feat: birch

engine/scene/Tree.ts

@@ -7,7 +7,7 @@ const TAU = Math.PI * 2
  *  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"
+  kind: "oak" | "spruce" | "birch"
   /** Trunk base, sitting on the ground. */
   position: Vec3
   growth: number
@@ -34,8 +34,10 @@ export namespace Tree {
     const rand = rng(tree.seed)
     if (tree.kind === "oak") {
       oak(tree.position, tree.growth, rand, trunk, foliage, lod)
-    } else {
+    } else if (tree.kind === "spruce") {
       spruce(tree.position, tree.growth, rand, trunk, foliage, lod)
+    } else {
+      birch(tree.position, tree.growth, rand, trunk, foliage, lod)
     }
   }
 
@@ -101,6 +103,48 @@ export namespace Tree {
     }
   }
 
+  function birch(base: Vec3, g: number, rand: () => number, trunk: Mesh, leaves: Mesh, lod: "full" | "impostor"): void {
+    // Silver birch: tall, slender, near-straight white trunk under an airy, high,
+    // slightly drooping canopy of small leaf tufts -- a lean silhouette between the
+    // broad oak and the conical spruce (the white bark texture does the rest).
+    const h = lerp(1, 8.5, g)
+    const rTrunk = lerp(0.03, 0.16, g)
+    const canopyY = base.y + h * 0.75
+    const blobR = h * 0.22
+    if (lod === "impostor") {
+      limb(trunk, base, { x: base.x, y: base.y + h * 0.9, z: base.z }, rTrunk, rTrunk * 0.5, 3)
+      blob(leaves, { x: base.x, y: canopyY, z: base.z }, blobR * 1.1, rand, 4, 2)
+      return
+    }
+    limb(trunk, base, { x: base.x, y: base.y + h * 0.88, z: base.z }, rTrunk, rTrunk * 0.35, 5)
+
+    const spread = h * 0.22
+    // Sparse small blobs clustered high, biased downward so the crown droops.
+    const blobs = 2 + 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() - 0.6) * spread,
+        z: base.z + Math.sin(angle) * rad,
+      }
+      blob(leaves, center, blobR * (0.7 + rand() * 0.4), rand)
+    }
+    // Grown birches trail a few thin, near-horizontal drooping twigs.
+    if (g > 0.5) {
+      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: 0.6, z: Math.sin(angle) })
+        const start = { x: base.x, y: base.y + h * 0.7, z: base.z }
+        const end = Vec3.add(start, Vec3.scale(dir, h * 0.22))
+        limb(trunk, start, end, rTrunk * 0.4, rTrunk * 0.15, 4)
+        blob(leaves, end, blobR * 0.55, rand)
+      }
+    }
+  }
+
   /** 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))