UI Follow Scene Objects?

How do I make a UI that follows an object (eg, a healthbar that always stays the same size no matter how far the object is from the camera?

My current setup that isn't working:

I have a Screen Space - Camera canvas set to "Scale with screen size" 1136 x 640 and the slider is set to Height (1).

There is a cube in the scene.

I have an image in the canvas with a script on it that moves its position every frame with the following code:

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class TrackObject : MonoBehaviour {
5.     public GameObject Obj;
6.  
7.     public Camera mCamera;
8.     private RectTransform rt;
9.  
10.     void Start ()
11.     {
12.         rt = GetComponent<RectTransform>();
13.     }
14.    
15.     void Update ()
16.     {
17.         if (Obj != null)
18.         {
19.             Vector2 pos = RectTransformUtility.WorldToScreenPoint(mCamera, Obj.transform.position);
20.             rt.position = pos;
21.         }
22.     }
23. }

But what I'm currently seeing is the UI element move at half the speed of the objects in the camera. What am I doing wrong?

 

Figured it out. I have to run the screen point through RectTransformUtility.ScreenPointToLocalPointInRectangle() to get the proper local position of the UI element.