Cam Transform Vector confusion???

Unity Engine
1

Can someone please explain the following code snippet to me ??
More specifically the creation of the right Vector

        Vector3 forward = mainCam.transform.TransformDirection(Vector3.forward);//forward of camera to forward of world
        forward.y = 0f;
        forward = forward.normalized;
        Vector3 right = new Vector3(forward.z, 0.0f, -forward.x);
        Vector3 targetDirection = (horizontal * right + vertical * forward);
        if (targetDirection.sqrMagnitude > 1f)
        {
            targetDirection = targetDirection.normalized;
        }
//Quaternion targetRotation = Quaternion.LookRotation(targetDirection, Vector3.up) * camRotation;
  Quaternion targetRotation = Quaternion.LookRotation(targetDirection, Vector3.up);
  // Create a rotation that is an increment closer to the target rotation from the player's rotation.
  Quaternion newRotation = Quaternion.Lerp(rigidbody.rotation, targetRotation, slerpDampTime * Time.deltaTime);

The target direction is the gamepad right stick input.
The script allows the gamepad to stay oriented with the rotation of the camera but I do not understand why the right vector was created this way ( - forward.x ??? )
I have tried to map it out pen paper and tracing source but it is not clicking :frowning:

2

It’s a quick trick to create a vector that is perpendicular to a given vector in 2D.

If we calculate the dot product of vectors (x, y) and (y, -x), we’ll get zero

(x, y) dot (y, -x) = xy + y(-x) = xy - xy = 0

which means the vectors are perpendicular to each other.

3

Thank you.
What would be an equivalent way to code this without using tricks? Can we use a cross product ? Why is this needed anyway? Just trying to understand completely but no matter thanks.

4

Its just to effectively rotate the forward vector by 90 degrees
You could go the hard way and create a 90 degree rotation around the up axis, and rotate the vector
Ot you could cross the forward vector with up
But why bother when you can just swap the x and z values. Its not a bad trick.

1 Like
5

  1. If we didn’t know the trick, we could draw a picture and “guess” the same formula from it:
    1772170--112337--WP_001022.jpg

  2. Or we could solve it analytically:
    1772170--112343--WP_001023.jpg
    which gives us two vectors (y, -x) and (-y, x) perpendicular to (x, y), one looks to the right and another to the left.

  3. I personally would use my own helper methods that use cross product:

public static Vector3 CalculateUp(Vector3 forward, Vector3 right)
{
    return Vector3.Cross(forward, right).normalized;
}

public static Vector3 CalculateForward(Vector3 right, Vector3 up)
{
    return Vector3.Cross(right, up).normalized;
}

public static Vector3 CalculateRight(Vector3 up, Vector3 forward)
{
    return Vector3.Cross(up, forward).normalized;
}
  1. Or we could rotate the forward vector:
Vector3 right = Quaternion.Euler(0, 90, 0) * forward;

Vector3 targetDirection = (horizontal * right + vertical * forward);

Horizontal and vertical variables, I believe, are taken from the player’s input. The forward axis is taken from the camera and the right axis is calculated from it.

Actually, if the camera doesn’t roll, I think we could just take the right axis from the camera as well:

Vector3 right = mainCam.transform.right;

And even if it rolls, we could use the same trick as with the forward axis:

Vector3 right = mainCam.transform.right;
right.y = 0;
right.Normalize();
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