25.

Let me now also handle devicePixelRatio cap 2, antialias true.

Also title/description: "Squish Lab — a soft-body playground: grab, stretch, throw, inflate and pop wobbling pressure blobs..."

Edge highlight visual polish: add a faint fresnel-ish rim via material sheen? MeshPhysicalMaterial sheen works. Use sheen 0.5, sheenColor white-ish. OK.

One more feature for "wow": colorful point lights that blobs carry? Each body could have a small PointLight — expensive with many bodies; skip (env lighting + dir light enough).

Post chain: RenderPass → UnrealBloomPass → OutputPass.

Let me write the file now. It'll be long (~1100-1300 lines). I'll write it in one go with the write tool, then extract physics core for node tests.

Key detail — physics core must be THREE-free. Geometry building uses THREE (that's outside core markers). The SoftBody class constructor takes plain arrays {positions, indices} → builds particles/edges/faces. World/step functions operate on plain data. Collision obstacles as plain objects.

Let me define the physics core API:

World step:

Grab applied in collide phase as positional constraint.

Now pressure acceleration detail: per substep, compute V; if V < 1e-6 skip. P = kp * (q/V - q/V0). kp tuned: let's think of magnitude. Sphere r=1.6 → V0 = 17.16. If compressed to 0.9V0 → ΔP = kpq(1/(0.9V0) - 1/V0) = kpq/V0 * (0.111/0.9)≈ kpq0.0072 per unit... force on face vertex = P * n/6 where |n| = 2area. Face area for r=1.6 sphere with 5120 faces: total area 4πr²=32.2 → face ≈ 0.0063. So per-vertex force ≈ P0.0063/63 (each vertex ~6 faces) ≈ P0.019. Acceleration = Fh²... with h=dt/8=0.00208, h²=4.3e-6. So to get noticeable restoring, kp must be large: we want ΔP such that pressure accel can counter gravity over... Let's just make kp a tunable constant, e.g., kp = 9000, then ΔP at 10% compression = 9000*(1/0.9-1)q/V0 = 90000.1111/17.16 ≈ 58. Per-vertex force ≈ 580.019 ≈ 1.1 → accel pos delta = 1.14.3e-6 per substep... tiny?? Wait — applying force as Δpos = Fh² per substep, over 8 substeps/frame ≈ Fdt² = 1.12.7e-4 = 3e-4 per frame; over 1 second (60fps) ≈ 0.018... too weak vs gravity 9.8dt per frame velocity buildup... Hmm units: acceleration a=F/m; Δv = at. Gravity 9.8. Pressure force 1.1 → a=1.1 (m=1). To resist gravity need F≈9.8 per vertex. So kp should be ~ 80000? Let's compute required: F_vertex ≈ kp * (ΔV/V)/V0 * faceAreaTotalPerVertex... per-vertex force from ΔP: F_v = ΔP * (Σ|n|/6 over incident faces) ≈ ΔP * (6 faces * 20.0063)/6 = ΔP0.0126. Need ~9.8 → ΔP ≈ 780 at 10% compression → ΔP = kp0.111/V0 = kp0.00647 → kp ≈ 120,000. OK so kp ~ 1e5. These numbers are fine as constants; I'll set kp default 1.2e5 and expose nothing (stability clamps). Hmm but mass=1 per particle of 2562 — blob total mass 2562, pressure per-vertex... whatever, tuning by simulation test in node: I'll run the test to check equilibrium sag and stability, adjust kp.

Actually rather than precise derivation, node test will verify: simulate 3s; check no NaN; volume stays within [0.5, 2.5]×V0; height above floor. I can tweak kp if test shows collapse. Since I can run node quickly, iterate once or twice.

Clamp ΔP to [-0.5, 3]×(kp*q/V0) to avoid explosion.

Also add small global velocity damping and sleep? Not needed.

Volume via signed tetrahedron sum with cross products — standard.

Normals for rendering: accumulate cross per face to vertex normals then normalize.

Inter-body repulsion: r_p per body = 0.5avgEdge1.1. Compute per body once.

Spatial hash: Map<int, indices[]> with cell = floor(x/cs)+... key = (ix73856093 ^ iy19349663 ^ iz83492791) — use Map with string or computed key via ((ix92837111)^(iy689287499)^(iz283923481))|0 collisions fine. Simpler: key = ix + iy512 + iz262144 with offset — coordinate range ±30 → fits with offset 256. Use Map<number, number[]> rebuilt each frame; n up to 6k — OK.

Grab mechanics: on grab, store body & particle index & grabTarget (Vector3) & grabDepth plane. Per substep: move particle toward target: delta = target - pos; pos += delta * min(1, grabStiff) where grabStiff=0.35; prev adjust so velocity follows (prev = pos - delta0.9h?) Simpler: after moving pos, set prev = pos - (pos - prev)*0.5 to limit velocity gain, preventing explosion; throw still works since velocity builds while dragging.

Raycast to select particle: intersect mesh → face → pick nearest of face's 3 vertices (welded indices map). Good.

Now UI layout:

• top-left: title + subtitle.
• top-right: stats (FPS, bodies, particles).
• left panel (glass): sliders: Gravity (0–2, default 1), Inflation (0.5–1.6, default 1.0), Stiffness (0.1–1, default 0.85), Damping (0–0.5, default 0.06)? and buttons: Spawn Blob, Pop (X), Presets row: Bounce House / Zero-G / Floppy / Reset; toggles: Jelly Shader ✓, Bloom ✓, Slow-mo, Sound ✓, Wireframe.
• bottom center hint bar.

Mobile: panel collapsible? Keep simple: panel scrollable, small.

Colors: pastel palette: ['#ff8fb3','#ffd166','#8ef6d2','#8ecfff','#c9a7ff','#ffb37e'].

Now the code. Let me carefully write it.

Also remember to update /space/demos/index.html — insert new card after the comment <!-- New demos are added here -->.

Let me check how many lines the demo will be — aim ~1200. Write in chunks to avoid tool limits? The write tool can handle large content; previous run wrote 1562 lines at once apparently. I'll write in ~3 chunks using write then append via shell cat >> heredoc? Heredoc in execute_shell_command with quotes is risky with backticks/$ in JS template literals. Use read_write_file_in_shell_environment write mode for full content — but very large content in one call might be OK (the 040 demo was written that way per summary). I'll split into 2-3 write/append operations using the file write tool with mode: write for first chunk then... the write tool doesn't support append; line-range write could break. Safer: write full file in one call. If too large, split via heredoc with quoted delimiter 'EOF' to avoid expansion, and avoid backticks in JS (use string concat instead of template literals where risky — heredoc quoted 'EOF' doesn't expand backticks or $, so safe). Actually quoted heredoc delimiter prevents all expansion — safe for $ and backticks. But the command string itself passes through the tool as JSON — fine.

Plan: write file via 3 shell heredocs (cat > part1, cat >> part2, cat >> part3) to keep each call moderate. Or single write tool call with entire content — likely fine (large but acceptable). I'll do heredocs in 3 parts for safety.

Let me draft the full code mentally section by section, then write.

HTML/CSS

Standard: <div id="ui"> panels. Canvas container #scene. Import map.

JS module outline

PHYSICS CORE (marker)

Vec helpers inline within loops (avoid allocation).

SoftBody class:

Center particle: index n (extra). Recompute its rest position = centroid. Spokes from center to each vertex with rest = distance * 0.999? Center constraints help volume preservation. But 2562 extra constraints ×16 solves = 41k more — fine. Hmm — center spokes with stiff compliance may fight pressure inflation. Set spoke compliance higher (softer): compliance_spoke = 1e-6? Actually spokes mainly prevent collapse; pressure pushes out; compromise: spokes soft-ish. Or drop center entirely — pressure blobs are usually stable. I'll include center with moderate stiffness and test; if the node test shows weird volume oscillation I can soften. Keep it.

Wait, one risk: center particle in inter-body repulsion — exclude (mark body.hasCenter, repulsion loops over first n particles).

Integrate:

Wait invMass multiplies force but gravity shouldn't be scaled by invMass (it's accel). Since invMass=1 for all regular particles, just apply uniformly; center same. Fine: pos.y += vy + (acc.y - g)hh.

computePressureAcc:

Note: per-face vertex force = P * n / 6 (since face force = Parean̂ = Pn/2, divided by 3 verts → Pn/6). Yes.

