meat/engine/scene/Tree.ts

import { Vec3 } from "../math/Vec3"
import { STRIDE, type Mesh } from "./Mesh"

const TAU = Math.PI * 2

/** One procedural tree instance. `growth` 0..1 runs sapling -> full grown: it
 *  scales height and girth and adds canopy blobs (oak) / tiers (spruce). `seed`
 *  drives the per-tree random wobble so a forest doesn't look cloned. */
export type Tree = {
  kind: "oak" | "spruce"
  /** Trunk base, sitting on the ground. */
  position: Vec3
  growth: number
  seed: number
}

/**
 * Low-poly tree geometry, in the same faceted flat-shaded style as the rest of
 * the world. Two silhouettes carry the species read:
 *   oak    -- short tapered trunk, a couple of branches, a broad cluster of
 *             rounded canopy blobs (bushy, wider than tall).
 *   spruce -- tall thin trunk under stacked cones that narrow to a point
 *             (tiered, taller than wide).
 * `build` appends into caller-owned meshes so a whole forest batches into a few
 * draw calls: all trunks share one bark mesh, foliage splits oak vs spruce so
 * each can carry its own leaf/needle texture.
 */
export namespace Tree {
  /** Append one tree into the shared `trunk` (bark) mesh and the `foliage` mesh
   *  for its kind (oak leaf vs spruce needle). */
  /** `lod` "impostor" bakes a much cheaper stand-in (few tris, same textures +
   *  faceted look, same height/position) for far chunks; "full" is up close. */
  export function build(tree: Tree, trunk: Mesh, foliage: Mesh, lod: "full" | "impostor" = "full"): void {
    const rand = rng(tree.seed)
    if (tree.kind === "oak") {
      oak(tree.position, tree.growth, rand, trunk, foliage, lod)
    } else {
      spruce(tree.position, tree.growth, rand, trunk, foliage, lod)
    }
  }

  function oak(base: Vec3, g: number, rand: () => number, trunk: Mesh, leaves: Mesh, lod: "full" | "impostor"): void {
    const h = lerp(0.8, 7, g)
    const rTrunk = lerp(0.04, 0.32, g)
    const forkY = base.y + h * 0.5
    const canopyY = base.y + h * 0.72
    const blobR = h * 0.3
    if (lod === "impostor") {
      // One low-poly blob on a stubby trunk -- reads as an oak at distance.
      limb(trunk, base, { x: base.x, y: forkY, z: base.z }, rTrunk, rTrunk * 0.6, 3)
      blob(leaves, { x: base.x, y: canopyY, z: base.z }, blobR * 1.15, rand, 4, 2)
      return
    }
    limb(trunk, base, { x: base.x, y: forkY, z: base.z }, rTrunk, rTrunk * 0.6, 5)

    const spread = h * 0.32
    // Central blob plus, as it grows, a couple offset ones -> broad bushy crown.
    const blobs = 1 + Math.round(g * 2)
    for (let i = 0; i < blobs; i++) {
      const angle = rand() * TAU
      const rad = i === 0 ? 0 : spread * (0.5 + rand() * 0.5)
      const center = {
        x: base.x + Math.cos(angle) * rad,
        y: canopyY + (rand() - 0.4) * spread,
        z: base.z + Math.sin(angle) * rad,
      }
      blob(leaves, center, blobR * (0.7 + rand() * 0.4), rand)
    }
    // Grown oaks throw out a few branches, each tipped with a leaf tuft.
    if (g > 0.55) {
      const branches = 2 + Math.round(rand())
      for (let i = 0; i < branches; i++) {
        const angle = rand() * TAU
        const dir = Vec3.normalize({ x: Math.cos(angle), y: 1.2, z: Math.sin(angle) })
        const start = { x: base.x, y: base.y + h * 0.42, z: base.z }
        const end = Vec3.add(start, Vec3.scale(dir, h * 0.3))
        limb(trunk, start, end, rTrunk * 0.4, rTrunk * 0.2, 4)
        blob(leaves, end, blobR * 0.6, rand)
      }
    }
  }

  function spruce(base: Vec3, g: number, rand: () => number, trunk: Mesh, needles: Mesh, lod: "full" | "impostor"): void {
    const h = lerp(0.6, 9, g)
    const rTrunk = lerp(0.03, 0.2, g)
    const impostor = lod === "impostor"
    limb(trunk, base, { x: base.x, y: base.y + h, z: base.z }, rTrunk, rTrunk * 0.25, impostor ? 3 : 5)

