Retrieve object rotation from face normal

Question

I would like to rotate an object to match its ground. So i cast 3 Rays (at the corners) to calculate the normal of the plane below. Now i need to rotate the object accordingly but keep the y rotation (so in which direction it "faces") so just setting transform.up = normal does not work. I thought i could just use the dot product between the transform directions to rotate it (so xRotation = Vector3.Dot(normal, transform.forward) for x and zRotation = Vector3.Dot(normal, transform.right) for z) this should be the angles between the normal vector and the right/forward vector. But as the result my object just faces the sky that way to the idea is completely wrong. Do you know how i should proceed ?

user_5740243 · Answer

Here is the solution to your problem. Although there are different methods for doing this, I personally find it best to use the Cross axis. In fact, you need Vector3.Cross instead of Vector3.Dot. This code works in such a way that by multiplying the transform.right of the player on the Ground normal vector, Since this axis calculates the perpendicular direction, you can expect it to give a forward corresponding to the ground.

public LayerMask groundLayer; // The Layer Ground
public Vector3 offset = Vector3.up;

private void Update()
{
    if (Physics.Raycast(transform.position, Vector3.down, out var groundHit, 2f, groundLayer.value))
    {
        var cross = Vector3.Cross(transform.right, groundHit.normal);

        var _lookRotation = Quaternion.LookRotation(cross, groundHit.normal);

        transform.position = groundHit.point + offset; // the offset is OPTIONAL, you can adjust it manuel or remove

        transform.rotation = _lookRotation;
    }   
}

Result:

You can see the result below. Consider that you can delete the offset code and make it compatible with the ground with mechanisms such as CharacterController or Rigidbody.

Retrieve object rotation from face normal

1 Answers

Result: