fix: per-species scale correction for skinned mesh fish

Box3.setFromObject measures skinned geometry in skeleton space (and may
use a stale cached bounding box), which does not match the GPU's actual
rendered extent (mesh.matrixWorld * skinning * position). Add a
per-species baseScale correction factor (rendered / box3) to FishSpec and
fold it into buildFish's scale calc so each GLB normalizes to its true
size.

Measured per GLB with three.js's exact skinning formula: most models
are 1.0. The skinned blendswap oscar (1.35) and knifefish (2.58) deviate
and are corrected. The clownfish GLB is a full scene (water 'Surface'
45.3, 'TopLight' 15, 'Bubbles' 3.4) plus a small skinned body (~8.3);
box3 is dominated by the water plane, so a baseScale can't fix it without
blowing up the scene geometry - left at 1.0 pending a clean re-export.

src/main.ts

@@ -60,6 +60,31 @@ function getSpeciesUrl(sp: FishSpecies): string {
   };
   return urls[sp];
 }
+// Per-species correction factor between Box3.setFromObject's measured extent
+// and the model's true rendered extent (see FishSpec.baseScale). Box3 measures
+// skinned geometry in skeleton space (and may use a stale cached bounding box),
+// which does not match the GPU's actual output (mesh.matrixWorld * skinning *
+// position). baseScale = rendered / box3, so `targetLength / (box3 * baseScale)`
+// normalizes each fish to its real rendered size.
+//
+// Measured with three.js's exact skinning formula per GLB. Most models are 1.0.
+// The skinned blendswap oscar/knifefish deviate and are corrected here. The
+// clownfish GLB is a full *scene* (water "Surface" 45.3, "TopLight" 15, "Bubbles"
+// 3.4) plus a small skinned body (~8.3); box3 is dominated by the water plane, so
+// a baseScale can't fix it without blowing up the scene geometry — left at 1.0
+// until the GLB is re-exported as a clean fish.
+const SPECIES_SCALE: Record<FishSpecies, number> = {
+  angelfish: 1.0,
+  goldfish: 1.0,
+  guppy: 1.0,
+  betta: 1.0,
+  blue_tang: 1.0,
+  flower_horn: 1.0,
+  neon_tetra: 1.0,
+  oscar: 1.35,
+  knifefish: 2.58,
+  clownfish: 1.0,
+};
 function fishToSpec(f: FishData): FishSpec {
   return {
     url: getSpeciesUrl(f.species),
@@ -67,6 +92,7 @@ function fishToSpec(f: FishData): FishSpec {
     speed: [0.3 * f.genetics.speedMult, 0.7 * f.genetics.speedMult],
     orient: isQuaternius(f.species) ? { y: Math.PI / 2 } : undefined,
     genetics: f.genetics,
+    baseScale: SPECIES_SCALE[f.species],
   };
 }
 

src/scene/GLTFish.ts

@@ -20,6 +20,16 @@ export interface FishSpec {
   orient?: { x?: number; y?: number; z?: number };
   speed?: Range;
   genetics?: Genetics;
+  /**
+   * Correction factor between the extent reported by `Box3.setFromObject`
+   * and the fish's true rendered extent. `Box3.setFromObject` measures
+   * skinned geometry in skeleton space (and may use a stale cached
+   * bounding box), which does not match what the GPU actually renders
+   * (`mesh.matrixWorld * skinning * position`). baseScale = rendered / box3,
+   * so the final scale becomes `targetLength / (box3 * baseScale) =
+   * targetLength / rendered`, normalizing every fish to its real size.
+   */
+  baseScale?: number;
 }
 
 // ---- Helpers ----
@@ -132,7 +142,8 @@ function buildFish(gltf: GLTF, spec: FishSpec): Fish {
   const box = new THREE.Box3().setFromObject(model);
   const size = box.getSize(new THREE.Vector3());
   const longest = Math.max(size.x, size.y, size.z);
-  const scale = (spec.targetLength / longest) * rand(0.92, 1.08);
+  const baseScale = spec.baseScale ?? 1.0;
+  const scale = (spec.targetLength / (longest * baseScale)) * rand(0.92, 1.08);
   orient.scale.setScalar(scale);
   orient.add(model);