Solve edges XPBD:

Iterate edges iters times per substep. XPBD with lambda accumulation is more correct but simple PBD-style per-substep with small compliance works (Small Steps paper). I'll do plain PBD projection when compliance=0; with compliance use XPBD formula with per-substep lambda reset — implement XPBD properly with lambda array per edge? Simplify: use "effective stiffness" approach: correction *= k where k = 1 - compliance... Many demos just use k factor (stiffness 0..1) applied per iteration: delta = 0.5k. Stable and intuitive. I'll do that: stiffness s∈(0,1], correction = C * s split by masses. With 8 substeps × 2 iters, s=0.9 ≈ rigid. Slider maps 0.1..1 → s.

Collisions per body per substep:

• floor: y0=floorY+rp. if pos.y < y0: pos.y=y0; friction: prev.x += (pos.x-prev.x)mu? Let me do: vx=pos.x-prev.x etc; prev.x = pos.x - vxfricKeep where fricKeep=0.75 on contact; vy restitution: prev.y = pos.y + (pos.y-prev.y)rest (rest≈0.25) → gives bounce. Careful: vy = pos.y - prev.y (negative when falling). After clamp pos.y=y0, set prev.y = pos.y + vyrest → next v.y = -vy*rest (upward). Good.
• walls cylinder radius R: rr = pos.x²+pos.z²; maxR = R - rp; if rr > maxR²: scale back: f=maxR/√rr; pos.x*=f; pos.z*=f; damp radial velocity: prev similar reflect? Just positional clamp + slight velocity damp (prev = pos - v*0.5).
• sphere obstacles: dist to center < orad+rp → push out along n; reflect normal velocity with restitution 0.3.
• bar: as designed. Compute contact world normal; relative velocity: vrel = v_p - v_surf; if vrel·n < 0 → pos -= n*(vrel·n)h(1+rest)? Since we're position based, adjust prev: prev += n*(vrel·n)h(1+rest) making v_p' = v_p - n*(vrel·n)(1+rest). v_surf = ω × r = (-ωrz, 0, ω*rx) where r = contact - barCenter(axis through Y). Bar rotates about its center about Y axis.

Bar local transform: θ = angle; local x axis = (cosθ, 0, sinθ)? Rotate point to local: dx=p-c; lx = dx·axisX, lz = dx·axisZ, ly=dy. axisX=(cosθ,0,-sinθ)?? Let bar mesh rotate about Y: mesh.rotation.y = θ. Its local X axis in world = (cosθ, 0, -sinθ). Local Z = (sinθ, 0, cosθ). So lx = dxcosθ - dzsinθ... wait dx·axisX = dxcosθ + dz(-sinθ). lz = dxsinθ + dzcosθ. ly = dy. Clamp lx to [-L, L] → closest local point (lxC, 0, 0). dvec local = (lx-lxC, ly, lz); dist; if dist < barR+rp → push along world normal: nLocal = dvec/dist; nWorld = nLocal.xaxisX + nLocal.yY + nLocal.zaxisZ. push = barR+rp - dist; pos += nWorldpush. Contact velocity: v_surf = ω × r_contact, r_contact = contactWorld - c. For rotation about Y: v = ω × r = (ω ẑ?) ω=(0,ω,0); ω×r = (ωr.z? no: (0,ω,0)×(rx,ry,rz) = (ωrz*? compute: (ωyrz - ωzry, ωzrx - ωxrz, ωxry - ωyrx) = (ωrz, 0, -ωrx). So v_surf=(ωrz, 0, -ωrx) with r from bar center. OK.

• grabbed particle: pos += (target-pos)grabK (grabK≈0.4); prev = pos - v0.3 (tame). Actually keep prev unchanged mostly so velocity follows drag; maybe damp: prev += (target-pos)grabK0.9 (prev follows too, less velocity). Compromise: move both pos and prev toward target: pos += d0.5; prev += d0.35 → velocity = 0.15d/h per step — reasonable throw.

Inter-body repulsion once per frame:

To keep O(n) sane: for each particle iterate 27 cells — 6k×27 cell lookups with Map gets — 162k Map lookups/frame — might be ~2-4ms. Optimize: iterate cells and pairs within cell + half neighbor cells (13 forward) — standard. Or only check 14 cells (self + 13 forward) to avoid double-processing pairs. I'll do that.

Cell size = max rp2 among bodies ≈ depends. Compute per frame: cs = 2maxRp.

Key: ((x/cs)|0 + 512) + ((y/cs)|0 + 512)*2048 + ((z/cs)|0 + 512)*4194304 → number key in Map storing arrays.

Also pop burst particles (visual only, in three section).

THREE section

• init renderer (antialias, ACESFilmic, sRGB), shadows PCFSoft.

• scene, fog, camera (fov 45, pos (9,6,11)).

• PMREM RoomEnvironment → scene.environment.

• Lights: dir light (castShadow, 2048 map, tuned frustum), ambient/hemi.

• Ground: CircleGeometry r=30 with canvas texture (dark + grid + radial fade), receiveShadow; plus big soft disc glow? fine.

• Obstacles meshes: two spheres (MeshStandardMaterial, castShadow), bar capsule mesh (castShadow).

• Soft body mesh creation: BufferGeometry with position attr (from body.pos, but excluding center particle → geometry uses first n particles and faces index), normal attr computed manually, MeshPhysicalMaterial. castShadow true, receiveShadow false.

• weldGeometry function (THREE-based): input BufferGeometry (non-indexed or indexed), output {positions: Float32Array(unique), indices: Uint32Array}. Implementation: get pos attr; if index, iterate faces via index; weld by key from rounded coords (1e-4). Build map from key→newIndex; build indices array of welded face triples (need original face topology: iterate triplets of index (or sequential if non-indexed)).

• Shape builders: makeSphere (IcosahedronGeometry(1.6,4)), makeCapsule (CapsuleGeometry(0.85,1.3,6,14)), makeDonut (TorusGeometry(1.05,0.5,12,26)). All → weld → body params. Scale? default sizes fine.

• syncMesh(body): copy pos (first n) into geometry attr, compute normals (reuse face loop: zero normal array, accumulate cross, normalize), position.needsUpdate, computeBoundingSphere for raycast? Raycast uses boundingSphere — update it (computeBoundingSphere each frame — 2562 pts OK; or set large static boundingSphere radius manually to skip: geometry.boundingSphere = new THREE.Sphere(center from body centroid, radius dynamic)). Compute centroid + max dist — needed anyway? Centroid useful for HUD? Just compute bounding sphere properly each frame: centroid then max radius — cheap.

• Pop burst: class Burst {points, velocities, life}; update in loop.

• Audio engine: as designed.

• UI wiring: sliders → params; spawn; pop; presets; toggles.

• Pointer: raycaster on pointerdown (only if event on canvas): intersect body meshes; if hit → grab nearest vertex; else orbit (controls enabled default). Actually disable controls only when grabbing: controls.enabled = !grabbing. Hover: pointermove throttled (every ~50ms) raycast for hover highlight (emissive boost) — set activeBody.

• Keyboard: X pop active, B spawn, R reset.

• Loop: fixed dt accumulate? Use clock.getDelta clamped to 1/30, timeScale, stepWorld, sync meshes, update bursts, bar angle, HUD stats.

Bloom toggle: composer vs plain render. Jelly toggle: switch material params.

Stats update every 0.25s.

Edge case: when body popped while grabbed → clear grab.

Spawn: limit bodies to 9; if exceeding, pop oldest spawned.

Presets:

• Bounce House: gravity=1, inflation=1.15, stiffness=0.9, damping=0.08; ensure 3 blobs.
• Zero-G: gravity=0.03.
• Floppy: stiffness=0.35, inflation=0.95.
• Reset: remove spawned, reset hero pos/vel, defaults.

Reset hero: restore pos to initial layout (store initial pos array).

Now — audio details:

Impact detection: in collide floor/obstacle, accumulate impact = max(|vn|) per body per frame → after frame, if impact > 2 → thump(min(1,impact/12), pitch by body size). Collect via body.impact accumulator reset each frame. Physics core: when resolving floor contact, compute vn = (pos.y-prev.y)/h before clamp; if |vn|>1.5 record body.impact = max. Since audio in three section reads body.impact after step. Physics core stays pure (just records numbers).

Now the POP of a pressurized blob: burst particles from surface positions outward — fine.

HUD FPS: EMA of dt.

Alright — also <meta viewport>, prevent context menu on canvas (right-drag orbit pan OK but menu annoying → preventDefault contextmenu).

Camera controls: OrbitControls with damping, minDistance 4, maxDistance 40, maxPolarAngle slightly below horizon? allow under-floor? keep maxPolarAngle = 1.45π/2? Just Math.PI*0.495.

Title: "Squish Lab". Demo folder 041-squish-lab.

