Trouble Understanding Vector3.Angle() Result

Questions & Answers
1

Hello everyone, I was looking at this post and tried to make my own little test script that could tell the angle between a projectile’s velocity and the surface it hit.

The problem is, the angle returned is always either 90 or a result very close to 90 (89.xxx / 90.xxx), so I either set things up wrong, or I don’t understand the result (and I should note that the upper plane is rotated 45 degrees, and the lower plane is rotated to 10 degrees).

Any clarification is appreciated.

using UnityEngine;
using System.Collections;

public class zTestProjectiles : MonoBehaviour {
	//goes on the actual projectile prefab object, 
	//makes sure object "explodes" or bounces.
	public float autoBounceAngle; //the angle at and over which a shell will always bounce
	private Rigidbody thisRigidbody;

	void Awake()
	{
		thisRigidbody = gameObject.GetComponent<Rigidbody>();
	}

	void OnCollisionEnter (Collision other)
	{
		Debug.Log (gameObject.name + " hit: " + other.gameObject.name);
		Vector3 impactNormal = other.contacts[0].normal;
		Debug.Log ("impactNormal is: " + impactNormal);
		Debug.DrawRay(other.contacts[0].point, other.contacts[0].normal, Color.red, 2.0f);

		Debug.Log ("thisRigidbody.velocity is: " + thisRigidbody.velocity);

		CalculateImpactAngle(impactNormal);
	}

	//method to determine if the shell should explode or bounce on impact:
	private void CalculateImpactAngle(Vector3 impactNormalAngle)
	{
		Debug.Log ("CalculateImpactAngle called");

		float angle = Vector3.Angle (thisRigidbody.velocity, -impactNormalAngle);
		Debug.Log ("angle is: " + angle);
	}
}

34087-testscene.jpg
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2

Try the following:

float angle = Vector3.Angle (thisRigidbody.transform.position.up, transform.somereference.up);

Make a reference object that is flat and non kinematic, like a plane, and use this as an up reference. The angle function will return the angle from the rigidbody’s up to the reference’s up in radians.

Convert radians to degrees with (180/(2*Mathf.PI))

This is off the top of my head, so sorry if it’s not what you were after!

Cheers,

Popuppirate

3

You should not use your rigidbody velocity for this. Instead use the relativeVelocity field of your Collision object, like this:

void OnCollisionEnter(Collision other) {
    float angle = Vector3.Angle(other.relativeVelocity, other.contacts[0].normal));
    // do stuff with angle
}

This will give you the angle between your projectile and the other colliders surface normal.

lg