Render your scene by parts using render layers

The aim of this tutorial is to render a scene by parts for example an animation. In many cases, your project grows slowly and you can't start the rendering until your entirely finish the project. As a result, the rendering starts very late and close to the deadline which makes for example rendering big animations impossible to do.

But... big projects can often be divided by parts. I mean separate objects that won't change until the deadline like the environment, non-project objects, and designed objects you are sure they will be kept till deadline. Therefore, with render layers, you can render the steady parts first, quietly, and just the last part of your project before the deadline. This saves you a lot of time from when you need it.

As your model is not complete, you should take care of the interactions between the steady part and the project. Mainly transparency and shadows.

Separate parts
You should separate parts within layers. For example:
 * Layers 1-2 : Steady parts (environment, etc..)
 * Layers 3-4-5 : Project (still not finished)
 * Layer 6 : Transparency objects (Z transp or Ray) which interact between environment and project)
 * Layer 7 : lights and cameras

Render layers
Let's create render layers. These are presets of which layers to render.
 * Renderlayer "Environment" : tick layers 1-2-7
 * Renderlayer "Project" : tick layers 4-5-7
 * Renderlayer "Transparency" : tick layers 6-7

We also may create a full render layer to render the full project scene once a time.
 * Renderlayer "Whole scene" : tick layers (1-7)

Rendering will take in account all visible layers. That means that for example, objects in non renderable layers will anyway cast shadows on Renderlayers. In our case, tick (1-7). As a result, do not make visible layers where you put your bin or others objects you don't want to see on the final render.

Passes
For all renderlayers, tick all except "AllZ"

Shadows