Velocity relative to Local Axis

Question

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

Best Answer

Answer by aldonaletto · Dec 08, 2011 at 11:36 PM

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;

Add comment · Show 7 · Share

Oninji · Dec 09, 2011 at 12:06 AM 0

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

aldonaletto · Dec 09, 2011 at 02:34 AM 1

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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? $$anonymous$$odify the direction without changing the magnitude? Or keep the direction and modify the magnitude? Or none of above?

Oninji · Dec 10, 2011 at 11:47 PM 0

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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 $$anonymous$$ovSpeed 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 ($$anonymous$$ovSpeed) to the x and z velocity. But I'm not sure it's the best way to accomplish this.

aldonaletto · Dec 11, 2011 at 01:40 AM 3

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

   rigidbody.velocity = transform.forward * $$anonymous$$ovSpeed;

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.

Oninji · Dec 12, 2011 at 02:04 AM 0

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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.

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Velocity relative to Local Axis

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