How to check if the finger is touching area where object instance exist?

Question

There is a way to place objects on the surface by swiping finger over the screen (thanks to @ryemoss for an answer here )

The question now is how to avoid placing objects on top of each other - how can we check if the finger is touching the area where there is already an object instance exist, and place new objects only if the area is empty?

The answer I'm looking for should include the possibility to make objects overlap a bit if needed (touch each other's borders).

Answer 1

Here is a snippet from Unity Samples in the SDK (check here) that checks if an existing placed marker (gameobject) is touched during a touch event:

        Touch t = Input.GetTouch(0);
        Vector2 guiPosition = new Vector2(t.position.x, Screen.height - t.position.y);
        Camera cam = Camera.main;
        RaycastHit hitInfo;

        if (t.phase != TouchPhase.Began)
        {
            return;
        }

        if (m_selectedRect.Contains(guiPosition))
        {
            // do nothing, the button will handle it
        }
        else if (Physics.Raycast(cam.ScreenPointToRay(t.position), out hitInfo))
        {
            // Found a marker, select it (so long as it isn't disappearing)!
            GameObject tapped = hitInfo.collider.gameObject;
            if (!tapped.GetComponent<Animation>().isPlaying)
            {
                m_selectedMarker = tapped.GetComponent<ARMarker>();
            }
        }

What you need is handled by

if (Physics.Raycast(cam.ScreenPointToRay(t.position), out hitInfo))