utils.js

import * as THREE from 'three';
export const createSnowFlakesMesh = (geometry, material) => {
  const count = 200;
  const mesh = new THREE.InstancedMesh(geometry, material, count);
  mesh.castShadow = true;
  mesh.receiveShadow = true;

  for (let i = 0; i < count; i++) {
    const radius = Math.random();
    const coords = new THREE.Vector3().randomDirection().multiplyScalar(16);
    coords.y = Math.abs(coords.y);
    const matrix = new THREE.Matrix4();
    matrix.scale(new THREE.Vector3(radius, radius, radius))
    matrix.setPosition(new THREE.Vector3(...coords));
    mesh.setMatrixAt(i, matrix);
  }

  mesh.instanceMatrix.needsUpdate = true;

  mesh.userData.update = (delta) => {
    for (let i = 0; i < count; i++) {
      const matrix = new THREE.Matrix4();
      mesh.getMatrixAt(i, matrix);
      const dummy = new THREE.Object3D();
      matrix.decompose(dummy.position, dummy.rotation, dummy.scale);

      const shakeDirection = new THREE.Vector3().randomDirection().multiplyScalar(2 * delta);
      shakeDirection.y = delta * dummy.scale.x * -2;
      dummy.position.add(shakeDirection);
      dummy.rotateY(Math.random() * .1);

      const coords = new THREE.Vector3().randomDirection().multiplyScalar(16);
      coords.y = Math.abs(coords.y);
      if (dummy.position.y + dummy.scale.y / 2 < 0) {
        dummy.position.set(...coords);
      }
      dummy.updateMatrix()
      mesh.setMatrixAt(i, dummy.matrix);
    }
    mesh.instanceMatrix.needsUpdate = true;
  }

  return mesh;
}