OnCollisionStay Vs OnCollisionEnter

Question

Hi I've been using Unity for a while and for the life of me I can still not tell the difference between OnCollisionStay and OnCollisionEnter. I'm assuming that OncollisionEnter is called when a rigidbody (like a ball) makes contact with another rigidbody/collider such as a wall. But when I look at the example for OnCollisionStay, I get completely confused. Even if its called once per frame, If I for example jump up in the air and hit a ceiling above me, is it still called? What exactly are the differences? Here is what the Unity documentation says and the code I was using.

"OnCollisionEnter is called when this collider/rigidbody has begun touching another rigidbody/collider."

"OnCollisionStay is called once per frame for every collider/rigidbody that is touching rigidbody/collider."

if(Input.GetKeyDown(KeyCode.W) && OnGround == True)
{
    rigidbody.velocity.y = jumpHeight;
}

onGround = false;

function OnCollisionStay()
{
    onGround = true;
}

Answer 1

It is really pretty simple. Let's assume that the collision starts, the colliders keep intersecting for a period of time (several frames), and after a while they separate again. The events then are:

• OnCollisionEnter for the first frame only, and never again until after an OnCollisionExit
• OnCollisionStay for the entire duration (all frames)
• OnCollisionExit for the last frame only

Even if its called once per frame, If I for example jump up in the air and hit a ceiling above me, is it still called?

Sure it is. But then for that new collision. If there is a period in between where you're not colliding with anything however, you will not get OnCollisionStay events for that duration.

To get a deeper understanding of these events, it's perhaps best to create a simple demo scene in which you manually intersect two colliders, and have some text written to the console for the various events.

Answer 2

Bart's answer is correct so this answer is more of an example of how you would use the two.

So OnCollisionEnter will fire 1 per collision case lets say for example a ball hitting a wall and OnCollisionEnter could maybe do some damage to the wall or destroy the wall. (Like breakout)

//Ball
#pragma strict

function OnCollisionEnter (col : Collision)
{
    if(col.gameObject.name == "Wall")
    {
        Destroy(col.gameObject);
    }
}

However in OnCollisionStay you would use that if you have a character with hit points that steps into a poison cloud and removes continuous hit points or removes incremental timed hit points as long as OnCollisionStay continues to fire after a given amount of time. (I'm thinking castlevania SOTH)

//PoisonGasCloud
#pragma strict
function OnCollisionStay(collisionInfo : Collision)
{

    collisionTime = Time.deltaTime;

    if ( collisionTime > 5)
    {
        hero.hp-=10;
    }
}

Note:I usually code in C# so my UnityScript might be a little off.

Answer 3

• When collision start: OnCollisionEnter & OnCollisionStay
• When collision continue: OnCollisionStay
• When collision end: OnCollisionExit

Note 1: OnCollisionStay & OnCollisionExit will not be called simultaneously.

Note 2:OnCollisionEnter call before OnCollisionStay.