InstancedMesh

• count is fixed at construction and cannot be changed later; allocate the maximum up front.
• instanceMatrix holds 16 floats per instance and defaults to DynamicDrawUsage.
• setColorAt() creates instanceColor on first call and requires material support.

const geometry = new THREE.BoxGeometry(8, 1, 8);
const material = new THREE.MeshStandardMaterial({ color: 0xaeb8c2 });

const count = buildings.length;
const mesh = new THREE.InstancedMesh(geometry, material, count);

const matrix = new THREE.Matrix4();
const color = new THREE.Color();

buildings.forEach(function (b, i) {
  matrix.compose(
    new THREE.Vector3(b.x, b.height / 2, b.z),
    new THREE.Quaternion(),
    new THREE.Vector3(1, b.height, 1)
  );
  mesh.setMatrixAt(i, matrix);

  color.setHSL(0.58, 0.18, 0.55 + (b.height % 10) * 0.01);
  mesh.setColorAt(i, color);
});

mesh.instanceMatrix.needsUpdate = true;
if (mesh.instanceColor) {
  mesh.instanceColor.needsUpdate = true;
}
mesh.castShadow = true;
mesh.receiveShadow = true;
mesh.computeBoundingSphere();
scene.add(mesh);

• Unwritten instance slots keep identity matrices and render stacked at the origin.
• Instancing saves draw calls, not memory; a million instances still consumes large matrix buffers.