Question Displaying a texture with Graphics.Blit

This code is copied from the Unity documentation on Graphics.Blit:

Code (CSharp):

1. using UnityEngine;
2.  
3. public class Example : MonoBehaviour
4. { // Copies aTexture to rTex and displays it in all cameras.
5.  
6.     Texture aTexture;
7.     RenderTexture rTex;
8.  
9.     void Start()
10.     {
11.         if (!aTexture || !rTex)
12.         {
13.             Debug.LogError("A texture or a render texture are missing, assign them.");
14.         }
15.     }
16.  
17.     void Update()
18.     {
19.         Graphics.Blit(aTexture, rTex);
20.     }
21. }

As written, it produces the error message at Line 13 (so I guess you could say it works).

However, when I make

aTexture

and

rTex

public and drop appropriate assets into them, all I get when I run the scene is a view of the skybox, not the texture dropped into

aTexture

.

Adding this component to the GameObject with the main Camera does work:

Code (CSharp):

1. using UnityEngine;
2.  
3. // Blit a texture to the screen.
4.  
5. public class Blit : MonoBehaviour
6. {
7.     public Texture aTexture;
8.  
9.     void OnRenderImage(RenderTexture src, RenderTexture dst)
10.     {
11.         Graphics.Blit(aTexture, null as RenderTexture);
12.     }
13. }

It seems as though the first example is supposed to do the same thing, but I'm obviously missing something.

Can someone help me understand what the first example is doing?

Thanks!