Particle collision in Unity - Step-by-Step Execution

Concept Flow - Particle collision

Particle moves

↓

Check collision with other particles or objects

Yes↓

Calculate collision response

↓

Update particle velocity and position

↓

Continue simulation loop

No↓

Continue moving particle

The particle moves, checks for collisions, responds by updating velocity and position, then continues moving.

Execution Sample

Unity

void OnParticleCollision(GameObject other) {
    Vector3 normal = (transform.position - other.transform.position).normalized;
    velocity = Vector3.Reflect(velocity, normal);
}

This code detects collision and reflects the particle's velocity based on the collision normal.

Execution Table

Step	Particle Position	Other Object Position	Collision Detected?	Collision Normal	Velocity Before	Velocity After	Action
1	(0,0,0)	(1,0,0)	No	-	(1,0,0)	(1,0,0)	Particle moves forward
2	(0.5,0,0)	(1,0,0)	No	-	(1,0,0)	(1,0,0)	Particle moves forward
3	(1,0,0)	(1,0,0)	Yes	(-1,0,0)	(1,0,0)	(-1,0,0)	Reflect velocity on collision
4	(0.9,0,0)	(1,0,0)	No	-	(-1,0,0)	(-1,0,0)	Particle moves backward
Exit	-	-	No	-	-	-	No more collisions, simulation continues

💡 Particle no longer collides, simulation continues with updated velocity.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Particle Position	(0,0,0)	(0.5,0,0)	(1,0,0)	(1,0,0)	(0.9,0,0)	(0.9,0,0)
Velocity	(1,0,0)	(1,0,0)	(1,0,0)	(-1,0,0)	(-1,0,0)	(-1,0,0)
Collision Detected	No	No	Yes	No	No	No

Key Moments - 3 Insights

Why does the velocity change direction after collision?

Because the velocity is reflected using the collision normal vector, as shown in step 3 of the execution_table where velocity changes from (1,0,0) to (-1,0,0).

Why is there no collision detected at step 2 even though the particle is close to the other object?

Collision is detected only when positions overlap or intersect exactly, which happens at step 3. Step 2 position (0.5,0,0) is not colliding yet.

What happens if we don't update the velocity after collision?

The particle would continue moving in the same direction and pass through the object, ignoring the collision, unlike step 3 where velocity is updated to reflect the collision.

Visual Quiz - 3 Questions

Look at the execution_table, what is the velocity after step 3?

A (1,0,0)

B (-1,0,0)

C (0,1,0)

D (0,0,1)

💡 Hint

Check the 'Velocity After' column at step 3 in the execution_table.

At which step does the collision get detected?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Collision Detected?' column in the execution_table.

If the particle velocity was not reflected on collision, what would happen at step 4?

A Particle would continue moving forward

B Particle would stop moving

C Particle would move backward

D Particle would teleport

💡 Hint

Refer to the 'Velocity After' and 'Action' columns in the execution_table for step 3 and 4.

Concept Snapshot

Particle collision in Unity:
- Detect collision using OnParticleCollision.
- Calculate collision normal vector.
- Reflect velocity using Vector3.Reflect.
- Update particle velocity and position.
- Continue simulation with new velocity.

Full Transcript

In Unity, particle collision happens when a particle moves and hits another object. The system checks if the particle's position overlaps with another object's position. When a collision is detected, the particle's velocity is reflected based on the collision normal vector, which changes its direction. This reflection is done using Vector3.Reflect. After updating velocity, the particle continues moving with the new velocity. The execution table shows step-by-step positions, collision checks, and velocity changes. Key moments clarify why velocity changes and when collisions are detected. The visual quiz tests understanding of velocity changes and collision detection steps.

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