assets.js

// Test subjects, generated rather than downloaded.
//
// A splat reconstruction of a real object would do, but every published one we
// could use carries research-only or unstated terms, so the shapes here are made
// from scratch: each returns gaussian centres plus per-kernel sigma, which is all
// the simulation and the renderer need.

function rngFrom(seed) {
    let s = seed >>> 0 || 1;
    return () => {
        s ^= s << 13; s >>>= 0;
        s ^= s >> 17;
        s ^= s << 5; s >>>= 0;
        return s / 4294967296;
    };
}

/** Fill a shape by rejection sampling, so density is uniform in volume. */
function fill(inside, count, bounds, rng) {
    const pos = [], [lo, hi] = bounds;
    let guard = 0;
    while (pos.length < count * 3 && guard < count * 400) {
        guard++;
        const x = lo[0] + rng() * (hi[0] - lo[0]);
        const y = lo[1] + rng() * (hi[1] - lo[1]);
        const z = lo[2] + rng() * (hi[2] - lo[2]);
        if (inside(x, y, z)) pos.push(x, y, z);
    }
    return new Float32Array(pos);
}

const SHAPES = {
    cube: {
        label: 'Cube',
        make: (rng, n) => fill((x, y, z) =>
            Math.abs(x - 0.5) < 0.13 && Math.abs(z - 0.5) < 0.13 && y > 0.52 && y < 0.78,
            n, [[0.3, 0.45, 0.3], [0.7, 0.85, 0.7]], rng),
    },
    ball: {
        label: 'Ball',
        make: (rng, n) => fill((x, y, z) =>
            (x - 0.5) ** 2 + (y - 0.66) ** 2 + (z - 0.5) ** 2 < 0.145 ** 2,
            n, [[0.34, 0.5, 0.34], [0.66, 0.82, 0.66]], rng),
    },
    column: {
        label: 'Column',
        make: (rng, n) => fill((x, y, z) =>
            (x - 0.5) ** 2 + (z - 0.5) ** 2 < 0.09 ** 2 && y > 0.2 && y < 0.86,
            n, [[0.4, 0.2, 0.4], [0.6, 0.86, 0.6]], rng),
    },
    arch: {
        label: 'Arch',
        make: (rng, n) => fill((x, y, z) => {
            if (Math.abs(z - 0.5) > 0.09) return false;
            const r = Math.hypot(x - 0.5, y - 0.42);
            if (y > 0.42) return r > 0.14 && r < 0.24;
            return Math.abs(Math.abs(x - 0.5) - 0.19) < 0.05 && y > 0.2;
        }, n, [[0.24, 0.2, 0.4], [0.76, 0.7, 0.6]], rng),
    },
    tower: {
        label: 'Tower',
        make: (rng, n) => fill((x, y, z) => {
            const t = (y - 0.2) / 0.62;
            if (t < 0 || t > 1) return false;
            const half = 0.16 * (1 - t * 0.55);
            return Math.abs(x - 0.5) < half && Math.abs(z - 0.5) < half;
        }, n, [[0.3, 0.2, 0.3], [0.7, 0.85, 0.7]], rng),
    },
};

export const SHAPE_LIST = Object.entries(SHAPES).map(([id, s]) => ({ id, label: s.label }));

/**
 * Build a gaussian cloud for a shape. Sigma is tied to the spacing implied by the
 * particle count so the kernels just overlap — the same reason a splat
 * reconstruction looks solid rather than like beads.
 */
export function makeAsset(shape = 'cube', count = 4200, seed = 7) {
    const rng = rngFrom(seed);
    const spec = SHAPES[shape] || SHAPES.cube;
    const pos = spec.make(rng, count);
    const n = pos.length / 3;

    const sigma = new Float32Array(n * 3);
    const base = 0.5 * Math.cbrt(0.02 / Math.max(1, n));   // ~volume per particle
    for (let i = 0; i < n; i++) {
        const s = base * (2.6 + rng() * 0.8);
        sigma[i * 3] = s;
        sigma[i * 3 + 1] = s;
        sigma[i * 3 + 2] = s;
    }

    // Colour is banded by height so deformation is legible: a uniform blob tells
    // you nothing about which part of it stretched.
    const color = new Float32Array(n * 3);
    let minY = Infinity, maxY = -Infinity;
    for (let i = 0; i < n; i++) { minY = Math.min(minY, pos[i * 3 + 1]); maxY = Math.max(maxY, pos[i * 3 + 1]); }
    for (let i = 0; i < n; i++) {
        const t = (pos[i * 3 + 1] - minY) / Math.max(1e-6, maxY - minY);
        color[i * 3] = 0.55 + 0.4 * t;
        color[i * 3 + 1] = 0.62 + 0.22 * Math.sin(t * 6.28);
        color[i * 3 + 2] = 0.95 - 0.45 * t;
    }
    return { positions: pos, sigma, color, count: n };
}