3D colliders in Unity - Step-by-Step Execution

Concept Flow - 3D colliders

Start: GameObject with Collider

↓

Physics Engine checks collisions

↓

Is Collider Trigger?

Yes→Trigger Event

↓

OnTriggerEnter()

↓

Collision Event

↓

OnCollisionEnter() called

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Respond to collision

↓

End

This flow shows how Unity checks 3D colliders on objects, decides if they trigger events or collisions, and calls the right functions.

Execution Sample

Unity

void OnCollisionEnter(Collision collision) {
    Debug.Log("Hit " + collision.gameObject.name);
}

void OnTriggerEnter(Collider other) {
    Debug.Log("Triggered by " + other.gameObject.name);
}

This code runs when a 3D collider hits another or enters a trigger, printing the other object's name.

Execution Table

Step	Event Type	Collider State	Function Called	Output
1	Collision detected	IsTrigger = false	OnCollisionEnter	Hit Enemy
2	Trigger detected	IsTrigger = true	OnTriggerEnter	Triggered by PowerUp
3	Collision detected	IsTrigger = false	OnCollisionEnter	Hit Wall
4	No collision or trigger	-	-	-

💡 No more collisions or triggers detected, so no functions called.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
collision.gameObject.name	-	Enemy	-	Wall	Wall
other.gameObject.name	-	-	PowerUp	-	PowerUp
Collider.IsTrigger	false	false	true	false	false

Key Moments - 2 Insights

Why does OnTriggerEnter run instead of OnCollisionEnter sometimes?

Because the collider's IsTrigger property is true, Unity calls OnTriggerEnter (see execution_table step 2). If IsTrigger is false, OnCollisionEnter runs (steps 1 and 3).

What happens if two colliders both have IsTrigger false and collide?

OnCollisionEnter is called on both objects involved in the collision, as shown in steps 1 and 3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what function is called at step 2?

A Update

B OnCollisionEnter

C OnTriggerEnter

D Start

💡 Hint

Check the 'Function Called' column at step 2 in the execution_table.

At which step does the collider have IsTrigger set to true?

A Step 2

B Step 1

C Step 3

D No step

💡 Hint

Look at the 'Collider State' column in the execution_table and variable_tracker.

If the collider's IsTrigger was false at step 2, what would happen?

A OnTriggerEnter would still be called

B OnCollisionEnter would be called instead

C No function would be called

D The game would crash

💡 Hint

Refer to the key_moments explanation about IsTrigger property and function calls.

Concept Snapshot

3D Colliders in Unity detect physical contact.
If IsTrigger is false, OnCollisionEnter runs on contact.
If IsTrigger is true, OnTriggerEnter runs when overlapping.
Use these to respond to hits or triggers in your game.
Set IsTrigger in the Collider component in Inspector.

Full Transcript

In Unity, 3D colliders are components that detect when objects touch or overlap. When two colliders meet, Unity checks if either is set as a trigger. If a collider's IsTrigger property is true, Unity calls OnTriggerEnter to handle the event. If IsTrigger is false, Unity calls OnCollisionEnter instead. This lets you decide if you want physical collisions or just trigger events. The example code shows how to print the name of the other object when these events happen. The execution table traces these events step by step, showing which function runs depending on the collider's trigger state. Understanding this helps you control game interactions clearly and avoid confusion about which event runs.