sky.js

// Time and weather. A world that is always noon in fair weather is a diagram of a
// world; moving the sun and letting it rain is what makes the same terrain read
// as a place you could be standing in at a particular hour.
//
// Climate is not chosen by the user — it is read back from the world itself, so a
// glacier world gets snow and a desert stays dry without anyone saying so.

import * as THREE from 'three';
import { GRID } from './world.js';

// key times of day: sun colour, sky colour, ambient, and how strong the sun is
const KEYS = [
    { t: 0.00, sun: '#243043', sky: '#0e1622', amb: '#1b2433', power: 0.12 },  // night
    { t: 0.22, sun: '#e8915c', sky: '#7d7a8e', amb: '#5c5f70', power: 0.75 },  // dawn
    { t: 0.32, sun: '#ffd9a8', sky: '#9fc4dc', amb: '#c8dcea', power: 1.30 },  // morning
    { t: 0.50, sun: '#fff3dd', sky: '#a8ccdf', amb: '#cfe3f2', power: 1.55 },  // noon
    { t: 0.72, sun: '#ffc07a', sky: '#b39a92', amb: '#a89aa0', power: 1.05 },  // afternoon
    { t: 0.82, sun: '#f2764a', sky: '#7c5f6b', amb: '#5f5460', power: 0.55 },  // dusk
    { t: 1.00, sun: '#243043', sky: '#0e1622', amb: '#1b2433', power: 0.12 },
];

function sample(t) {
    t = ((t % 1) + 1) % 1;
    let a = KEYS[0], b = KEYS[KEYS.length - 1];
    for (let i = 0; i < KEYS.length - 1; i++) {
        if (t >= KEYS[i].t && t <= KEYS[i + 1].t) { a = KEYS[i]; b = KEYS[i + 1]; break; }
    }
    const k = (t - a.t) / Math.max(1e-6, b.t - a.t);
    return {
        sun: new THREE.Color(a.sun).lerp(new THREE.Color(b.sun), k),
        sky: new THREE.Color(a.sky).lerp(new THREE.Color(b.sky), k),
        amb: new THREE.Color(a.amb).lerp(new THREE.Color(b.amb), k),
        power: a.power + (b.power - a.power) * k,
    };
}

/**
 * Read the climate off the finished world. Snow biomes and a high mean elevation
 * mean precipitation falls as snow; low moisture means it rarely falls at all.
 */
export function climateOf(world) {
    const cold = world.regions
        .filter(r => ['snow', 'alpine', 'tundra'].includes(r.key))
        .reduce((a, r) => a + r.coverage, 0);
    const arid = world.regions
        .filter(r => ['desert', 'badlands', 'mesa', 'canyon'].includes(r.key))
        .reduce((a, r) => a + r.coverage, 0);
    let mean = 0;
    for (let i = 0; i < world.moisture.length; i++) mean += world.moisture[i];
    mean /= world.moisture.length;

    return {
        cold, arid, moisture: mean,
        precipitation: cold > 0.3 ? 'snow' : (arid > 0.45 || mean < 0.12 ? 'none' : 'rain'),
    };
}

export class Sky {
    constructor(scene) {
        this.scene = scene;
        this.hemi = new THREE.HemisphereLight('#cfe3f2', '#4a4636', 1.2);
        this.sun = new THREE.DirectionalLight('#fff3dd', 1.5);
        scene.add(this.hemi, this.sun);
        scene.fog = new THREE.Fog('#9fc4dc', 140, 400);
        this.particles = null;
        this.kind = 'none';
    }

    /** @param {number} t 0..1 through the day, 0.5 = noon */
    setTime(t, radius = 150) {
        const s = sample(t);
        // the sun rides an arc; below the horizon it simply stops lighting anything
        const ang = (t - 0.25) * Math.PI * 2;
        this.sun.position.set(Math.cos(ang) * radius, Math.sin(ang) * radius, radius * 0.35);
        this.sun.color.copy(s.sun);
        this.sun.intensity = Math.max(0.02, s.power);
        this.hemi.color.copy(s.amb);
        this.hemi.intensity = 0.45 + s.power * 0.55;
        this.scene.background = s.sky.clone();
        this.scene.fog.color.copy(s.sky);
        return s;
    }

    /** Rain or snow as a drifting particle column that follows the camera. */
    setWeather(kind, extent = 220) {
        if (kind === this.kind) return;
        this.kind = kind;
        if (this.particles) {
            this.scene.remove(this.particles);
            this.particles.geometry.dispose();
            this.particles.material.dispose();
            this.particles = null;
        }
        if (kind === 'none') return;

        const count = kind === 'snow' ? 2600 : 4200;
        const pos = new Float32Array(count * 3);
        for (let i = 0; i < count; i++) {
            pos[i * 3] = (Math.random() - 0.5) * extent;
            pos[i * 3 + 1] = Math.random() * 90;
            pos[i * 3 + 2] = (Math.random() - 0.5) * extent;
        }
        const geo = new THREE.BufferGeometry();
        geo.setAttribute('position', new THREE.BufferAttribute(pos, 3));
        this.particles = new THREE.Points(geo, new THREE.PointsMaterial({
            color: kind === 'snow' ? '#ffffff' : '#a9c6d8',
            size: kind === 'snow' ? 0.55 : 0.28,
            transparent: true, opacity: kind === 'snow' ? 0.85 : 0.55,
            depthWrite: false,
        }));
        this.particles.frustumCulled = false;
        this.scene.add(this.particles);
        this.extent = extent;
    }

    stepWeather(dt, camera) {
        if (!this.particles) return;
        const p = this.particles.geometry.attributes.position;
        const fall = this.kind === 'snow' ? 4 : 34;
        const drift = this.kind === 'snow' ? 1.6 : 0.4;
        for (let i = 0; i < p.count; i++) {
            let y = p.getY(i) - fall * dt;
            let x = p.getX(i) + Math.sin((y + i) * 0.15) * drift * dt;
            if (y < -4) {
                y = 85 + Math.random() * 8;
                x = camera.position.x + (Math.random() - 0.5) * this.extent;
                p.setZ(i, camera.position.z + (Math.random() - 0.5) * this.extent);
            }
            p.setX(i, x); p.setY(i, y);
        }
        p.needsUpdate = true;
    }
}