1. Compositing and Rendering
Chakrit Watcharopas
Reference:
The Essential Blender: The Official Guide to 3D Creation with the Blender Open Source Suite  
Author Roland Hess Publisher No Starch Press Publication Date 
Also from:

2. Why Composite?

3. Why Composite? (2)
Although rendering might seem to be done, in a real production environment you would almost certainly have to
color correct or
composite the render with a pre-created background, or
possibly even put it together with several different renders, building a final image in layers
Though possible to add all elements of finished scene into a single render, it is an inefficient and time consuming approach to a production

4. Render Passes
Left to Right, Top to Bottom: Combined, Alpha, Z Buffer, Color, Diffuse, Specular, Shadow, Ambient Occlusion, Reflection.

5. Render Passes (2)
Blender’s Composite Nodes give the user access to data from its render passes
When Blender calculates things like shadows or specular highlights, it stores the results as pixel data
Images will be combined later using pixel math

6. Render Passes (3)
You can access each of these passes individually, modify them, and put them back together in whatever way you see fit
Above process is known as compositing
The primary definition of a composite is something made up of differing parts, combined to make a whole

7. Render Passes (4)
In Blender, when you press F12 and render your scene, what you’re seeing is a composite of render passes – a set of separate images combined together into one final
With CompositeNodes, you can control how these render passes get assembled, and what happens to them along the way

8. CompositeNodes
Combined – This is the renderer’s composite of all the render passes

9. CompositeNodes (2)
Alpha – This pass is an 8-bit graphic representing the combined render’s transparency information
White pixels represent full opacity and black pixels indicate full transparency

10. CompositeNodes (3)
Z-buffer – You might not be able to see the data represented of the scene depth here
Z-buffer information is stored as a 32 bit floating point number

11. CompositeNodes (4)
Color – This is an RGB color mask containing information for each object in the scene

12. CompositeNodes (5)
Diffuse – This pass contains all shading and color information from lighting calculations in the scene

13. CompositeNodes (6)
Specular – The information generated from all materials’ specular shaders are contained in this pass

14. CompositeNodes (7)
Shadow – For ray traced or buffered shadows, this pass stores the shadow data for the scene

15. CompositeNodes (8)
Ambient Occlusion (AO) – This pass is the result of a separate pass using the AO algorithm

16. CompositeNodes (9)
Reflection – Data generated from ray traced reflections are stored in this pass

17. Render Layers
In the Render Layers panel of the Scene buttons (F10) are buttons for activating the various passes
Left: Render Layers panel with render passes selected.
Right: Render Layer node with corresponding render pass points

18. Setting Up for Node Editing
Node editor with composite nodes button selected and “Use Nodes” button active.

19. Setting Up for Node Editing (2)
Once you have the Node editor active, you’ll need to select the Composite Nodes button at the bottom, and turn on the Use Nodes button

20. Setting Up for Node Editing (3)
The last step is turning on “Do Composite” in the Anim panel of the Scene buttons – if you don’t, all the node editing in the world won’t affect your final render
Anim panel with “Do Composite” active