Find the ARAnchor attached to specific ARPlane?

[RyanJVR]

Im using public ARAnchor AttachAnchor(ARPlane plane, Pose pose); to attach an ARAnchor to a ARPlane that is touched.

How do I retrieve that ARAnchor if I touch the same plane a second time? Tried if (hit.trackable is ARPlane plane) { var p_anchor = plane.gameObject.GetComponent<ARAnchor>(); } but p_anchor always ends null for me.

Regards Ryan

[tdmowrer]

How do I retrieve that ARAnchor if I touch the same plane a second time?

There is no way to ask "to what plane is this anchor attached?".

Tried if (hit.trackable is ARPlane plane) { var p_anchor = plane.gameObject.GetComponent<ARAnchor>(); } but p_anchor always ends null for me.

Planes and anchors have different transforms (on different GameObjects), so plane.GetComponent<ARAnchor>() should always return null.

When you "attach" an anchor to a plane, you get back a regular anchor; the only difference is in the way that it is updated internally. From https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@4.0/manual/anchor-manager.html#attaching-anchors:

Attaching an anchor to a plane affects the anchor update semantics. This type of anchor will only change its position along the normal of the plane to which it is attached, thus maintaining a constant distance from the plane.

If this relationship is important to you, you can keep a Dictionary of anchors to planes in order to look up the plane to which an anchor is attached.

[RyanJVR]

Thank you for your response tdmowrer. I was using this example and hoping attachAnchor() worked a similar way.

void AnchorContent(Vector3 position, GameObject prefab)
{
    // Create an instance of the prefab
    var instance = Instantiate(prefab, position, Quaternion.identity);

    // Add an ARAnchor component if it doesn't have one already.
    if (instance.GetComponent<ARAnchor>() == null)
    {
        instance.AddComponent<ARAnchor>();
    }
}

Will look into the Dictionary route. Thanks again. Ryan

[tdmowrer]

Attaching an anchor to a plane requires a method call on the ARAnchorManager because it has to know what ARPlane to attach to at the time it is created. "Regular" anchors only need a transform, so simply calling AddComponent is enough.

Will look into the Dictionary route.

For inspiration:

using System.Collections.Generic;
using UnityEngine;
using UnityEngine.XR.ARFoundation;
using UnityEngine.XR.ARSubsystems;

public class SomeBehaviour : MonoBehaviour
{
    [SerializeField]
    ARRaycastManager m_RaycastManager;

    [SerializeField]
    ARAnchorManager m_AnchorManager;

    Dictionary<ARPlane, ARAnchor> m_Anchors = new Dictionary<ARPlane, ARAnchor>();

    List<ARRaycastHit> m_RaycastHits = new List<ARRaycastHit>();

    void Update()
    {
        var screenTapped = Input.touchCount > 0 && Input.GetTouch(0).phase == TouchPhase.Ended;
        if (screenTapped && m_RaycastManager.Raycast(Input.GetTouch(0).position, m_RaycastHits, TrackableType.PlaneWithinPolygon))
        {
            var hit = m_RaycastHits[0];
            if (hit.trackable is ARPlane plane)
            {
                // Get the existing anchor attachment if it exists
                if (!m_Anchors.TryGetValue(plane, out var anchor))
                {
                    // We don't already have one. Create a new one?
                    anchor = m_AnchorManager.AttachAnchor(plane, hit.pose);

                    // Add it to the map for next time
                    m_Anchors.Add(plane, anchor);
                }

                // Do something with the attached anchor...
            }
        }
    }
}

[RyanJVR]

Thank you so much

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