An example of having a 2d arrow on the screen follow a 3d object in the world as a kind of radar hint about where that object is (for Unity3D).

Unity3DArrow

using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class ArrowScript : MonoBehaviour {

	public GameObject lookat;
	public Camera camera;
	public const float EDGE = 0.3f;
	public const float DIST = 100;
	public Color color = new Color(1,1,1,1);
	
	void Start () {
	
	}
	
	void Update () {
	
		Vector3 viewpos = camera.transform.InverseTransformPoint( lookat.transform.position );
		viewpos.Normalize();
	
		// get a position from -0.5 to 0.5 in xy space - which actually doesn't do what it claims sigh.
		//Vector3 viewpos = camera.WorldToViewportPoint( lookat.transform.position );
		//viewpos.x = (viewpos.x - 0.5f);
		//viewpos.y = (viewpos.y - 0.5f);
		
		// fade if not needed
		if(((viewpos.x >-EDGE && viewpos.x < EDGE) && (viewpos.y >-EDGE && viewpos.y < EDGE)) && viewpos.z > 0) {
			color.a = color.a > 0.0f ? color.a - 0.01f : 0.0f;
			renderer.sharedMaterial.color = color;
		} else {
			color.a = color.a < 0.5f ? color.a + 0.01f : 0.5f;
			renderer.sharedMaterial.color = color;
		}

		// rotate the arrow around lookat axis
		float angle = Mathf.Atan2(viewpos.y, viewpos.x) * Mathf.Rad2Deg + 90;
		transform.localRotation = Quaternion.AngleAxis(angle, Vector3.forward) * Quaternion.Euler (270,0,0);
	
		// constrain the arrow to the screen; it should fade away when on screen; we allow it to come off the edge in that case
		if(viewpos.x > EDGE ) viewpos.x = EDGE;
		if(viewpos.x <-EDGE ) viewpos.x = -EDGE;
		if(viewpos.y > EDGE ) viewpos.y = EDGE;
		if(viewpos.y <-EDGE ) viewpos.y = -EDGE;
		
		// place the arrow
		transform.localPosition = new Vector3( viewpos.x * DIST, viewpos.y * DIST, DIST/2.0f );

	}
}