Unity instantiate object at a certain direction based of a spatial anchor

Question

Having a spatial anchor as a reference point, I wanted to create an object that does not change its location that reference to the spatial anchor.

When creating and saving the initial location of the object that will be spawned later using prefab. I make use of the difference of the x,y,z coordinate between the spatial anchor and the object and saved it in the cloud. After that, making use of the difference of the x,y,z to load the prefab back to it original position.

when creating the object based off the spatial anchor

However, upon restarting the application at a different point, the prefab will be shifted based on the start up position of the hololens. Based on what I know, the initial position of the hololens when it starts up in (0,0,0). Hence, the spatial anchor coordinate will be different and causing the prefab to be loaded at a different direction.

upon starting up the app at different location

Is there any way or solution that I can implement to make the prefab load at the same place as where it is created without it being affected by the hololens location?

Answer 1

If your prefab is a child of the anchor you could do something like this to save the position relative to the anchor:

var localPos = spawned.transform.localPosition;
var localRot = spawned.transform.localEulerAngles;
var localSca = spawned.transform.localScale;
        
var prefab = new Prefab
{
   AnchorId = anchorId,
   PosX = localPos.x,
   PosY = localPos.y,
   PosZ = localPos.z,
   RotX = localRot.x,
   RotY = localRot.y,
   RotZ = localRot.z,
   ScaX = localSca.x,
   ScaY = localSca.y,
   ScaZ = localSca.z
};

Save prefab to the cloud.

And then when you are starting a new session and you locate the anchor, you'd spawn the prefab as a child of the anchor and set the transform from the data you stored previously:

spawned.transform.localPosition = new Vector3(prefab.PosX, prefab.PosY, prefab.PosZ);
spawned.transform.localRotation = Quaternion.Euler(prefab.RotX, prefab.RotY, prefab.RotZ);
spawned.transform.localScale = new Vector3(prefab.ScaX, prefab.ScaY, prefab.ScaZ);