Anchors sample: attach anchors to planes if a plane is hit

Assets/Scenes/Anchors.unity

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Assets/Scripts/AnchorCreator.cs

     {
         public void RemoveAllAnchors()
         {
+            Logger.Log($"Removing all anchors ({m_Anchors.Count})");
             foreach (var anchor in m_Anchors)
             {
                 m_AnchorManager.RemoveAnchor(anchor);
         {
             m_RaycastManager = GetComponent<ARRaycastManager>();
             m_AnchorManager = GetComponent<ARAnchorManager>();
-            m_Anchors = new List<ARAnchor>();
+        }
+
+        ARAnchor CreateAnchor(in ARRaycastHit hit)
+        {
+            // If we hit a plane, try to "attach" the anchor to the plane
+            if (hit.trackable is ARPlane plane)
+            {
+                var planeManager = GetComponent<ARPlaneManager>();
+                if (planeManager)
+                {
+                    Logger.Log("Creating anchor attachment.");
+                    return m_AnchorManager.AttachAnchor(plane, hit.pose);
+                }
+            }
+
+            // Otherwise, just create a regular anchor at the hit pose
+            Logger.Log("Creating regular anchor.");
+            return m_AnchorManager.AddAnchor(hit.pose);
         }
 
         void Update()
             if (touch.phase != TouchPhase.Began)
                 return;
 
-            if (m_RaycastManager.Raycast(touch.position, s_Hits, TrackableType.FeaturePoint))
+            // Raycast against planes and feature points
+            const TrackableType trackableTypes =
+                TrackableType.FeaturePoint |
+                TrackableType.PlaneWithinPolygon;
+
+            // Perform the raycast
+            if (m_RaycastManager.Raycast(touch.position, s_Hits, trackableTypes))
-                // Raycast hits are sorted by distance, so the first one
-                // will be the closest hit.
-                var hitPose = s_Hits[0].pose;
-                var anchor = m_AnchorManager.AddAnchor(hitPose);
-                if (anchor == null)
+                // Raycast hits are sorted by distance, so the first one will be the closest hit.
+                var hit = s_Hits[0];
+
+                // Create a new anchor
+                var anchor = CreateAnchor(hit);
+                if (anchor)
-                    Logger.Log("Error creating anchor");
+                    // Remember the anchor so we can remove it later.
+                    m_Anchors.Add(anchor);
-                    m_Anchors.Add(anchor);
+                    Logger.Log("Error creating anchor");
                 }
             }
         }
-        List<ARAnchor> m_Anchors;
+        List<ARAnchor> m_Anchors = new List<ARAnchor>();
 
         ARRaycastManager m_RaycastManager;
 

README.md

 
 This sample shows how to create anchors as the result of a raycast hit. The "Clear Anchors" button removes all created anchors. See the [`AnchorCreator.cs`](https://github.com/Unity-Technologies/arfoundation-samples/blob/master/Assets/Scripts/AnchorCreator.cs) script.
 
+This script can create two kinds of anchors:
+1. If a feature point is hit, it creates a normal anchor at the hit pose using the [AddAnchor](https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@4.0/api/UnityEngine.XR.ARFoundation.ARAnchorManager.html#UnityEngine_XR_ARFoundation_ARAnchorManager_AddAnchor_UnityEngine_Pose_) method.
+1. If a plane is hit, it creates an anchor "attached" to the plane using the [AttachAnchor](https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@4.0/api/UnityEngine.XR.ARFoundation.ARAnchorManager.html#UnityEngine_XR_ARFoundation_ARAnchorManager_AttachAnchor_UnityEngine_XR_ARFoundation_ARPlane_UnityEngine_Pose_) method. 
+
 ## Scale
 
 This sample shows how to adjust the apparent scale of content in an AR scene. It does this by moving, rotating, and scaling the `ARSessionOrigin` instead of the content. Complex scenes often can't be moved after creation (e.g., terrain), and scale can negatively affect other systems such as physics, particle effects, and AI navigation. The `ARSessionOrigin`'s scale feature is useful if you want to make your content "appear" at a position on a detected plane and to scale, for example, a building sized object to a table-top miniature.