Description for index: "A soft-body playground — grab, stretch, fling, inflate and pop jiggly pressure-simulated blobs; XPBD spring lattice + ideal-gas pressure, floor and obstacle collisions, a spinning paddle, inter-blob bumping, bloom-lit jelly materials, generative squish audio and four presets."

Now write the code. I'll be careful about correctness since only node tests will validate.

Let me write out significant functions fully now in my head as I type the file. I'll write the file in 3 chunks via heredocs.

Chunk 1: HTML head, CSS, body UI, import map, start of module: imports, config, audio, physics core start. Chunk 2: physics core rest + three setup + builders. Chunk 3: interaction, UI, loop, end tags.

Let me be careful with heredoc: use cat > file <<'HTMLEOF1' etc. Content includes backticks and ${} template literals — quoted heredoc keeps them literal.

Physics constants:

• GRAV default 9.8 * gravityScale.
• kPress: let me compute better. Per-vertex mass 1, hero 2562 verts, weight total = 25629.8 ≈ 25k... pressure force total on surface = P * area_total = P32.2. To hold against gravity need P32.2 ≈ 25k → P ≈ 780 at equilibrium compression ~few %. P = kpgas*(1/V - 1/V0) ≈ kpgas(ΔV/V²) for small ΔV... ΔV/V=2% → (1/V - 1/V0) = (V0-V)/(VV0) ≈ 0.02/V01.02 ≈ 0.00117/1.716e1? For r=1.6: V0=17.16 → 0.02/17.16 = 0.001166. So kp = 780/0.001166 ≈ 670k. Hmm large. But per-substep application integrates fine regardless. Let me set kp = 7e5 default, expose clamp Pmax = kpgas/V0 (huge → clamp at 3× ≈ 120k — force = Parea per face vertex... stability: per substep Δpos = Fh². Max F per vertex ≈ Pmax3? P0.01263≈ 1200000.038 ≈ 4500... wait that's huge: Δpos per substep = 4500*4.3e-6 = 0.019 — OK bounded. Fine, constraints clean up.

Hmm wait, actually gravity for verlet: a = g ≈ 9.8 → per substep Δpos from gravity = 9.8*4.3e-6 = 4.2e-5. Pressure at equilibrium must produce similar per-vertex force ≈ 9.8 → consistent with above (need P≈780).

Numbers fine. I'll set kPress = 700000 and verify via node test (rest height ≈ should rest on floor, slightly squashed, not bouncing forever). If unstable (oscillation), add pressure damping or lower kp. Node test will tell (track volume variance & max |v|).

Also initial drop: hero spawns at y=3.4 center, radius 1.6 → falls 1.8 → impact. Restitution 0.25 → bouncy but settles. Damping 0.06/... global damping factor per second: dampF=1-damping*h with damping≈2.0? Verlet damping: v = (1-dampingh); for damping=2 → per second v *= exp(-2) ≈ 0.135 — strong. Let damping default 1.2, slider 0..4 labeled "Bounciness"? Label "Damping". OK.

Restitution floor 0.3. Walls 0.4.

Bar: ω=0.9, L=3.2, r=0.4, center (0, 0.9, 5.0)? Also maybe second small static sphere obstacles at (-4.5,1.1,-2.5) r1.3 and (4.8,0.9,-1.8) r1.1. Floor y=0.

Walls R=13.

Hero: icosphere r=1.7 at (0,3.6,-1).

For inter-body rp: sphere rp = avgEdge0.55 ≈ (edge len for r=1.7 detail4: circumference-based ~ 0.13) → rp≈0.07. Small — blobs will visually overlap before repelling? No: surface particles repel at 2rp=0.14 — surfaces nearly touch visually. Good.

Torus pressure: torus self-intersect? fine.

Capsule spawn: capsule axis vertical (Y). CapsuleGeometry axis is Y. Good.

Donut axis: TorusGeometry lies in XY plane, axis Z — rotate geometry to lie flat (rotateX 90°) so axis Y. Fine.

Inflation slider scales body.gas for ALL bodies (relative to their creation gas). Store body.baseGas; gas = baseGas * inflation.

Stiffness slider → body.stiff all.

Now grabbed particle + XPBD: grab sets pos directly — with mass 1 fine.

Raycast on 5120-tri mesh on pointerdown only — fine.

Normals: manual function:

Position attr copy: for i<n*3: attr.array[i]=body.pos[i].

boundingSphere: compute centroid of first n; max d². set geometry.boundingSphere.center/radius. Note geometry in local space = world space (mesh at origin, no transform). Meshes stay at identity; particles hold world coords. Raycaster works (identity matrix).

Shadow: dir light from (8,14,6), shadow camera box ±16, mapSize 2048, bias -0.0005.

Ground canvas texture: 1024², bg #0c0e14, grid lines rgba(120,140,255,0.07) every 64px, radial highlight center rgba(80,100,180,0.15) fade. Also add vignette ring? fine.

Environment intensity: scene.environmentIntensity? r170 supports scene.environmentIntensity? environmentIntensity added r163? It's fine to set materials' envMapIntensity instead (0.7).

Background: color #07080d + subtle star field? Add 400 point stars sphere shell for depth — cheap & pretty. Yes: Points with additive, size 1.2, opacity 0.6, radius 60-90 shell. Nice.

Bloom: strength 0.5, radius 0.6, threshold 0.75.

Composer toggling: useBloom flag.

Jelly material base params:

Non-jelly: transmission 0, thickness 0, transparent true opacity 0.92, roughness 0.28.

Toggle function updates all body materials + needsUpdate.

Hover highlight: material.emissiveIntensity 0.06→0.35 lerp? simple set.

Grabbed visual: small glowing sphere marker at grab point (Mesh basic additive) shown while grabbing. Nice.

OK also FPS meter and particle count.

One more consideration: OrbitControls import path & grabbing interplay: pointerdown listener on renderer.domElement with capture=true, run raycast before controls? OrbitControls listens on same element; if we disable controls in our pointerdown (capture phase runs first if registered with {capture:true} and controls registered without capture — capture listeners on same target run in registration order? For same target, capture flag doesn't reorder vs bubble on the target itself (target phase: listeners fire in registration order regardless of capture). Hmm. Simpler: register our pointerdown before creating OrbitControls? Controls check this.enabled at event time; if our listener runs first and sets enabled=false, controls ignore. Registration order: if we addEventListener before new OrbitControls, ours runs first. I'll do that: add pointerdown after creating controls but use capture:true on window? Events: pointerdown on canvas → our listener (target phase, registered earlier if we register before controls creation). To guarantee order, create controls AFTER registering our handler? Our handler references controls... set a flag wantGrab in early handler, then in same handler controls may already...

Cleaner: raycast on pointerdown via listener registered with {capture: true} on renderer.domElement BEFORE controls instantiation... but we need controls variable — declare let controls; assign later; handler checks if (grabbed) controls.enabled=false — but controls processes the same event right after (its own listener, non-capture, registered later): same-target listeners fire in registration order → ours (registered before new OrbitControls) fires first → sets controls.enabled=false → controls' handler checks enabled → returns.

So: create renderer, camera; addEventListener('pointerdown', onPointerDown) (no capture needed); then controls = new OrbitControls(...). In onPointerDown, if raycast hits: set controls.enabled=false (controls var assigned by the time user clicks).

pointermove/up on window for drag end.

Right-click drag: OrbitControls pans — fine; contextmenu preventDefault.

Also disable controls' rotate while grabbing only.

Touch: same logic works with touch pointers.

Now write everything. Also markers for physics extraction test:

// ====== PHYSICS CORE BEGIN ====== ... // ====== PHYSICS CORE END ======

Test file plan (/tmp/squish/test.mjs):

• import core from './core.mjs' (extracted + export line appended).
• build a test sphere: implement tiny icosphere generator in pure JS? Simpler: generate UV sphere in test (lat/lon grid with welded poles) — write quick function makeUVSphere(r, w, h): positions + indices; weld not needed if constructed indexed. Use w=24,h=18.
• Tests:
  1. Volume of sphere ≈ 4/3πr³ within 2%.
  2. Edge build count = expected (manifold edges = 3F/2).
  3. Free fall: drop body, after 0.5s center Y decreased; no NaN.
  4. Floor rest: simulate 4s; all particle y >= -1e-3; kinetic energy small (< threshold); volume within [0.75, 1.3]×V0 (squashed but not collapsed, not exploded).
  5. Inflation: set gas=1.3 → after 2s volume > 1.05×V0.
  6. Squash recovery: apply downward impulse, volume min > 0.5 V0, returns to [0.8,1.2].
  7. Inter-body: two overlapping blobs separate (distance between centroids increases).
  8. No NaN after 2000 steps with bar collision (place body at bar).
• node --check on module extracted from HTML.

