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Greetings,

Can I add velocity to an object relative to it's local axis?

This is the snippet of code:

rigidbody.velocity.z = MovSpeed;

But it's relative to the world, I want it to be relative to the local object.

Here is the full script.

private var lookRotationPoint = Vector3;

public var MovSpeed : float = 10.00;

    function Update()
    {        
        var ray : Ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        var hit : RaycastHit;
        if (Physics.Raycast(ray, hit))
        {
            if (hit.collider.gameObject.tag == "Plane")
            {
                var lookRotationPoint = hit.point - transform.position;
                transform.rotation = Quaternion.LookRotation(lookRotationPoint.normalized);
                transform.rotation = Quaternion.Euler(0, transform.rotation.eulerAngles.y, 0);
            }

         if(Input.GetMouseButton(1))
         {
              "Code goes here"
         }
         if(!Input.GetMouseButton(1))
                    {
         rigidbody.velocity = Vector3(0,0,0);
         }
        }    
    }

Answer 1

0

Convert the current velocity to local space, modify the z component and convert it back to world space:

  var locVel = transform.InverseTransformDirection(rigidbody.velocity);
  locVel.z = MovSpeed;
  rigidbody.velocity = transform.TransformDirection(locVel);

But if you just want to add/subtract a velocity in the forward direction, use transform.forward:

  rigidbody.velocity += transform.forward * MovSpeed;

answered Dec 08 '11 at 11:36 PM

aldonaletto
41.4k ● 16 ● 42 ● 197

It works, but halfly. The velocity total doesn't follow the fix value and provoke a slingshot effect.

Dec 09 '11 at 12:06 AM Oninji

You're modifying only the z local component, what alters the magnitude and the direction; furthermore, transform.forward and rigidbody.velocity may have completely different directions - thus local space (a transform related concept) may not be what you're looking for.
What exactly do you want to do? Modify the direction without changing the magnitude? Or keep the direction and modify the magnitude? Or none of above?

Dec 09 '11 at 02:34 AM aldonaletto

I want to move the object on a constant pace toward it's own rotation. Thus I want to control velocity through the script and keep it's x,z velocity equal to the MovSpeed variable.

I also thought of translating the Y rotation degree into X,Z distribution value (ie 90Y rotation = 0.5x + 0.5z), the distribute the total velocity wanted (MovSpeed) to the x and z velocity. But I'm not sure it's the best way to accomplish this.

Dec 10 '11 at 11:47 PM Oninji

I suppose that this may be what you're looking for:

  rigidbody.velocity = transform.forward * MovSpeed;

If you rotate the object, transform.forward follow the direction change, thus this should do what you want. It's advisable to set rigidbody.freezeRotation to true - the object still can be rotated, but physics rotational effects will be disabled, avoiding weird reactions to collisions.

Dec 11 '11 at 01:40 AM aldonaletto

Yes, it behaves exactly how I wanted. Better then my patch-up where I've translated Y rotation to X,Z using Cos and Sin maths.

Erf, and just saw I've used a similar code in an old project, just didn't saw the relation.

Anyway, thank you very much sir.

Dec 12 '11 at 02:04 AM Oninji

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Velocity relative to Local Axis

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