Fix incorrect BVH (#93)

* move centroid check

* add reflecting material that tests bvh

* better comment

scenes/renderer-test/main.js

@@ -196,6 +196,32 @@ function init() {
     model.add(mesh);
   }
 
+  let unreadyMat;
+  {
+    // Create a test (non-buffer) Geometry
+    const geo = new THREE.BoxGeometry(20, 6, 6);
+    const mat = new THREE.MeshStandardMaterial();
+    mat.roughness = 0.2;
+    mat.metalness = 0.0;
+    mat.color.set(0x993311);
+    unreadyMat = mat;
+    const mesh = new THREE.Mesh(geo, mat);
+    mesh.position.set(0, 3, 30);
+    model.add(mesh);
+  }
+
+  // background mirror
+  // verifies BVH used in reflections
+  {
+    const geo = new THREE.PlaneBufferGeometry(40, 16);
+    const mat = new THREE.MeshStandardMaterial();
+    mat.roughness = 0.0;
+    mat.metalness = 1.0;
+    const mesh = new THREE.Mesh(geo, mat);
+    mesh.position.set(0, 8, 40);
+    model.add(mesh);
+  }
+
   // ground plane
   {
     const geo = new THREE.PlaneBufferGeometry(1000, 1000);
@@ -219,20 +245,6 @@ function init() {
     model.add(mesh);
   }
 
-  let unreadyMat;
-  {
-    // Create a test (non-buffer) Geometry
-    const geo = new THREE.BoxGeometry(6, 6, 6);
-    const mat = new THREE.MeshStandardMaterial();
-    mat.roughness = 0.2;
-    mat.metalness = 0.0;
-    mat.color.set(0x993311);
-    unreadyMat = mat;
-    const mesh = new THREE.Mesh(geo, mat);
-    mesh.position.set(0, 3, 30);
-    model.add(mesh);
-  }
-
   scene.add(model);
 
   THREE.DefaultLoadingManager.onLoad = () => {

src/renderer/bvhAccel.js

@@ -149,9 +149,6 @@ function recursiveBuild(primitiveInfo, start, end) {
     }
     const dim = maximumExtent(centroidBounds);
 
-    if (centroidBounds.max[dim] === centroidBounds.min[dim]) {
-      return makeLeafNode(primitiveInfo.slice(start, end), bounds);
-    }
 
     let mid = Math.floor((start + end) / 2);
 
@@ -167,7 +164,11 @@ function recursiveBuild(primitiveInfo, start, end) {
     // surface area heuristic method
     if (nPrimitives <= 4) {
       nthElement(primitiveInfo, (a, b) => a.center[dim] < b.center[dim], start, end, mid);
+    } else if (centroidBounds.max[dim] === centroidBounds.min[dim]) {
+      // can't split primitives based on centroid bounds. terminate.
+      return makeLeafNode(primitiveInfo.slice(start, end), bounds);
     } else {
+
       const buckets = [];
       for (let i = 0; i < 12; i++) {
         buckets.push({