Let me write the code now. I'll aim for clean, commented code.

One subtlety for the physics core running both in browser & node: no THREE references inside markers. The bar obstacle and everything plain. SoftBody takes {positions (Float32Array), indices (Uint32Array)}.

Data for obstacles: world.obstacles = { spheres: [{x,y,z,r}], bar: {x,y,z,halfLen,radius,omega,theta} }.

stepWorld(world, dt):

Wait — integrate computes pressure acc which needs volume — per substep ok.

Order: integrate (modifies pos) → constraints → collisions. Multiple iterations of constraints inside solveConstraints (2).

Grab: world.grab = {body, index, tx,ty,tz, k} applied in collideBody.

collideBody returns nothing; sets body.impact = max(body.impact, impactThisSubstep).

Impact detection detail (floor): before clamp, vy = (pos.y - prev.y)/h; if pos.y < y0 && vy < -1.2 → impact = max(impact, -vy).

Restitution: after pos.y=y0: prev.y = pos.y + (pos.y - prev.y)rest → wait vy=pos-prev; to invert with rest: new vy' = -vyrest → prev.y = pos.y + vy*rest? vy negative (falling): pos.y - prev.y = -3 → prev.y = pos.y + (-3)0.3 = pos.y - 0.9 → new vy = +0.9 upward. Yes: prev.y = pos.y + vyrest where vy = old (pos.y-prev.y) computed BEFORE clamping pos (use pre-clamp values: vy measured pre-clamp; but if particle penetrated deep, vy from prev still fine).

Friction on contact: vx,vz = (1 - mu) with mu=0.25: prev.x = pos.x - vx0.75.

Sphere obstacle similar with normal.

Bar: compute v relative normal; if approaching, reflect with rest 0.4 + add surface velocity: vn_new = vn_surf + (vn_surf - vn)rest → prev adjust: prev -= n(vn_new - vn)h? v = (pos-prev)/h → prev = pos - vh. To set normal comp to vn_new: delta_v = (vn_new - vn)n; prev -= delta_vh.

Also positional push-out.

Grabbed particle skip collisions? Apply collisions anyway (clamp), then grab constraint after.

interBody hash: exclude center particles (body.centerIndex). Use arrays: build list of {b, i}. Implementation with Map<number, Int32Array-like arrays> — use plain JS arrays in Map.

Repulsion: dist < r1+r2+margin(0.0) → overlap → push each half along delta; also small velocity smoothing skip.

Bodies count small → could O(n*m) centroid-based repulsion? Nah, surface detail matters; hash it is. Particles up to ~6k → fine.

Now compute rp per body: avg edge length * 0.6.

Also grab raycast needs world-space mesh — identity.

Alright, also the "pop" requires removing mesh & disposing, spawn burst.

Burst implementation:

Presets detail: applying preset sets slider DOM values too.

Now, write files. Also favicon? skip.

Also index.html in demos root: insert card:

Insert right after <!-- New demos are added here --> line (matching previous pattern where 040 card directly follows the comment).

Let me confirm format of the 040 card quickly before writing... The grep showed the comment then immediately <a class="demo-card" href="./040-lumen-lab/index.html">. I'll match indentation (12 spaces for comment, cards similar). Let me view that snippet when editing.

Now let me carefully compose the entire index.html. I'll write it in 3 heredoc chunks. Let me draft the JS fully and consistently. I need to be careful with variable naming consistency.

Full code draft:

CSS: dark, glass panels, accent #7ef0d4 (mint) or pink #ff8fb3. Accent gradient title. Range styling. Buttons.

Panels:

• .panel-top-left: h1 "Squish Lab", p subtitle.
• .panel-controls (left, below title): sections.
• .panel-stats (top-right).
• .hint (bottom-center).

Let me write out the module JS in full now — I'll be methodical.

Hmm: centroid of torus is inside hole — center particle there still works (spokes to ring). OK.

Volume with center added — faces only reference first n. Fine.

Hmm — world.gravity is scale; actual accel = 9.8scale. Pass gAcc = world.gravity (already in m/s²) — set world.gravity = 9.8scale at UI level. Keep world.gravity in m/s² (default 9.8).

Note invMass all 1 — skip multiply.

Constraints:

Wait spoke correction sign: dx from center to i. If d > rest, need to move them together: center moves toward i (+= dxdiff), i moves toward center (-= dxdiff). Yes as written.

Collisions:

Careful: vy = (y - prevY)/h (negative when falling). After clamp y=floorY. Want new vy' = -vyrest → prevY_new = y - vy'h = y + vyresth. Since vy computed with invH: prev[i3+1] = floorY + vyresth. Let me store vyRaw = (y - prev[i3+1]) (per-step displacement) to avoid invH: vyRaw negative. prevNew = floorY + vyRawrest. Impact magnitude = -vyRawinvH. Good, use raw displacements.

Friction: vxRaw = x - prevX; prevX = x - vxRawfricKeep → next displacement = vxRawfricKeep. Good.

Since v = pos - prev; to change v by Δv, prev -= Δv. Reflecting radial: vr_new = -vrwallRest → Δvr = -vr(1+wallRest) → prev -= Δvrn = prev + vr(1+wallRest)*n. Yes as written.

Wait after updating pos x,y,z changed — recompute vx with updated x: v = pos - prev uses updated pos — that includes push — slight energy add, negligible. OK.

**************************************************************************************************************************************************er.

***************************************************************************************************************************ps.

The exit code 1 was from grep -c returning 0 matches (grep exits 1 when no match) — that's expected, file is 1149 lines and contains no heredoc markers. Good.

Now there's one bug I need to check: in the file, I used pointer and raycaster in onPointerDown before they are declared with const later (raycaster/pointer declared after controls). onPointerDown is an arrow function assigned to const — it's only called on user events, by which time the consts are initialized. TDZ only matters at call time, not definition time. Fine.

But wait — onPointerDown references controls — declared after the addEventListener but before any event can fire. Fine.

grabMarker referenced in onPointerDown — declared after controls. Events fire after script finishes. Fine.

Also activeBody declared with let in interaction section but used earlier in removeBody (defined earlier in chunk 2) — function hoisting: removeBody is a function declaration referencing activeBody... activeBody is declared with let later — TDZ issue only if removeBody is called before declaration executes. spawnHero→spawnBody→... spawnBody doesn't call removeBody. spawnRandom calls removeBody — only from UI. OK. Same with grab variable referenced in removeBody — declared in interaction section before any call. OK.

params used in spawnBody (params.stiffness) — params declared in UI section, spawnBody called first time in spawnHero at "INITIAL SCENE" which is after UI section. OK.

Now extract physics core and test in node.

Core syntax OK. Now write the test file with a UV-sphere generator and run physics tests.

21 passed, 5 failed. Let me analyze:

1. rest volume — test 2: restVolume0 vs 4/3π·3.375 for r=1.5: 4/3π(1.5³)=4/3π·3.375≈14.137. Hmm the check Math.abs(b.restVolume0 - 14.137) < 0.3 failed. But test 1 volume passed for unit sphere within 2%. For r=1.5 sphere with W=16,H=12 (coarser), the polyhedron volume is notably less than smooth sphere? With H=12, W=16: inscribed volume maybe ~5-8% less → 14.137*0.93 = 13.15, diff 0.99 > 0.3. Probably test tolerance issue. Let me verify by computing volume directly. Likely the mesh is just coarse. I'll relax the test to use relative error < 5%.

2. rest volume ratio 0.26 — the resting blob collapses to 26% of rest volume! Pressure too weak vs gravity with mass=1 per particle and r=1.5 sphere. My earlier estimate said kp≈700k... but I forgot: total mass = n particles. For W=20,H=15 sphere: n = 2 + 1920 = 382 particles... wait H=15 → rows j=1..14 → 2+1420=282 particles, mass 282, weight 2764. Surface area 4π·2.25=28.3. Needed P ≈ 2764/28.3 ≈ 98. Hmm that's less than my 780 estimate (which was for 2562 particles!). For hero blob 2562 particles weight = 25100, area 4π(2.2)² (r=1.25*1.3=1.625... hero scale 1.3×1.25=1.625 r; area 33.2) → P ≈ 756. With kp=700000: P at compression δ=V0-V: P = kp·(1/V − 1/V0) ≈ kp·δ/V0². V0 = 4/3π(1.625)³ ≈ 17.97. For the test sphere r=1.5, V0≈14.1 (mesh ~13.3). P = 700000·δ/(13.3²) = 3955·δ. Need 98 → δ ≈ 0.025?? That suggests kp fine... but observed collapse to 0.26 V0 means pressure is NOT balancing gravity. Something else is wrong.

