Build "Citrus Jelly" — a single self-contained HTML file (inline JS/CSS, no libraries) with a real-time WebGPU soft-body jelly shaped like a half-moon citrus slice that the user can grab, stretch and let go.
Object
- A rounded half-moon slice: half-disk profile extruded into a thick slab with softly rounded edges; flat cut edge on the bottom, arc on top.
- Three material zones by radius: translucent fruit flesh inside, a pale pith band, and a thin glossy rind along the arc (the cut edge is all flesh).
- Inside the jelly: thin milky radial membranes between the segments (inset from both faces), a few seeds, small air bubbles with a faint rim and a pin-point highlight, and tiny sub-surface glints like sugar crystals.
Physics (XPBD tetrahedral soft body)
- Mesh: 2D Bowyer–Watson Delaunay of the profile, extruded into prisms and split into tets. Render mesh embedded in sim tets with barycentric coordinates.
- Constraints:
- co-rotational per-tet shape matching (warm-started quaternion, Müller 2016), which recovers inverted tets;
- tet volume constraints for near-incompressibility;
- hard edge strain limits (max 1.8×, min 0.35× rest length).
- Fixed timestep, "small steps" XPBD (many substeps, 1 iteration each).
- Damping: relative-velocity damping along edges, light air drag, and settling that bleeds off residual jitter.
- Floor contact with Coulomb-style positional friction (static and kinetic). Never crush the held patch into the floor; lift the patch instead.
- Grab and pull:
- pick a surface point with the mouse or touch;
- the patch around it is attached softly;
- reach is bounded, so stretching is strong but never unbounded;
- on release the slice wobbles back.
- Safety: reset gently if any value becomes NaN.
Rendering (WebGPU / WGSL)
- Procedural photo studio: pale cyclorama paper sweep, overhead softbox, back-left strip light, front-right fill card.
- Jelly material:
- refraction of the scene through the slice;
- view-ray thickness from a back-face depth pass, used for Beer–Lambert absorption;
- Fresnel studio reflections and a key highlight.
- Soft PCSS-style floor shadows, tinted by the jelly color.
- Post: exposure, Khronos PBR Neutral tone mapping, sRGB, dither. Optional mesh-overlay debug view.
UI (minimal editorial style, English)
- Corner label "Material Studies / No. 010", big title "Citrus / Jelly.", hint "Grab. Stretch. Let go."
- Variety swatches: Orange, Blood Orange, Lemon. Each changes the flesh, pith and rind colors.
- Sliders: Firmness (log-mapped edge compliance) and Internal damping.
- Buttons:
- "Give it a nudge": a small hop with a twist;
- Reset;
- Pause.
- Keys: Space to pause, N to nudge, R to reset.
- Fallback screen "This slice needs WebGPU." when the browser has no WebGPU.
- Responsive: works on mobile with touch and no horizontal scroll.
