How to get the forward direction of a normal?

Unity Engine
1

I have a Bike that travels down a race track, the track is one long mesh. I have a rectangular object, let’s call him Ben, that sits under the bike that has the below script on it, the bike will eventually Lerp it’s Y rotation to Ben’s Y rotation.

Ben casts a red ray straight down, obtains the normal vector of the object below, and matches it’s rotation.
It’s working fine for the X and Z values, but not for Y, which is the one I need. Ben also casts a green ray sideways to detect the edge of the road.

How do I rotate Ben to have the same Y rotation as the normal directly below him?

Or rather, how do I make Ben face down the track? I want his Y rotation perfectly aligned with the white lines on the road as it passes over them.

function Update ()
{
    OrientThisToObjectBelow(transform.TransformDirection(-Vector3.up));
}

function OrientThisToObjectBelow (dir : Vector3)
{
    if (Physics.Raycast (transform.position, dir, hit, 150, LayerMask))
    {
        Debug.DrawLine(transform.position, hit.point, Color.red);
        transform.up = hit.normal;
    }
}

2

Depending on Ben’s world up direction, this should rotate him according to the direction of the hit normal:

transform.rotation = Quaternion.LookRotation(hit.normal);
3

Hmm, that produces the below result. And even during my testing it doesn’t seem as though Ben adopts the Y rotation of the normal below him.

4

“How do I rotate Ben to have the same Y rotation as the normal directly below him?”

You mean the ground? Even with all your pictures, it’s still quite confusing what you want. What is your end goal?

5

Below the bike is the road, which is one long mesh made up of several faces, I’ve used the EasyRoads 3d package to generate the road.

I want to cast a ray downwards from the bike, hit a face on the road mesh, and use that face’s normal to align my bike with it on the bikes Y axis. So that the bike is always facing in the direction of face’s Z axis.

So that if I set the bike to always drive forward, it will follow the road no matter how the road bends or turns, because the bike is always aligned with whatever face it is passing over on the road.

6

The normal isn’t going to “spin” when you rotate the plane on the up axis. You’ll have to align the bike with something like the forward vector of the plane itself.

7

Ah, yep this was my mistake, for some reason I thought a normal had an X and Z direction. Thank you everyone for your help anyway it’s much appreciated!

8

The topic is a bit old, but I guess I’d share my findings with this.
I figured it’s very easy when you instantiate an empty game object as a ‘helper’.

private void Awake()
{
     ClimbNormalObj = new GameObject();
     ClimbNormalObj.gameObject.name = "Climbing Helper";
     ClimbNormalObj.transform.parent = transform;
}

ClimbNormalObj.transform.position = Hit.point;
ClimbNormalObj.transform.rotation = Quaternion.LookRotation(Hit.normal);

You can now pass through a public transform of that to return the forward of the normal.

public Transform GetNormalRef
{
    get { return ClimbNormalObj.transform; }
}