Wait — the bug: in integrateBody, pressure force applied per substep with Δpos = F·h² where h = dt/8. Per frame total Δ ≈ 8·F·(dt/8)² = F·dt²/8. Gravity Δ same form, so equilibrium when F_pressure per vertex ≈ g = 9.8. Fine.

Hmm, but the pressure force per vertex: F = P·n/6 accumulated over ~6 faces → P·(6·|n|)/6 = P·|n| where |n| = 2·faceArea. Face area for 282-vert sphere: 28.3/560 faces ≈ 0.05. So F_v ≈ P·0.1. Need F_v ≈ 9.8 → P ≈ 98. OK matches.

So why collapse? Let me reconsider: solveConstraints — edges contract toward rest length. If the blob is compressed by gravity (squashed), edges at the bottom are compressed... the springs resist compression. But nothing here restores volume except pressure. Pressure P should push out...

Wait, actually, I bet the issue is the pressure clamp: P = kp·(1/V − 1/V0). At V=0.26V0: P = 700000·(1/(0.26·13.3) − 1/13.3) = 700000·(0.289 − 0.0752)·... wait 1/(0.26·13.3)=1/3.46=0.289; 1/13.3=0.0752 → P = 700000·0.214 = 150k. Clamped to 3·P0 = 3·700000/13.3 = 158k. So P=150k → F_v = 15k?! That's enormous vs gravity 9.8. The blob should EXPLODE outward, not stay collapsed. Unless... something else counteracts: the edge springs! When the blob inflates beyond rest shape, edges are stretched and pull back with stiffness — at s=0.9, iterations 2, substeps 8 → very stiff springs. Springs at rest length L0 resist expansion: to inflate the blob back from 0.26V0, edges must stretch beyond L0, and stiff springs prevent that. So the blob finds equilibrium where spring compression... no wait — if volume is BELOW rest, then edges are compressed below L0 → springs push OUTWARD too, plus pressure pushes out with 15k force... nothing should hold it at 0.26 V0!

Unless the mesh INVERTED — normals flipped → volume computed negative?... computeVolume would be negative → V>1e-6 check fails → no pressure at all! And if the sphere inverts (turns inside out), pressure sign flips. Collapse to 26% + residual motion high suggests chaos/inversion.

Hmm wait, but actually let me reconsider the edge solve: diff = (d - rest)/d * 0.5 * s. If d > rest (stretched), diff > 0: pos[a] += dx*diff (a moves toward b) → d decreases. Correct. If compressed, pushes apart. Correct.

Spokes similar. OK.

Let me think about the pressure integration magnitude more carefully — per substep, Δpos = F·h². h = (1/60)/8 = 0.00208, h² = 4.34e-6. F_v = P·0.1. For P=98: F_v=9.8, Δpos per substep = 4.25e-5. Per frame: 8 substeps → but Verlet: velocity accumulates: it's correct physics — acceleration a = F/m = 9.8 m/s². OK.

BUT WAIT — the clamp uses P0 = kPress/restVolume. At rest V=V0 → P=0. Hmm fine.

Let me think about the drop test more concretely — maybe the blob bounces hard, inverts on impact, then pressure sign... Let me just debug numerically: run the settle test and print V over time, plus check for inversion (signed volume negative at some point).

Also failure 4: rests on floor at sane height (3.31) — centroid at 3.31 after 5.5s of sim: it never came down?! It fell (first 30 steps showed y<4) then... it went back UP? Centroid 3.31 means it's floating/bouncing high. With huge pressure forces and clamp 3P0... Actually if it inverts (volume sign flips negative), pressure = 0, then it should just fall like a springy bag. Centroid 3.31 — maybe it's bouncing violently (residual motion 0.15 per-substep displacement = 0.15/0.002 = 75 m/s?!) — residual motion 0.15 per substep → velocity = 0.15·480 = 72 m/s. It's EXPLODING/jittering with huge energy. So pressure forces are injecting energy like crazy. Something wrong.

Ah — I think I see it: the pressure force is computed once per substep from the volume at substep start, then applied as acceleration. Fine. But the real issue could be the clamp allowing enormous P at small V → 150k·0.1 = 15k m/s² acceleration → Δv per substep = 15k·0.00208 = 31 m/s per substep. Wild oscillation between spring constraints → energy pumping → explosion. The oscillation: overcompressed (V small) → huge outward force → overshoot → V > V0 → P negative (pull in) down to -0.6P0 clamp = -105k·0.1 = -10.5k → huge inward → oscillation divergence. Classic explicit-integration instability with stiff pressure.

Fix options:

1. Apply pressure force more softly: scale kp down and rely on smaller per-substep steps. The stability condition: pressure "stiffness" in position terms: at equilibrium, dF/dV ≈ kp·|n_v|/V0² ... To keep stable, need per-substep position response << overlap: F·h² should be small relative to particle spacing. F_v·h² ≈ P·0.1·4.34e-6. For P=3P0=158k → Δ=0.069 per substep vs particle spacing ~0.2 (r=1.5, 282 verts → spacing ~0.19). 0.069 is 35% of spacing per substep — too big, oscillation.

Better: scale pressure so that even at clamp, Δpos per substep ≤ ~5% spacing. → F_v·h² ≤ 0.01 → F_v ≤ 2300 → P·0.1 ≤ 2300 → P ≤ 23k. And equilibrium needs P ≈ 100–800 depending on particle count. So set clamp Pmax smaller and lower kp... but kp also needs to give enough restoring force vs gravity: P_eq for hero ≈ 760.

Approach: make pressure mass-aware: compute total weight W = g·count. Set target equilibrium pressure Peq = W / surfaceArea... too fancy. Alternative: normalize pressure force by particle count — force per vertex should scale with g (per-particle weight = g·1). The needed P scales with n/area. Since kp·(1/V−1/V0) is geometry dependent, simplest robust fix: tune kp so physics stable, and reduce clamp, and add pressure damping (use relative volume velocity to damp).

Practical approach used by many: apply pressure as a positional constraint rather than force: i.e., a global volume constraint solved in the constraint loop: ΔV = V − V0target; move each vertex along its "volume gradient" (sum of face normals) by λ·(V0−V)/Σ|∇V|² — this is exactly PBD volume conservation (like "balloon" constraint from Müller's PBD papers — overpressure factor k). That's unconditionally stable! Volume constraint: C = V − k·V0 (k = overpressure). Gradient ∇_i V = (1/6)Σ_{faces incident} (p_j × p_k)... For each vertex, accumulate n_face/6? Let me derive: V = Σ_f (p_a·(p_b×p_c))/6. ∂V/∂p_a = (p_b×p_c)/6 = n_f/6 (unnormalized face normal /6... n_f = (p_b−p_a)×(p_c−p_a) and p_b×p_c differs...). Actually ∂V/∂p_a for face (a,b,c) = (p_b × p_c)/6. Hmm that's position-dependent, not the face normal. But commonly approximated by face normal/3·? The standard "balloon" PBD (e.g. in Position Based Dynamics for balloons / "Unified particle physics") uses per-vertex gradient = sum of incident face normals / 6? Let me recall: V = (1/6)Σ p_a·(p_b×p_c). dV/dp_a = (p_b×p_c)/6. For a closed mesh, Σ (p_b×p_c)/6 over incident faces equals Σ n_f/6 where n_f = (p_b−p_a)×(p_c−p_a)/2? Difference is a telescoping term that sums to zero over closed fan... I believe for closed meshes ∂V/∂p_i = (1/6)·Σ_{f∋i} n_f where n_f is the unnormalized cross (b−a)×(c−a). Let me not prove; widely used in code (e.g., "Balloon" in PBD tutorials) as grad = Σ n_f/6 with n_f unnormalized cross. Then correction Δp_i = −k_s·C·grad_i / Σ|grad|², C = V − k_over·V0. This is stable like any PBD constraint.

I'll replace the pressure force with a volume constraint solved once per substep in solveConstraints (after edges, before spokes maybe). With overpressure factor op = 1.0 default... inflation handled via restVolume scaling (setInflation already scales restVolume0). Plus a bit of overpressure for bounciness: target V = restVolume·1.0. Compliance via stiffness factor kv (0..1) per solve — use kv = min(1, pressureStiff·something). Apply full correction (kv=0.2..0.5 per substep is fine since 8 substeps).

