InstancedMesh trees are randomly rotated and half vanish

• InstancedMesh count is 5000, yet each tree has an arbitrary orientation and scale unrelated to what was set.
• Moving the camera a few hundred metres makes an entire stand of trees disappear, and it returns when you move back.
• setColorAt colours appear only after instanceColor.needsUpdate is set.

1. Reduce count to 2 and read matrix 0 back with getMatrixAt to verify the writes.
2. Check that Matrix4.compose received a Quaternion rather than Euler angles.
3. Verify instanceMatrix.needsUpdate = true runs after the loop.
4. Compare the bounding sphere with the 800 m instance spread.

Drive a reusable Object3D (position, rotation, scale, updateMatrix) into setMatrixAt, then set instanceMatrix.needsUpdate and call computeBoundingSphere() so frustum culling covers the instance spread.

var m = new THREE.Matrix4();
mesh.getMatrixAt(0, m);
console.log('第 0 个实例平移', new THREE.Vector3().setFromMatrixPosition(m));

var treeGeo = new THREE.ConeGeometry(0.6, 3, 6);
var treeMat = new THREE.MeshStandardMaterial({ color: 0x2e7d32 });
var count = 5000;

var trees = new THREE.InstancedMesh(treeGeo, treeMat, count);
var dummy = new THREE.Object3D();

for (var i = 0; i < count; i++) {
  dummy.position.set(
    (Math.random() - 0.5) * 800,
    0,
    (Math.random() - 0.5) * 800
  );
  dummy.rotation.y = Math.random() * Math.PI * 2;
  dummy.scale.set(1, 0.8 + Math.random() * 0.6, 1);
  dummy.updateMatrix();

  trees.setMatrixAt(i, dummy.matrix);
}

trees.instanceMatrix.needsUpdate = true;
trees.computeBoundingSphere();      // 关键：按实例分布重算
scene.add(trees);

• Reusing one dummy Object3D without resetting rotation and scale each iteration makes later instances inherit the previous transform.
• Calling computeBoundingSphere before writing the matrices leaves the sphere at the base geometry size.