Particle collision in Unity

Introduction

Particle collision lets particles in a game hit objects or each other. This makes effects like sparks or smoke look real.

When you want sparks to bounce off walls in a game.

When smoke particles should disappear after touching the ground.

When raindrops hit the ground and create splashes.

When you want fire particles to react when hitting a character.

Syntax

Unity

using UnityEngine;

public class ParticleCollisionHandler : MonoBehaviour
{
    private ParticleSystem particleSystem;

    void Start()
    {
        particleSystem = GetComponent<ParticleSystem>();
    }

    void OnParticleCollision(GameObject other)
    {
        // Code to run when particles collide with 'other'
        Debug.Log($"Particle collided with {other.name}");
    }
}

The method OnParticleCollision is called automatically by Unity when particles hit another object.

You must have a ParticleSystem component on the same GameObject for this to work.

Examples

Basic collision detection that logs the name of the object hit.

Unity

void OnParticleCollision(GameObject other)
{
    Debug.Log($"Particle hit {other.name}");
}

Check if the particle hit an object tagged as 'Enemy' and respond.

Unity

void OnParticleCollision(GameObject other)
{
    if (other.CompareTag("Enemy"))
    {
        Debug.Log("Particle hit an enemy!");
    }
}

Ignore collisions if the particle system is not playing.

Unity

void OnParticleCollision(GameObject other)
{
    // No collision if particle system is stopped
    if (!particleSystem.isPlaying) return;
    Debug.Log($"Particle collided with {other.name}");
}

Sample Program

This script logs a message when the particle system starts and whenever a particle hits another object. Attach it to a GameObject with a ParticleSystem and enable collision in the ParticleSystem settings.

Unity

using UnityEngine;

public class ParticleCollisionExample : MonoBehaviour
{
    private ParticleSystem particleSystem;

    void Start()
    {
        particleSystem = GetComponent<ParticleSystem>();
        Debug.Log("Particle system started.");
    }

    void OnParticleCollision(GameObject other)
    {
        Debug.Log($"Particle collided with {other.name}");
    }
}

Important Notes

Time complexity: Particle collision checks run every frame and depend on the number of particles and colliders.

Space complexity: Minimal extra memory is used for collision events.

Common mistake: Forgetting to enable 'Collision' in the ParticleSystem's settings, so OnParticleCollision never triggers.

Use particle collision when you want particles to interact with the environment. For simple visual effects without interaction, you can skip collision to save performance.

Summary

Particle collision lets particles detect and react when they hit objects.

Use OnParticleCollision(GameObject other) method to handle collisions.

Remember to enable collision in the ParticleSystem component for this to work.

Practice

1. What is the main purpose of the OnParticleCollision method in Unity's particle system?

easy

A. To stop the particle system from playing

B. To change the color of particles over time

C. To emit new particles continuously

D. To detect when particles collide with other objects

Solution

1. Step 1: Understand the role of OnParticleCollision

   This method is a special Unity callback triggered when particles hit other objects in the scene.

2. Step 2: Identify its main use

   It is used to detect collisions of particles, allowing you to respond to those events in code.

3. Final Answer:

   To detect when particles collide with other objects -> Option D

4. Quick Check:

   Particle collision detection = To detect when particles collide with other objects [OK]

Hint: Remember: OnParticleCollision detects particle hits [OK]

Common Mistakes:

• Confusing particle emission with collision detection
• Thinking it changes particle appearance
• Assuming it stops the particle system

2. Which of the following is the correct signature for the OnParticleCollision method in a Unity C# script?

easy

A. void OnParticleCollision(GameObject other)

B. void OnParticleCollision(Collider other)

C. void OnParticleCollision(Particle other)

D. void OnParticleCollision()

Solution

1. Step 1: Recall Unity's method signature

   The OnParticleCollision method receives a GameObject parameter representing the object hit by particles.

2. Step 2: Match the correct parameter type

   Only void OnParticleCollision(GameObject other) uses GameObject as the parameter, which is correct.

3. Final Answer:

   void OnParticleCollision(GameObject other) -> Option A

4. Quick Check:

   Parameter type is GameObject = void OnParticleCollision(GameObject other) [OK]

Hint: OnParticleCollision always takes GameObject parameter [OK]

Common Mistakes:

• Using Collider instead of GameObject
• Omitting the parameter
• Using Particle type which doesn't exist

3. Consider this Unity C# script attached to a GameObject with a Particle System:

void OnParticleCollision(GameObject other) {
    Debug.Log("Hit: " + other.name);
}

What will happen when particles collide with another object named "Wall"?

medium

A. The console will print: Hit: Wall

B. The particle system will stop emitting

C. Nothing will happen because OnParticleCollision is not called

D. An error will occur because other.name is invalid

Solution

1. Step 1: Understand the method behavior

   When particles hit an object, OnParticleCollision is called with that object as other.

2. Step 2: Analyze the Debug.Log statement

   The code prints "Hit: " plus the name of the collided object, which is "Wall".

3. Final Answer:

   The console will print: Hit: Wall -> Option A

4. Quick Check:

   Collision triggers log with object name = The console will print: Hit: Wall [OK]

Hint: OnParticleCollision logs object name on hit [OK]

Common Mistakes:

• Assuming particle system stops on collision
• Thinking method is never called
• Believing other.name is invalid

4. Given this code snippet in a Unity script:

void OnParticleCollision(GameObject other) {
    int count = other.GetComponent<int>();
    Debug.Log(count);
}

What is the problem with this code?

medium

A. OnParticleCollision must return a value

B. GetComponent<int>() is invalid because int is not a component

C. The method should use Collider instead of GameObject

D. Debug.Log cannot print integers

Solution

1. Step 1: Check GetComponent usage

   GetComponent<T> expects a Component type, but int is a primitive type, not a component.

2. Step 2: Identify the error cause

   Using GetComponent<int>() causes a compile-time error because int is invalid here.

3. Final Answer:

   GetComponent<int>() is invalid because int is not a component -> Option B

4. Quick Check:

   GetComponent requires Component type = GetComponent<int>() is invalid because int is not a component [OK]

Hint: GetComponent needs a Component type, not primitives [OK]

Common Mistakes:

• Thinking OnParticleCollision must return a value
• Confusing parameter type with Collider
• Believing Debug.Log can't print integers

5. You want to reduce the damage caused by particles when they collide with enemies, but only if the enemy's health is above 50. Which approach correctly uses OnParticleCollision to achieve this?

hard

A. Check the particle system's emission rate and reduce damage if emission is low.

B. Use OnParticleCollision() without parameters and reduce damage always.

C. In OnParticleCollision(GameObject other), get the enemy's health component, check if health > 50, then reduce damage accordingly.

D. Use OnParticleCollision(Collider other) and reduce damage without checking health.

Solution

1. Step 1: Use correct method signature

   The method must be OnParticleCollision(GameObject other) to get the collided object.

2. Step 2: Access enemy health and apply condition

   Retrieve the health component from other, check if health > 50, then reduce damage only in that case.

3. Final Answer:

   In OnParticleCollision(GameObject other), get the enemy's health component, check if health > 50, then reduce damage accordingly. -> Option C

4. Quick Check:

   Conditional damage based on health = In OnParticleCollision(GameObject other), get the enemy's health component, check if health > 50, then reduce damage accordingly. [OK]

Hint: Check enemy health inside OnParticleCollision before damage [OK]

Common Mistakes:

• Using wrong method signature without parameters
• Ignoring enemy health condition
• Using Collider parameter instead of GameObject