Actually careful: full PBD volume solve with kv=1 per substep × 8 substeps/frame — very stiff, could fight edges — but PBD handles it. Use kv=0.5.

Energy behavior: volume constraint in PBD is dissipative-ish but for balloon it works well and is super stable. Also gives nice "inflate" behavior (target volume scaling = inflation).

Then gravity sag: volume conserved but shape squishes at bottom — edges handle that. Blob rests with slight flattening.

Let me also reconsider: do we still need center spokes? Keep them (soft) — helps prevent inversion.

Also the -0.6P0 clamp logic disappears.

Implementation:

Check sign: C>0 (too big) → shrink → Δp = −C·g/Σg²: g points outward → negative moves inward. Correct.

volStiff: 0.4 per substep.

Rest volume target = body.restVolume (scaled by inflation). For "pressure" feel — slight overinflate for tautness? Use target exactly restVolume; inflation slider changes target via setInflation (already scales restVolume).

Note this makes kPress unused — remove from integrate (keep acc for gravity only; simplify: no acc needed, integrate directly with gravity). Keep acc array? Just integrate gravity inline.

Hero at 2562 particles: volume constraint cost = one face loop per substep → 5120×8 = 41k — fine.

Also failure 3's "settles residual 0.15" — should resolve with stable pressure.

Failure 5 stomp collapse 0.19 — volume constraint will strongly resist → good.

Failure 2 test tolerance: change to relative 6%.

Failure 4 (height 3.31) — likely due to explosion; recheck after fix.

Also reconsider restitution bounce energy: prev adjust with rest 0.32 fine.

Let me patch the code:

In SoftBody constructor: add this.volGrad = new Float32Array(n*3) (note pos array gets extended with center AFTER; volGrad only n — fine, exclude center from volume solve; center follows via spokes).

Wait — volume gradients only for surface verts; center not part of faces. Good.

Add body.volStiff = 0.4.

In integrateBody: remove pressure computation (acc). Just verlet + gravity.

In solveConstraints: after edge loop & spokes per iteration... order: edges, volume, spokes — per iteration. Volume solve per iteration (2×/substep = 16/frame). Cost: 16×5120 faces = 82k — fine.

Hmm but doing volume solve twice per substep with volStiff 0.4 → effectively 0.64 per substep — quite stiff. OK.

Update core.mjs and index.html both — better: patch index.html, re-extract. Since core is extracted via sed from index.html, edit index.html then regenerate core.mjs.

Edits:

1. Remove pressure block from integrateBody.
2. Add volGrad in constructor + volStiff.
3. Add solveVolume function; call in solveConstraints loop.
4. Remove kPress from SoftBody (or leave harmless). Leave but unused — cleaner to remove. Also PHYS.kPress keep but unused? Remove usage; keep constant removal. I'll remove kPress references.

Also test 2 fix: relative tolerance 6%.

One more: test 3 threshold ke < 0.02 — with volume constraint the resting jitter should be small. Keep.

Also with volume solve the resting volume ratio should be ≈1.0 exactly (target V0) → test bounds 0.7–1.35 fine; height ~ close to r... floor rest height for r=1.5 sphere ≈ 1.5 (slightly squashed 1.3–1.45). Test expects 0.7–1.6 ✓.

Inflation test: setInflation(1.25) → target 1.25V0 → volume should approach ≈1.25 → test expects >1.05 & <1.6 ✓. Deflate 0.8 → ratio ~0.8, test expects <0.95 & >0.5 ✓.

Stomp: minV should stay high now (>0.5 easily) ✓.

Grab test: fine.

Let me apply the patches via replace_in_file tool on /tmp/squish/index.html.

Now update solveConstraints to call solveVolume, and constructor to add volGrad + volStiff, remove kPress. Also remove acc usage (constructor allocates acc — can leave or remove; remove to keep clean but must remove references). Let me patch constructor: replace acc allocation & kPress line; add volGrad/volStiff.

Wait — there's an ordering bug in the constructor: this.volGrad = new Float32Array(n * 3) is created before the center-particle extension, which is fine (volGrad only covers surface verts). But note this.acc reallocation after center extension — acc no longer used by integrateBody; constructor allocates this.acc twice (initial + after center). Harmless but let me clean: remove acc allocations. Actually they're harmless; but the second allocation this.acc = new Float32Array((n+1)*3) — keep or remove, no functional impact. Leave it (scratch, could be useful). Actually cleanliness: remove both acc lines. Quick replaces.

Now patch solveConstraints to call solveVolume inside the iteration loop.

Also remove unused acc lines and PHYS.kPress to keep clean. Let me do two quick replaces: remove this.acc = new Float32Array(src.length); // per-substep force accumulator (pressure) and this.pos = pos2; this.prev = prev2; this.acc = new Float32Array((n + 1) * 3); this.invMass = im2; → remove acc part. And remove kPress from PHYS? Keep PHYS.kPress removal optional — it's unused; remove it to avoid confusion.

Progress: 24/26. Volume now holds (rest volume ratio passed). Remaining failures in test 3:

• settles (residual motion 0.2078) — after 5.5s there's still significant residual per-substep motion (0.21 → v ≈ 0.21/0.00208·... = 100 m/s?? No: per-substep displacement 0.21 → velocity = 0.21·(1/h) = 0.21·480 = 100 m/s. That's not settling at all — the blob is still bouncing violently or jittering.
• rests on floor at sane height (2.05) — centroid 2.05 for r=1.5 sphere → too high; it's likely still bouncing.

Wait — kPress refs left: 1. Where? Maybe in a comment? Let me check. Also the residual motion: could be the volume constraint pumping energy? Or the drop from y=4 with restitution keeps bouncing? Damping 1.4/s should kill it. Residual motion 0.2078 per substep is huge.

Hypothesis: the volume constraint and edge constraints fight, creating oscillation. OR the restitution model creates a feedback: prev[i3+1] = y + vy*rest where vy is per-substep displacement — fine.

Or: volume solve with volStiff 0.4 twice per substep × 8 substeps → effectively extremely stiff, causing jitter against gravity+floor. At rest on floor, gravity pulls down each substep, floor pushes back, volume keeps shape... jitter should be damped by global damping 1.4/s.

Let me debug: run a small script printing centroid Y, volume ratio, residual at intervals for the settle test.

Actually wait — maybe the issue is different: the sphere is dropped from y=4, falls, hits floor... bounces. Restitution 0.32. Bounce height decays ×0.32 per bounce... after ~5s should settle. Residual 0.2/substep means something is actively energizing. The volume constraint: when squashed on floor, C = V−V0 < 0 → pushes vertices outward along gradient. This is positional; applied twice per substep with volStiff 0.4 → strong. Combined with stiff edges... could create persistent jiggle, but damping should eat it.

Let me actually check the leftover kPress reference and debug numerically.

The blob never settles: it keeps bouncing with impact ~70 m/s every second?! Impact 74 m/s at t=1 — the initial drop from y=4 gives impact speed √(2·9.8·2.5) ≈ 7 m/s, not 74! Something injects massive energy on floor contact. resid ~0.21 per substep = ~100 m/s jitter — but cy stays ~2 — so it's not flying apart; particles oscillate in place violently. V/V0 ~0.9 — compressed 10% persistently.

Energy injection source: the floor restitution/friction prev manipulation interacting with constraint projections, OR the volume constraint vs floor fight: volume solve pushes bottom vertices down (to restore volume) → below floor → floor clamps up → repeat. This is a classic constraint-fight oscillation. But damping 1.4/s should still bleed energy... The oscillation magnitude stays ~0.2/substep.

Wait, impact 74 m/s?? That's enormous. Let me think: impact is recorded as -vy*invH where vy = y - prev per substep. If a particle oscillates with amplitude A per substep... impact 74 m/s means per-substep displacement 0.154 downward at floor contact. So particles are slammed into the floor at 74 m/s — by what? The volume constraint! When V < V0 (compressed 10%), the volume solve pushes ALL vertices outward along their gradients. For the bottom vertices, outward = downward → pushed into the floor hard every substep; floor clamps back; net oscillation at huge effective velocity.

But why does V stay 10% compressed rather than equilibrating? Because gravity + floor: the blob resting on floor naturally flattens; flattening reduces enclosed volume; volume constraint tries to restore by pushing sides out and top up / bottom down. At equilibrium the blob should bulge sideways enough that V ≈ V0 with moderate push. The persistent 10% deficit suggests the volume constraint can't win against something — or it's fighting edges: pushing vertices out stretches edges beyond rest → edges pull back (stiff 0.9). Equilibrium at 10% deficit with continuous residual oscillation.

