Force an AR object to always stand upright in Unity / Vuforia

Question

I have a Unity AR project using Vuforia engine. What I am trying to achieve is to have the AR object always stand upright in the view whether the image target is horizontal on a table or or vertical on a wall.

Currently the object is sitting on the image target no matter which orientation

Hope that makes sense,

Thanks

Answer 1

I always use Vector3.ProjectOnPlane for this and then you can simply assign axis directions to Transform.up and Transform.right (below I explain why right and not maybe forward)

public void AlignObject(Transform obj, Transform imageTarget)
{
    obj.position = imageTarget.position;

    // Get your targets right vector in world space
    var right = imageTarget.right;

    // If not anyway the case ensure that your objects up vector equals the world up vector
    obj.up = Vector3.up;

    // Align your objects right vector with the image target's right vector
    // projected down onto the global XZ plane => erasing its Y component
    obj.right = Vector3.ProjectOnPlane(right, Vector3.up); 
}

The assumption for this is: The target is usually never rotated in the Z axis. If you want it upright on a wall you would usually rotate it around its X axis.

Therefore we can assume the the image target will never be rotated more then 90° on the Z axis (in which case the mapped vector would flip about 180°) and thus if we map the right vector down onto the global XZ plane it always still points in the correct direction regardless of any rotations in Y and X axes.

If we would use the forward instead we take the risk that due to tracking inaccuracies the vertical targets forward vector actually points a tiny little bit towards us so when we map it down onto the XZ plane it points backwards not forwards and the object is flipped by 180°.

So using right works for a horizontal and a vertical target.