You can segment your scene using render layers. If you have foreground, midground, and background objects in your scene, you can render them using three different layers. This can save a lot of time when rendering if there is no interaction between the objects; for example, in a forest scene, you would render three different layers separately: a static background (sky, trees), midground (a cottage), and foreground (characters). Rendering the static background once and then compositing later can speed up your workflow and render time dramatically.

A more complex example might involve different effects that you want to composite. One layer has raytracing turned on just for those objects that need it (raytracing can be a very time-consuming process). A second layer has glow lights which will be composited with certain objects to produce halo effects; and a third and fourth have the shadow and specular information for later compositing.

Consider the following example of two planes in a dogfight.

This image is made up of five composited layers that are rendered separately. The background is one layer, as is the foreground airplane and the midground airplane.

The two other layers are for effects: the midground bullets with their glow and blur are rendered on a separate layer, as are the motion-blurred propellers.

All these layers are composited together with different blend modes in order to create the final image you see here. This simplifies the workflow and makes it easy to re-render parts of the scene with different options.

A larger production may use different renderers for different passes as well as modifications to lights, objects, and layer overrides. For example:

• A Beauty layer rendered in the Maya software renderer using production quality.
• A Matte layer rendered with Maya hardware renderer; this could be lower quality as only the alpha channel matters. Hardware particle effects could be rendered on this layer as well.
• A Reflection layer rendered in mental ray using bright white lights. If there weren’t reflections in all parts of the scene, this could be only rendered for part of the scene (for example, only the middle 100 frames).
• A Glow layer rendered in the Maya software renderer for glowing objects. It can be rendered extra bright knowing it will be dialed down as needed in the compositor.
• A “Hot” layer rendered with mental ray (for example, an exaggerated specular achieved by assigning white Blinn to all objects, tweaking the specularity, and setting key lights bright and slightly red).
• A “Cool” layer rendered with mental ray final gather and no other lights. As well, an exaggerated diffuse is rendered from this layer, achieved by assigning a white Lambert to all objects and tweaking the diffuse).
• A Dirt layer as an ambient occlusion pass rendered by mental ray.
• Render post processes like Fur and Paint Effects are also rendered separately.