Hmm, but a real water balloon: volume is conserved, shape flattens, sides bulge. The edge springs here are structural (they want the original sphere shape). The edges resist the bulge → volume deficit remains → volume constraint keeps pushing → oscillation.

Options:

1. Reduce edge stiffness effect when stretched? No — springs should allow shape change; the problem is that edges at rest length define a sphere with V0; flattening requires edges to stretch (same edge lengths can't tile a flattened shape with V0 enclosed... actually geodesic sphere: to flatten, bottom faces must deform — edges stretch on sides).
2. The real issue is oscillation/instability, not equilibrium: the per-substep corrections are large. Volume correction: λ = −C/Σg²·volStiff. C = −0.1·V0 = −0.1·13 ≈ −1.3. Σg² over 282 verts: each g_i = Σ n_f/6 over ~6 faces; |n_f| = 2·area ≈ 0.1; so |g_i| ≈ 6·0.1/6 = 0.1 → Σg² ≈ 282·0.01 = 2.82. λ = 1.3/2.82·0.4 = 0.184. Per-vertex Δ = g·λ ≈ 0.018 per solve, ×2 iters ×8 substeps = 0.29/frame?! That's HUGE — particles spacing 0.19. Yes, this is the energy source: volume correction moves vertices 0.018 per solve — but then edges pull back most of it; net oscillation 0.018·... The recorded residual 0.2/substep — consistent.

Fixes: a) Lower volStiff a lot (0.05) → gentler, less oscillation but still pressure-ish. Then gravity sag: volume slowly... at rest it would still fight edges. Residual oscillation scales with volStiff → 0.05 gives resid ~0.026/substep = 12 m/s — still not settled. b) Add stronger damping when in floor contact (friction higher) — treats symptom. c) Make the volume constraint the ONLY volume driver and edges weaker: reduce default stiffness? Floppy blob. d) Damp the system more aggressively overall (damping 3/s). Settles but looks sluggish underwater-ish.

Actually the cleanest: the oscillation comes from stiff constraints fighting — edges (stiff) vs volume (stiff). Real solution: make edges NOT fight volume: springs should have zero rest-length preference for shear? No...

Alternative approach — go back to pressure force but made stable: the explicit pressure force was unstable at stiffness needed. But now I realize the instability earlier was partially the same fighting. Hmm, earlier with kp=700k it collapsed to 26% then exploded...

Let me think about what the classic, proven-stable configuration is: Matyka's pressure soft bodies use: edge springs (damped springs, force-based) + pressure force, with semi-implicit Euler and small dt — stable with proper damping because springs are damped (relative velocity damping along edges). My PBD version lacks damping inside constraints, and constraint fighting pumps energy.

Simplest robust fix within PBD: add velocity-level damping between neighboring particles (viscosity): after constraints, for each edge, damp relative velocity (move prev toward equalizing normal relative velocity). PBD viscosity: for each edge, compute relative velocity v_ij, remove a fraction of the component along the edge... that kills internal oscillation without affecting rigid motion. This is cheap (one more edge loop per frame, not per substep) and very effective: internal jiggle modes get eaten.

Implement applyInternalDamping(body, h, amount): after all substeps (once per frame): v_i = pos−prev; for each edge: dv = v_b − v_a; project along edge dir n̂: rel = dv·n̂; transfer: v_a += n̂·rel·0.5·amount; v_b −= ... (equal masses, conserves momentum). Set via prev adjustment: prev_a −= n̂·rel·0.5·amount... wait v = pos − prev; to change v_a by Δ, prev_a −= Δ. With amount ~0.35 per frame at 60fps → strong internal viscosity. This should kill the oscillation while keeping bounce.

Also reduce volStiff to 0.25 to soften the fight, and maybe raise iterations so constraints converge better together? Keep iters 2.

Additionally, increase global damping default slightly (1.4 fine).

Also the floor restitution 0.32 keeps it bouncing longer; with viscosity the bounce of the whole body remains (rigid-ish mode unaffected much since all particles share velocity... actually during impact deformation, viscosity eats the deformation energy → less bouncy over time — good).

Let me also reconsider equilibrium volume deficit ~0.9: with viscosity, the fight still exists but static: residual ~0. Fine — blob sits slightly squashed. Actually V/V0=0.9 means visibly deflated-ish; acceptable ("soft"). Could raise volStiff later if needed; test bounds 0.7–1.35 OK.

Implement viscosity in core + call once per frame in stepWorld (after substep loop). Apply to edges. Also apply to spokes? Edges enough.

Then re-run tests. Set thresholds: settle residual < 0.02 per substep (= ~0.6 m/s avg... hmm 0.02·480 = 9.6 m/s — too loose; resid includes gravity micro-jitter. Let me aim resid < 0.01 and see).

Edge case: viscosity applied once per frame with amount 0.35: stability fine (projection).

Let me patch:

Wait sign: v_a_new = v_a + n̂·imp where imp = rel·0.5·amount, rel = (v_b−v_a)·n̂. To increase v_a by Δ: prev_a -= Δ. Yes as written. Conserves total momentum (equal masses).

Call in stepWorld after substeps: for each body applyInternalDamping(body, 0.3).

Also volume: maybe use volStiff 0.3.

Let me patch and retest.

Still residual 0.2177. The viscosity once per frame at 0.3 isn't enough, OR the energy source is something else. resid ≈ same as before. Impact 74 m/s at t=1 — before any settling. Let me instrument deeper: run with viscosity 0 and various volStiff to isolate. Actually let me think again about the impact magnitude: 74 m/s right at first bounce. Drop from 4 → sphere surface hits at ~7 m/s. 74 m/s means per-substep displacement 0.154. Where does a bottom particle get 0.154/substep downward velocity? Gravity per substep adds gh = 9.8·h² = 9.8·4.34e-6 = 4.25e-5 position units. No.

The volume constraint: λ·g per solve. Let me compute: C = V−V0. After impact squash, say V = 0.9V0 → C = −1.3, Σg²... earlier estimate per-vertex Δ ≈ 0.018 per solve ×2 per substep = 0.036/substep. resid 0.2? Hmm bigger. When the ball is squashed a lot at first impact (V maybe 0.6V0), C = −5.2, Δ = 0.074 per solve → 0.15/substep — matches impact 74! So volume solve slams particles around. The recorded impact is then inflated by volume-solve-induced motion — floor impact detection sees the volume-solve pushing particles down into the floor as "impact speed 74 m/s".

So the volume constraint is simply too strong per solve when the blob deforms. It needs to be gentler per substep but the oscillation is sustained by continuous fighting with stiff edges.

Better idea: make the volume constraint the volume's only strong defender but solve it with proper convergence alongside edges: The fight is edges-want-sphere vs volume-wants-V0. But wait — they should be compatible! A perfect sphere satisfies both. Squashed on the floor: the deformed shape that keeps all edge lengths is approximately sphere-like; enclosing V0 while resting on the floor requires the bottom to flatten → edges must stretch. In equilibrium, edges stretch somewhat, volume slightly low (~0.95), sides bulge. The static equilibrium exists. The problem is dynamic oscillation around it.

Why doesn't global damping settle it? damping 1.4/s removes velocity by factor e^{-1.4} ≈ 0.25 per second — over 5 s plenty. But the oscillation is driven each substep by constraint corrections (position-level), which effectively re-inject velocity (pos moved but prev not → becomes velocity next step). The driven oscillation persists as long as constraints fight. Viscosity at 0.3/frame removes ~30% of relative edge velocity per frame → should be strong! Unless... bug in my viscosity: it's applied AFTER substeps, using pos/prev which reflect per-substep displacement h-scale. rel computed in per-substep displacement units — fine, transfer proportional — fine.

Hmm wait — actually let me check: is the oscillation internal (jiggle) or is the whole blob bouncing? cy oscillates 1.8→2.2 over seconds with minY up to 0.68 at t=5 — THE WHOLE BLOB IS BOUNCING repeatedly (minY 0.68 → off the floor!). So it's not just jiggle: the blob bounces persistently ~0.5m high. Restitution 0.32 should decay bounce quickly... unless energy injected by the volume constraint during floor contact: each impact squashes blob → volume solve pushes out hard → blob launches upward (like a superball, restitution effectively >1). The volume constraint converts squash into launch — realistic for a bouncy ball! But it never decays because the volume solve is elastic (position-based, no energy loss) and edges elastic too; global damping 1.4/s vs bounce period ~0.7s: each bounce loses ~40%... but measured bounce height stays ~0.4-0.5m across 6 seconds — barely decaying.

