Rotating Star

/* rotating-star: a thick five-pointed star extruded in 3D, spinning and catching the light. */
import { Grid, palette, gradient, KEYS, TAU } from "../lib/kit.js";

/* tiny 3D kit for this piece: view space has +z away from the viewer. */
const LIGHT = (() => { const v = [-0.72, 0.55, -0.42], l = Math.hypot(v[0], v[1], v[2]); return [v[0] / l, v[1] / l, v[2] / l]; })();
/** Lambert plus a specular glint, 0..1, for a unit view-space normal. */
function lightOf(n, amb = 0.1, spec = 0.55, pw = 18) {
  const d = n[0] * LIGHT[0] + n[1] * LIGHT[1] + n[2] * LIGHT[2];
  const rz = 2 * d * n[2] - LIGHT[2];
  const s = d > 0 && rz < 0 ? Math.pow(-rz, pw) : 0;
  return Math.min(1, amb + Math.max(0, d) * (1 - amb) * 0.78 + s * spec * 0.6);
}
/** Filled triangle in screen cells with a depth test; points are [x, y, depth], larger depth is nearer. */
function tri(g, a, b, c, ch, co) {
  const area = (b[0] - a[0]) * (c[1] - a[1]) - (b[1] - a[1]) * (c[0] - a[0]);
  if (Math.abs(area) < 1e-6) return;
  const x0 = Math.max(0, Math.floor(Math.min(a[0], b[0], c[0]))), x1 = Math.min(g.cols - 1, Math.ceil(Math.max(a[0], b[0], c[0])));
  const y0 = Math.max(0, Math.floor(Math.min(a[1], b[1], c[1]))), y1 = Math.min(g.rows - 1, Math.ceil(Math.max(a[1], b[1], c[1])));
  for (let y = y0; y <= y1; y++) for (let x = x0; x <= x1; x++) {
    const px = x + 0.5, py = y + 0.5;
    const w0 = ((b[0] - px) * (c[1] - py) - (b[1] - py) * (c[0] - px)) / area;
    const w1 = ((c[0] - px) * (a[1] - py) - (c[1] - py) * (a[0] - px)) / area;
    const w2 = 1 - w0 - w1;
    if (w0 < -0.02 || w1 < -0.02 || w2 < -0.02) continue;
    g.setZ(x, y, w0 * a[2] + w1 * b[2] + w2 * c[2], typeof ch === "function" ? ch(w0, w1, w2) : ch, co);
  }
}
/** A depth-tested line between projected points, nudged toward the viewer by bias. */
function seg(g, a, b, ch, co, bias = 0) {
  const n = Math.max(1, Math.ceil(Math.max(Math.abs(b[0] - a[0]), Math.abs(b[1] - a[1])) * 1.5));
  for (let i = 0; i <= n; i++) {
    const f = i / n;
    g.setZ(a[0] + (b[0] - a[0]) * f, a[1] + (b[1] - a[1]) * f, a[2] + (b[2] - a[2]) * f + bias, typeof ch === "function" ? ch(f) : ch, co);
  }
}
/** Character for a line drawn in screen direction dx, dy (tall cells). */
function slopeChar(dx, dy) {
  const a = Math.atan2(dy * 2, dx), s = Math.abs(Math.sin(a)), c = Math.abs(Math.cos(a));
  if (s < 0.38) return "─";
  if (c < 0.38) return "│";
  return (dx > 0) === (dy > 0) ? "╲" : "╱";
}
const rx = ([x, y, z], a) => { const c = Math.cos(a), s = Math.sin(a); return [x, y * c - z * s, y * s + z * c]; };
const ry = ([x, y, z], a) => { const c = Math.cos(a), s = Math.sin(a); return [x * c + z * s, y, -x * s + z * c]; };
const rz = ([x, y, z], a) => { const c = Math.cos(a), s = Math.sin(a); return [x * c - y * s, x * s + y * c, z]; };
const sub = (a, b) => [a[0] - b[0], a[1] - b[1], a[2] - b[2]];
const cross = (a, b) => [a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2], a[0] * b[1] - a[1] * b[0]];
const unit = (v) => { const l = Math.hypot(v[0], v[1], v[2]) || 1; return [v[0] / l, v[1] / l, v[2] / l]; };
/** Draw a mesh: V in view space, F as index lists (fans), each face lit flat. Normals face away from the origin when orient is set. */
function mesh(g, V, F, P, paint, edgeCo, orient = true) {
  const S = V.map(P);
  for (let f = 0; f < F.length; f++) {
    const idx = F[f];
    let n = unit(cross(sub(V[idx[1]], V[idx[0]]), sub(V[idx[2]], V[idx[0]])));
    const c = [0, 0, 0];
    for (const i of idx) { c[0] += V[i][0] / idx.length; c[1] += V[i][1] / idx.length; c[2] += V[i][2] / idx.length; }
    if (orient && n[0] * c[0] + n[1] * c[1] + n[2] * c[2] < 0) n = [-n[0], -n[1], -n[2]];
    const [ch, co] = paint(lightOf(n), f, n);
    for (let i = 1; i + 1 < idx.length; i++) tri(g, S[idx[0]], S[idx[i]], S[idx[i + 1]], ch, co);
    if (edgeCo) for (let i = 0; i < idx.length; i++) {
      const a = S[idx[i]], b = S[idx[(i + 1) % idx.length]];
      seg(g, a, b, slopeChar(b[0] - a[0], b[1] - a[1]), edgeCo, 0.012);
    }
  }
}

