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I have this simple code to rotate an object (gun barrel) towards a target object (rocket ship).

The gun barrel rotates towards the ship but is always 90 degrees off. I have fixed this in my code as you can see by subtracting 90 degrees from the angle but I would like to know why it is happening and if there is a more elegant solution.

I have checked the rotation of all objects involved but that doesn't seem to change anything. Any help is much appreciated!

using UnityEngine;

public class RotateTowardsObject : MonoBehaviour {

    public float speed = 5f;        // how fast this rotates towards the target
    public Transform target;        // target to follow

    public void Update()
    {
        Vector2 direction = target.position - transform.position;                                               // direction between this position and target position
        float angle = Mathf.Atan2(direction.y, direction.x) * Mathf.Rad2Deg;                                    // calculate the angle
        angle = angle - 90f;                                                                                    // ** Temporary fix(?) for rotation being +90 degrees off
        Quaternion rotation = Quaternion.AngleAxis(angle, Vector3.forward);                                     // create rotation from quaternion
        transform.rotation = Quaternion.Slerp(transform.rotation, rotation, speed * Time.deltaTime);            //  set rotation acceleration towards target
    }
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  • \$\begingroup\$ What direction is the barrel pointing at by default? If it's not right, you need to offset it. \$\endgroup\$ – Tyyppi_77 Jun 8 '18 at 11:49
  • \$\begingroup\$ It is pointing straight up initially. I tried rotating the gun object 90 degrees in the Unity editor but it makes no difference. It will snap back to being 90 degrees off when I run the game. \$\endgroup\$ – Sam Luxton Jun 8 '18 at 11:50
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As per the unit circle math, angle 0 means "pointing right". Your initial orientation is up, at 90 degrees, so that's why you need to offset by 90 degrees so that your zero is at top, and -90 at right.

You can just keep doing this, or rotate your model to be oriented towards right by default. I find that the best practice, as then you can just swap content in and out of different systems. It's a good practice to make everything that needs to be rotated to face right in the future.

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  • 2
    \$\begingroup\$ Ha! I have a note in my Utilities "AngleTo" code that says "// TODO: Why does this need to be rotated pi/2" or something like that; silly in hindsight given I advocate the unit circle all the time \$\endgroup\$ – blurry Jun 8 '18 at 15:33

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