Hmm — damping 1.4/s over 6s → velocity ×e^{-8.4} ≈ 2e-4. Something must be injecting energy. THE VOLUME SOLVE: when squashed (V<V0), it pushes vertices out along gradient. The push direction on the TOP hemisphere is up — raising cy. This is energy transfer from constraint "potential" — fine and physical (stored elastic energy) — but it shouldn't increase total energy over cycles. Position-based corrections are projective; they can add energy when the configuration at solve time has been moved by another constraint (the floor). Floor clamps y≥rp (removes downward motion), then volume solve pushes down-bottom/out/top-up; next substep gravity... The interplay floor→volume→floor can pump.

Practical fix: increase viscosity drastically and apply it per substep (inside the substep loop) so jiggle never builds; also make floor restitution lower (0.15) and friction higher; also cap the volume correction per solve (clamp |λ·g_i| to e.g. 0.25×edge length). And reduce volStiff when far from target? Let me just try: viscosity 0.5 applied every substep, restitution 0.18, volStiff 0.25 with clamp.

Actually, hmm, let me reconsider impact recording: speed = -vy*invH where vy is the raw per-substep displacement at the moment of floor clamp — includes volume-solve push. For audio purposes it's fine (loud squish on hard squash). The test recorded impact 74 m/s though — that is per-substep displacement 0.154 = 74·0.00208. That's the volume solve pushing bottom vertices into the floor. With clamp, next substep it gets clamped and velocity... energy pump.

Cleanest robust approach honestly: dampen volume-solve with volume velocity — i.e., target V adjusted by dV/dt term: C = V − V0 + kv·(V − Vprev)/... PBD constraint damping is tricky.

Alternative pragmatic approach: accept some bounciness (it's a bouncy jelly demo after all!) but make the test drop from lower height and expect eventual settle. The real question: does it EVER settle? Debug shows bounce persisting ~same height at t=4,5,6 — non-decaying = bad (perpetual motion).

Let me try: per-substep viscosity 0.25 + restitution 0.15 + clamp volume correction. I'll iterate numerically in debug script before editing the html again — faster loop: copy core to a bench file, tweak constants via exported objects. Constants are baked in functions... viscosity amount & restitution: PHYS.restitution used inside collideBody (PHYS mutable → override PHYS.restitution in debug). volStiff per body. Viscosity: world.viscosity. Per-substep vs per-frame requires code change though.

Let me first try quick wins via debug: PHYS.restitution=0.1, world.viscosity=0.6, body.volStiff=0.2, world.damping=2.0, and see if it settles.

83% of time used. I need to finish fast. The oscillation is persistent — but let me think quickly about the real cause.

cy ≈ 2.0 with maxMinY after 4s ≈ 0.9-1.5 — the blob bounces ~1m high FOREVER. Since viscosity 1.0 and damping 4 don't kill it, the energy source is the volume constraint itself: when squashed at floor contact, it pushes top up and bottom down; floor clamp removes bottom motion; net upward impulse each contact — a pump. PBD volume balloons are known-bouncy. The usual cure: make restitution zero AND let the volume constraint be softer. But visc 1.0 should kill internal relative motion in one frame...

Wait, maybe the pump is more basic: the volume solve is applied TWICE per substep inside iterations (16×/frame). With volStiff 0.1 × 16 = strong. And each solve uses stale gradient — fine.

Honestly, maybe my test expectation is too strict for a "bouncy jelly" demo. A perpetually gently-bouncing blob is actually... toy-like but arguably alive. cy~2.0 bouncing 1m high forever = looks broken though.

Quick decisive fix: apply global damping to velocity proportional to how much the volume is compressed? Hacky.

Better quick fix: restitution 0 + extra damping while in floor contact (strong rolling/contact damping): in collideBody floor branch, scale tangential AND vertical-prev motion... Actually simplest and very effective: on floor contact set prev.y = y (kill all vertical velocity, restitution 0) and apply friction 0.5. The volume-pump then can't convert squash into upward launch? It still pushes top up (position) → becomes upward velocity next substep. Hmm.

The REAL pump: volume solve adds positional energy. Cure: clamp per-solve volume correction to a tiny step (e.g., max 0.02×edge per vertex) → pump rate limited, damping wins. With substeps 8 and 2 solves each, recovery from deep squash takes ~0.2s — visually fine (soft).

Let me try quickly in debug: clamp correction. Need code change in core (solveVolume). I'll patch index.html, regenerate core, and test with moderate params. Also set restitution 0.15 default, viscosity per frame 0.5.

Actually — even simpler and bulletproof: keep volume solve, but after constraints+collisions, re-sync prev toward pos for the volume-added component? Too complex.

Try clamp first. λ applied: pos[i] += g[i]λ. Compute per-vertex |gλ| clamp to maxStep = 0.3*body.rp (≈0.05). Test settle.

Still bouncing (maxMinY 0.34–0.57, resid 0.2). The clamp helped volume (V/V0 0.95) but the bounce persists. resid ≈ 0.2 in all configs — suspiciously constant! Even with visc 1.0. Something is structurally forcing per-substep displacement of 0.2...

OH WAIT. I bet I know what it is now: the spokes. The center particle! The center particle falls with gravity too, and its spokes pull surface particles... no. Hmm.

Actually let me reconsider: resid 0.2 constant regardless of damping — a driven oscillation at the substep level. What drives it? Let me instrument: is the oscillation in jiggle (relative) or bulk? Measure resid of CENTER particle vs surface relative to center.

Actually — a much more likely culprit: the volume solve correction changes sign each iteration? No...

Hold on, let me reconsider the floor contact: prev[i3+1] = y + vy*rest. vy = y - prev (negative). y=floorY. prev_new = floorY + vy·rest → BELOW floor (vy negative)! Next substep: v = pos − prev = floorY − (floorY + vy·rest) = −vy·rest > 0 upward — intended bounce. But ALSO gravity etc. Then it falls again. Restitution 0.32 → bounce decay ×0.32 per bounce...

Hmm, unless the volume constraint recompresses...

Let me just instrument the debug to print minY & resid at every 10 steps within one second after settling attempt — see the oscillation frequency/structure. Also print center particle's resid vs surface resid relative to center.

Both bulk (0.27/frame) and jiggle (0.34/frame) oscillate in a steady state — a limit cycle. minY oscillates 0.22–0.37 (per-frame values, so the blob bottom pulses). The blob hovers around cy 1.9 — for r=1.5 sphere that's fine (squashed). It's a driven limit cycle: volume pushes out, edges push in, floor clamps... per-frame bulk 0.27/frame = 16 m/s of velocity amplitude — huge. But cy only oscillates ±0.03?? bulk 0.27 per FRAME displacement with cy barely moving — so the bulk motion must reverse within a frame — i.e., it's high-frequency substep-level oscillation.

Hmm wait — bulk residual measured on the CENTER particle: 0.27 per substep (pos−prev is per-substep since prev is updated each substep... after the frame, prev is from the last substep). So center particle moves 0.27 in the last substep = 130 m/s?! And cy stable. That's an internal resonance: center particle oscillating wildly. The spokes connect center to all surface particles; the center is a single particle holding 282 spokes — each substep it gets yanked around. The center is the oscillator!

The spokes: 282 spokes pull the center with stiffness s·spokeStiff = 0.9·0.3=0.27 each, sequential Gauss-Seidel → the center effectively has huge collective stiffness ↔ violent oscillation with gravity. The center particle is the problem child.

Fix options:

• Remove center particle entirely. Pressure + edges should suffice (classic pressure soft bodies don't need it). Volume constraint prevents inversion now (it restores V strongly).
• Or give the center particle a large mass (invMass 0.05) so it moves gently.

Quick: drop the center/spokes completely. Test. If collapse/inversion issues appear in stomp test, revisit.

Let me patch: constructor — remove center extension & spokes; solveConstraints — remove spoke loop; collideBody — count = n. Simplest: make center handling optional flag useCenter:false. I'll just remove.

Also then count = n.

Let me edit: constructor block from "// --- center particle + spokes ---" through spokeRest assignment — remove; get count() { return this.n; }. solveConstraints: remove spoke section. collideBody unchanged (uses body.count).

Do it via python for precision.

************************************************************************************************************************************************************************************************ne.

All 26 tests pass. Now:

1. Full-module syntax check: extract the <script type="module"> content and node --check it.
2. Deploy to /space/demos/041-squish-lab/index.html.
3. Update /space/demos/index.html.

Do all in one command. Extract module script: between <script type="module"> and </script> (last occurrence). Use awk/sed. The importmap script is also type="importmap" — the sed for 'script type="module"' start to next '' works.