Calculating the difference between forward direction and velocity direction to set the body roll of my vehicle.

Question

Hi all,

As a relatively new Unity developer I'm having trouble coming up with a solution, possibly due to my vague understanding of the relationship between quaternions and euler angles.

Adding body roll to the model of my vehicle by calculating the difference in y angle between the angle the vehicle is facing and the direction of velocity/momentum. By adding it to the model instead of the parent/controlling object I was hoping to avoid over-complicating the steering process.

Currently my pseudo-code for this: (Code at its most basic level, disregarding euler to quaternion.)

 rollAngle = ( parent.forward.y - parent.rigidbody.velocity.y ) / 2; 
 
 transform.eulerAngle = new Vector3 (0, 0, rollAngle);

Should mean that when the direction of the vehicle is perpendicular to its momentum on the y-axis it will produce a 45 degree body roll.

alt text

So to sum up, I'm looking to take the y rotation of my object and subtract the y rotation of its velocity divide the value by two and then use it to add body roll to my object on the z axis. I think I need to convert both to eulerAngles and subtract them that way, but I haven't yet discovered a way to do so.

Thanks for taking the time to help me out!

Answer 1

Answer by Glurth · Oct 22, 2015 at 03:46 PM

Your code looks like its does ALMOST exactly this: "To to sum up, I'm looking to take the y rotation of my object and subtract the y rotation of its velocity divide the value by two and then use it to add body roll to my object on the z axis. "

With the one exception: you are REPLACING the rotation of the object with this, rather than adding to it.

  transform.eulerAngle = new Vector3 (0, 0, rollAngle);

If you wish to rotate the existing orientation, by that angle try something likethis:

  transform.rotation = transform.rotation * Quaternion.Euler(0, 0, rollAngle);

(it uses this operator to combine the rotatations: http://docs.unity3d.com/ScriptReference/Quaternion-operator_multiply.html)

Edit: note-the above operator "adds" the second Quaternion to the First. If you wish to reverse the rotation of the second Quaternion to do a "subtraction", use http://docs.unity3d.com/ScriptReference/Quaternion.Inverse.html ,like so:

  transform.rotation = transform.rotation * Quaternion.Inverse(Quaternion.Euler(0, 0, rollAngle));

(silly in this case, because you could have just used rollAngle * -1 to begin with)

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LewisGerschwitz · Oct 22, 2015 at 04:48 PM 0

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Thank you for the tip, but I feel like you haven't fully understood what I'm trying to do.

My issue isn't with adding the rollAngle to my object, it's with calculating the rollAngle.

Glurth · Oct 22, 2015 at 05:14 PM 1

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I am indeed NOT quite clear on what the problem is.

Is this the part you are having trouble with: " calculating the difference in y angle between the angle the vehicle is facing and the direction of velocity/momentum."?

If so, you should be able to use http://docs.unity3d.com/ScriptReference/Vector3.Angle.html, which takes two vectors, and returns the angle between them. Sounds like you might want to trim the input vectors and set their Z coordinate to 0, assuming you only want the angle on the XY plane.

LewisGerschwitz Glurth · Oct 22, 2015 at 06:10 PM 0

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Thanks, that might be exactly what I'm looking for.

Edit: Got it working;

 private Vector3 eulerAngles;
 private Vector3 velocityVec;
 private Vector3 forwardVec;
 public float angle;
 private int cross;
 
 velocityVec= transform.parent.GetComponent<Rigidbody>().velocity.normalized;
 velocityVec.y = 0;
 
 forwardVec= transform.parent.forward.normalized;
 forwardVec.y = 0;
 
 angle = Vector3.Angle(velocityVec, forwardVec);
 cross = Vector3.Cross(velocityVec, forwardVec).y < 0 ? 1 : -1;
 
 angle = Mathf.Clamp(angle, -45, 45); //Clamp to stop it over-rolling on sharp turns
 
 eulerAngles = new Vector3(transform.parent.rotation.eulerAngles.x, transform.parent.rotation.eulerAngles.y, cross*angle);
 
 transform.eulerAngles = eulerAngles;

Calculating the difference between forward direction and velocity direction to set the body roll of my vehicle.

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