【VRChat】Optimization with VRCObjectPool: Managing Bullet and Effect Creation

Overview

Gimmicks where you rapid-fire a gun to shoot lots of bullets, or use magic to scatter flashy particles, are elements that make worlds fun. However, processing that creates (Instantiate) and destroys (Destroy) objects many times in a short period like this causes serious performance degradation in Unity, especially in the VRChat environment.

Object Pooling is a classic and very effective technique for solving this problem. The VRChat SDK provides a dedicated component called VRCObjectPool that makes this mechanism easy to use.

This article explains how to combine the VRCObjectPool component with UdonSharp to implement performance-excellent gimmicks.

How Object Pooling Works

Object pooling is based on the following concept:

1. Preparation (Pooling): At world start, Instantiate as many objects as you'll need (e.g., 100 bullets) in advance, hide them all (SetActive(false)), and store them in a "pool."
2. Retrieval (Spawning): When an object is needed (e.g., when firing a gun), instead of Destroy & Instantiate, take out one standby object from the pool and display it (SetActive(true)) for use.
3. Return (Returning): When an object is no longer needed (e.g., when a bullet hits a wall), instead of Destroy, hide it again and return it to the pool to wait for its next turn.

This prevents the high-load Instantiate and Destroy calls from occurring at arbitrary times during world execution, concentrating the load at initial load time.

Step 1: Setting Up the VRCObjectPool Component

1. Create Pool Management Object: Create an empty GameObject in the scene and name it something like "BulletPool."
2. Add VRCObjectPool: Add the VRCObjectPool component to the "BulletPool" object.
3. Place Pooled Objects:
   • Place as many as needed of the objects you want to pool (e.g., bullets) in the scene (e.g., 100).
   • Attach UdonSharp scripts that control bullet behavior (described below), Rigidbody, Collider, etc. to these objects.
   • Set all objects to inactive (SetActive(false)) from the start.
4. Configure VRCObjectPool:
   • Pool: Drag & drop all the bullet objects placed in the scene to register them.

Important: VRCObjectPool is network synchronized, and only the pool object's owner can call TryToSpawn() or Return(). If other players want to spawn, they need to acquire ownership first with Networking.SetOwner().

Step 2: Script for Pooled Objects

A script that defines how bullets retrieved from the pool behave and how they return to the pool.

Script: PooledBullet.cs

using UdonSharp;
using UnityEngine;

public class PooledBullet : UdonSharpBehaviour
{
    [Tooltip("VRCObjectPool this object belongs to")]
    public VRCObjectPool pool;
    public float lifeTime = 5.0f; // Auto-return to pool after 5 seconds
    public float speed = 10.0f;

    private Rigidbody rb;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }

    // Called when object is retrieved from pool and enabled
    void OnEnable()
    {
        // Reset life timer and schedule return to pool after specified time
        SendCustomEventDelayedSeconds(nameof(ReturnToPool), lifeTime);
        // Apply force in forward direction
        if (rb != null)
        {
            rb.velocity = transform.forward * speed;
        }
    }

    // Called when colliding with walls, etc.
    void OnCollisionEnter(Collision collision)
    {
        // Write processing here for hit effects, etc.

        // Return to pool
        ReturnToPool();
    }

    // Method to return object to pool
    public void ReturnToPool()
    {
        if (pool != null)
        {
            pool.Return(this.gameObject);
        }
        else
        {
            // If pool is not set, simply hide
            this.gameObject.SetActive(false);
        }
    }
}

Key Point: This script needs to be attached to the bullet Prefab, and the "BulletPool" object in the scene needs to be assigned to the Pool field.

Step 3: Script for the Object Spawner (Gun, etc.)

A script attached to the player-operated gun that handles retrieving bullets from the pool and firing them.

Script: ObjectPoolGun.cs

using UdonSharp;
using UnityEngine;
using VRC.SDKBase;
using VRC.SDK3.Components;

public class ObjectPoolGun : UdonSharpBehaviour
{
    [Tooltip("VRCObjectPool for bullets")]
    public VRCObjectPool bulletPool;
    [Tooltip("Transform serving as the muzzle")]
    public Transform muzzle;

    // When Use button is pressed while holding the gun
    public override void OnPickupUseDown()
    {
        // Confirm you are the owner of this gun
        if (!Networking.IsOwner(this.gameObject)) return;

        // Acquire pool ownership (required to call TryToSpawn)
        Networking.SetOwner(Networking.LocalPlayer, bulletPool.gameObject);

        // Attempt to retrieve one object from the pool
        GameObject bullet = bulletPool.TryToSpawn();

        // If an object was retrieved from the pool
        if (bullet != null)
        {
            // Set bullet position and orientation to match the muzzle
            bullet.transform.SetPositionAndRotation(muzzle.position, muzzle.rotation);
            // This calls the bullet's OnEnable, and the bullet flies
        }
        else
        {
            // If pool was empty
            Debug.LogWarning("Out of ammo! No available objects in pool.");
        }
    }
}

Final Unity Setup

1. Set up VRCPickup, Udon Behaviour, etc. on the gun model.
2. Assign ObjectPoolGun.cs to the gun's Udon Behaviour.
3. Assign the "BulletPool" object in the scene to the Bullet Pool field, and an empty GameObject serving as the muzzle to the Muzzle field.

Summary

• Frequent object creation/destruction is high-load, so using object pooling for reuse is the standard for performance optimization.
• In VRChat, use the VRCObjectPool component.
• Pool Manager (VRCObjectPool): Register objects pre-placed in the scene to the Pool array.
• Spawner (gun, etc.): First acquire pool ownership with Networking.SetOwner(), then retrieve objects with pool.TryToSpawn(). Consider the case where the return value is null (out of ammo).
• Pooled Object (bullet, etc.): Perform initialization processing (velocity settings, etc.) in OnEnable, and call pool.Return(gameObject) in OnCollisionEnter or with a timer to return itself to the pool.

Object pooling may seem complex at first glance, but once you understand the mechanism, it's a very powerful technique that can be applied to various gimmicks. It's essential knowledge especially when implementing particle or shooting-type gimmicks.