Unity3d - Displaying output of one camera as background of other camera

Unity is an integrated authoring tool for creating 3D video games or other interactive content such as architectural visualizations or real-time 3D animations. Unity's development environment runs on Microsoft Windows and Mac OS X, and the games it produces can be run on Windows, Mac, Xbox 360, PlayStation 3, Wii, iPad, iPhone, as well as the Android platform. It can also produce browser games that use the Unity web player plugin, supported on mac and Windows but not Linux. The web player is also used for deployment as Mac widgets.

I was working with a problem where I needed to display the output of one camera as the background image of the other camera. The concepts used in this method are applicable at lot of places during the game development like showing the mirror effect in car races or the score board. 

I would suggest to go through the following links before using this method. The following links will give a rough idea about the Camera, GUI Texture, Render Texture and Layers in Unity.

1. http://unity3d.com/support/documentation/ScriptReference/Camera.html

2. http://unity3d.com/support/documentation/Components/class-Camera.html

3. http://unity3d.com/support/documentation/Components/class-RenderTexture.html

4. http://unity3d.com/support/documentation/Components/class-GuiTexture.html

5. http://unity3d.com/support/documentation/ScriptReference/GUITexture.html

6. http://unity3d.com/support/documentation/ScriptReference/RenderTexture.html

7. http://unity3d.com/support/documentation/Components/Layers.html

Setup your scene with the Main Camera showing your game objects. After this follow these steps to create the background using a different Camera  -

1. Create a new camera (GameObject->Create Other-> Camera), and name it "Background Camera".

2. Create a new GUI Texture (GameObject->Create Other->GUI Texture), and name it "Background Image".

3. Click the "Layer" dropdown menu in the Background Image's inspector pane, and select "Add Layer".

4. In the next free "User Layer" slot, create a new layer name called "Background Image". This will be directly under the layer named "Terrain" if you haven't yet added any others.

5. Select your Background Image in the hierarchy, and give it the desired texture, and set the x, y, width and height under "Pixel Inset" so that it fills the screen appropriately.

6. Near the top of the inspector window, Use the layer dropdown menu to assign your "Background Image" layer that you defined earlier to this game object.

7. Now select your Background Camera in the hierarchy, and adjust these settings in the inspector:

   * Un-check Flare Layer and Audio Listener (but leave GUILayer enabled)

   * Set Clear Flags to Solid Color

   * Set Depth to -1

   * Set Culling Mask, first to "Nothing", and then to "Background Image"

8. Now Select your other (Main) camera and in the inspector:

   * Set its Clear Flags to "Depth Only"

   * Click its culling mask setting, and un-check "Background Image". This should result in the culling mask displaying as "Mixed..."

9. Now your system should display the Background GUI Texture as the background of the main camera. Since, this GUI Texture is the background for the present camera, you can change it dynamically from the script to display any other video texture or Render Texture from other camera. Render Texture is  used to render the camera view to a texture which can be displayed somewhere else.

10. Create a new Render Texture by Assets->Create->Render Texture. Give a proper name to this render texture and edit the properties according to your needs.

11. Now setup the scene and a camera whose output you want to display as the background in the Main Camera.

12. Go to the 'Target Texture'  option of this new camera and assign the render texture you created to it.

13. Now make a script which assigns this render texture as GUI Texture to the Background Image you created earlier.