tower / files / src / terrain / falls.ts
falls.ts
80bcc97a 2026-09-17 Pavel Safronov | 1 | /** |
| 2 | * Waterfall sheets. See world-generation.md §4.3 and design.md §1.9.2. | |
| 3 | * | |
| 4 | * A fall is where a river's surface steps down (compile.ts). This builds the falling | |
| 5 | * water: a sheet leaving the lip at the river's speed, dropping under gravity, and — | |
| 6 | * because everything that falls in Tower carries the spin — drifting **antispinward** as | |
| 7 | * it goes, by exactly the free-fall drift canon/motion.ts computes. A kilometre-high fall | |
| 8 | * lands 133 m antispinward of the lip. The waterfall is the compass (§1.9.2); this is the | |
| 9 | * first time the view draws one. | |
| 10 | * | |
| 11 | * Pure geometry, no Three.js, so the lean can be tested against the canon it claims. | |
| 12 | */ | |
| 13 | import { RADIUS_M } from '../canon/constants.js' | |
| 14 | import { freeFallDriftM } from '../canon/motion.js' | |
| 15 | import type { CFall } from './compile.js' | |
| 16 | ||
| 17 | export interface FallSheet { | |
| 18 | /** The lip's centre, km, tower frame: where the mesh is placed. */ | |
| 19 | origin: [number, number, number] | |
| 20 | /** Vertices relative to the origin, km. */ | |
| 21 | positions: Float32Array | |
| 22 | /** u across the sheet 0..1, v down it 0..1. */ | |
| 23 | uvs: Float32Array | |
| 24 | indices: Uint16Array | |
| 25 | heightM: number | |
| 26 | } | |
| 27 | ||
| 28 | export const FALL_ROWS = 32 | |
| 29 | export const FALL_COLS = 6 | |
| 30 | ||
| 31 | /** | |
| 32 | * Where a drop of water that left the lip at `offset` metres across it is, `y` metres | |
| 33 | * down: in (z, a) on the unrolled floor and in elevation. | |
| 34 | */ | |
| 35 | export function fallPoint(f: CFall, offset: number, y: number): { z: number; a: number; elevationM: number } { | |
| 36 | const t = Math.sqrt((2 * y) / 9.80665) | |
| 37 | const forward = f.speedMS * t | |
| 38 | // Across the flow: (dz, da) turned a quarter to spinward. | |
| 39 | const acrossZ = -f.da, acrossA = f.dz | |
| 40 | return { | |
| 41 | z: f.z + f.dz * forward + acrossZ * offset, | |
| 42 | // Antispinward is -a: the drift is subtracted. | |
| 43 | a: f.a + f.da * forward + acrossA * offset - freeFallDriftM(y), | |
| 44 | elevationM: f.topM - y, | |
| 45 | } | |
| 46 | } | |
| 47 | ||
| 48 | export function fallSheet(f: CFall): FallSheet { | |
| 49 | const height = f.topM - f.bottomM | |
| 50 | const cos = Math.cos(f.a / RADIUS_M), sin = Math.sin(f.a / RADIUS_M) | |
| 51 | const r0 = (RADIUS_M - f.topM) / 1000 | |
| 52 | const origin: [number, number, number] = [r0 * cos, r0 * sin, f.z / 1000] | |
| 53 | const positions = new Float32Array((FALL_ROWS + 1) * (FALL_COLS + 1) * 3) | |
| 54 | const uvs = new Float32Array((FALL_ROWS + 1) * (FALL_COLS + 1) * 2) | |
| 55 | let k = 0 | |
| 56 | for (let row = 0; row <= FALL_ROWS; row++) { | |
| 57 | // Rows bunched toward the lip, where the sheet curves most. | |
| 58 | const v = (row / FALL_ROWS) ** 1.6 | |
| 59 | for (let col = 0; col <= FALL_COLS; col++) { | |
| 60 | const u = col / FALL_COLS | |
| 61 | const p = fallPoint(f, (u - 0.5) * f.widthM, v * height) | |
| 62 | const theta = p.a / RADIUS_M | |
| 63 | const r = (RADIUS_M - p.elevationM) / 1000 | |
| 64 | positions[3 * k] = r * Math.cos(theta) - origin[0] | |
| 65 | positions[3 * k + 1] = r * Math.sin(theta) - origin[1] | |
| 66 | positions[3 * k + 2] = p.z / 1000 - origin[2] | |
| 67 | uvs[2 * k] = u | |
| 68 | uvs[2 * k + 1] = v | |
| 69 | k++ | |
| 70 | } | |
| 71 | } | |
| 72 | const idx: number[] = [] | |
| 73 | for (let row = 0; row < FALL_ROWS; row++) { | |
| 74 | for (let col = 0; col < FALL_COLS; col++) { | |
| 75 | const a = row * (FALL_COLS + 1) + col | |
| 76 | const b = a + 1, c = a + FALL_COLS + 1, d = c + 1 | |
| 77 | idx.push(a, c, b, b, c, d) | |
| 78 | } | |
| 79 | } | |
| 80 | return { origin, positions, uvs, indices: new Uint16Array(idx), heightM: height } | |
| 81 | } |