    // Stacked cones: widest low, shrinking to a point up top -> conical tiers.
    // The impostor keeps the first two tiers at low sides (same seed => aligned).
    const tiers = impostor ? 2 : 2 + Math.round(g * 3)
    const sides = impostor ? 4 : 6
    const bottom = base.y + h * 0.1
    const span = h * 0.9
    for (let i = 0; i < tiers; i++) {
      const t = i / tiers
      const y = bottom + t * span * 0.82
      const radius = lerp(h * 0.3, h * 0.05, t) * (0.9 + rand() * 0.2)
      const coneH = (span / tiers) * 1.9
      cone(needles, { x: base.x, y, z: base.z }, coneH, radius, sides)
    }
  }

  /** A tapered tube between two points (trunk or branch), `sides`-gonal. */
  function limb(mesh: Mesh, a: Vec3, b: Vec3, ra: number, rb: number, sides: number): void {
    const axis = Vec3.normalize(Vec3.sub(b, a))
    const [u, v] = basis(axis)
    const len = Vec3.length(Vec3.sub(b, a))
    const start = mesh.verts.length / STRIDE
    for (let i = 0; i <= sides; i++) {
      const angle = (i / sides) * TAU
      const dx = u.x * Math.cos(angle) + v.x * Math.sin(angle)
      const dy = u.y * Math.cos(angle) + v.y * Math.sin(angle)
      const dz = u.z * Math.cos(angle) + v.z * Math.sin(angle)
      const s = i / sides
      mesh.verts.push(a.x + dx * ra, a.y + dy * ra, a.z + dz * ra, s * 1.5, 0)
      mesh.verts.push(b.x + dx * rb, b.y + dy * rb, b.z + dz * rb, s * 1.5, len * 0.5)
    }
    for (let i = 0; i < sides; i++) {
      const p = start + i * 2
      mesh.indices.push(p, p + 2, p + 3, p, p + 3, p + 1)
    }
  }

  /** A cone standing on a base ring, apex `height` above it (one spruce tier). */
  function cone(mesh: Mesh, base: Vec3, height: number, radius: number, sides: number): void {
    const start = mesh.verts.length / STRIDE
    mesh.verts.push(base.x, base.y + height, base.z, 0.5, 0)
    for (let i = 0; i <= sides; i++) {
      const angle = (i / sides) * TAU
      mesh.verts.push(base.x + Math.cos(angle) * radius, base.y, base.z + Math.sin(angle) * radius, (i / sides) * 2, 1)
    }
    for (let i = 0; i < sides; i++) {
      // Wound so the outer surface faces out, matching the backface-cull sign.
      mesh.indices.push(start, start + 2 + i, start + 1 + i)
    }
  }

  /** A lumpy low-poly sphere (one oak canopy blob). Per-ring radius wobble keeps
   *  it organic without cracking the longitude seam. */
  function blob(mesh: Mesh, center: Vec3, radius: number, rand: () => number, seg = 5, rings = 3): void {
    const start = mesh.verts.length / STRIDE
    for (let r = 0; r <= rings; r++) {
      const phi = (r / rings) * Math.PI
      const cy = Math.cos(phi)
      const cr = Math.sin(phi)
      const scale = radius * (0.85 + rand() * 0.3)
      for (let s = 0; s <= seg; s++) {
        const theta = (s / seg) * TAU
        mesh.verts.push(
          center.x + cr * Math.cos(theta) * scale,
          center.y + cy * scale,
          center.z + cr * Math.sin(theta) * scale,
          (s / seg) * 2,
          (r / rings) * 2,
        )
      }
    }
    const row = seg + 1
    for (let r = 0; r < rings; r++) {
      for (let s = 0; s < seg; s++) {
        const p = start + r * row + s
        mesh.indices.push(p, p + 1, p + row + 1, p, p + row + 1, p + row)
      }
    }
  }

  /** Two unit vectors spanning the plane perpendicular to `axis`. */
  function basis(axis: Vec3): [Vec3, Vec3] {
    const ref = Math.abs(axis.y) < 0.99 ? { x: 0, y: 1, z: 0 } : { x: 1, y: 0, z: 0 }
    const u = Vec3.normalize(Vec3.cross(ref, axis))
    return [u, Vec3.cross(axis, u)]
  }

  function lerp(a: number, b: number, t: number): number {
    return a + (b - a) * t
  }

  /** Deterministic 0..1 generator (mulberry32) seeded per tree. */
  function rng(seed: number): () => number {
    let a = seed >>> 0
    return () => {
      a = (a + 0x6D2B79F5) | 0
      let t = Math.imul(a ^ (a >>> 15), 1 | a)
      t ^= t + Math.imul(t ^ (t >>> 7), 61 | t)
      return ((t ^ (t >>> 14)) >>> 0) / 4294967296
    }
  }
}