const GOLD = gradient(["#2a1800", "#7a4a00", "#d08a10", "#ffd04a", "#fff6c8"], 12);
export const meta = {
  slug: "rotating-star",
  name: "Rotating Star",
  category: "solids",
  note: "A thick five-pointed gold star extruded in 3D, spinning on its axis and catching the light on its bevels.",
  tags: ["3d", "star", "award", "loader", "hero"],
  cols: 56, rows: 30, fps: 30,
  palette: palette([...GOLD, "#fffbe8", "#3a2a10"]),
};

const RAMP = ":-=+*#%@@@";

export default function create() {
  const { cols, rows } = meta;
  const g = new Grid(cols, rows);
  const VERTS = [], FACES = [];
  const ring = [];
  for (let i = 0; i < 10; i++) {
    const a = (i / 10) * TAU, r = i % 2 ? 0.46 : 1.15;
    ring.push([Math.sin(a) * r, Math.cos(a) * r]);
  }
  const Z = 0.26;
  ring.forEach(([x, y]) => VERTS.push([x, y, -Z]));           // 0-9 front
  ring.forEach(([x, y]) => VERTS.push([x, y, Z]));            // 10-19 back
  VERTS.push([0, 0, -Z - 0.22], [0, 0, Z + 0.22]);            // 20, 21 raised centres for a bevel
  for (let i = 0; i < 10; i++) {
    const j = (i + 1) % 10;
    FACES.push([20, i, j], [21, 10 + j, 10 + i], [i, j, 10 + j, 10 + i]);
  }
  const D = 6, SC = 17;
  const P = (v) => { const k = D / (v[2] + D); return [cols / 2 + v[0] * k * SC, rows / 2 - v[1] * k * SC * 0.5, k]; };
  const paint = (l) => [RAMP[Math.min(9, Math.floor((0.12 + l * 0.88) * 10))], KEYS[Math.min(11, Math.floor((0.2 + l * 0.8) * 12))]];
  return (t) => {
    g.clear();
    const u = t * 0.9 + 1.4, spin = u - 0.42 * Math.sin(2 * u);   // lingers face-on, sweeps through edge-on
    const V = VERTS.map((v) => rx(ry(v, spin), Math.sin(t * 0.7) * 0.25));
    mesh(g, V, FACES, P, paint, null);
    return g.frame();
  };
}