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.