Render Passes and Compositing

Compositors of all levels know how tedious and technical editing and manipulating a scene can become. Each shot is broken down and edited to an acute detail to where the audience can not determine what is computer generated, and what is not. The compositor may need to adjust the hue of an object, edit the brightness of a reflection, or add a blur toward z-space. Instead of having to re-render the 3D scene relentlessly to achieve the desired result, render passeses help ease those blemishes without the hassle of spending time and resources on re-renders. Render passes is the process of individually rendering out the elements of a full beauty pass; such as the ambient occlusion, specular, reflections, refractions, and so on.

Autodesk Maya 2016 and Mental Ray will be the examples provided for render pass examples. There are numerous amounts of 3D programs and render plug-ins available. Use whatever you are comfortable with using.

The procedure involves opening up the render settings and clicking on the "Passes" tab. If the passes tab does not appear, venture into preferences and open up the "rendering" tab and check "Show Maya Legacy Tab". There is a render pass tab under "Scene" and hidden under "Cameras", but that's for the new MILA materials. If you use MILA materials then go on ahead and use those settings.

Typically, what is needed from the render pass is diffuse, reflection, specular, ambient occlusion, matte, and a depth pass. Depending on the scene, these passes will change. For example, scatter passes for subsurface light scattering and a fur pass. If the scene being worked on has more than one object it is recommened to put each object on a separate layer. By doing so, each object can have a render pass.

Autodesk has great resources for better understanding render passes and rendering with Mental Ray. There's more detail about what each pass is, and how they all interact with each other.

Multi-Render Passes

Mental Ray Tabs in Maya 2016

Set Up Mental Ray Scene Pass

About The Render Settings Mental Ray Tabs in Maya 2016

Render Pass Attribute Editor

Introduction to Multi-Render Pass: A Simple Workflow Example

Compositing Workflow Within Blackmagic Fusion

Now that the passes have rendered, it is time to composite the pieces together. Compositing software such as The Foundry's Nuke, Blackmagic's Fusion, and even Adobe After Effects are used to assemble the passes. Following the previous render pass example, there is a diffuse, reflection, specular, ambient occlusion, matte, and depth pass. The set-up is roughly the same for all three programs. After Effects is layer-based and the transfer mode can be easily changed via a dropdown tab by the layer in the timeline. Nuke and Fusion are node-based and require a "merge" node to change the transfer (apply in Fusion) mode. The diffuse pass is the base to our final product, the diffuse apply mode stays as is. Reflections are next and are set to "screen"; followed by specular set to "screen"; then ambient occlusion set to "multiply". If darker shadows are preferred, duplicate another AO layer (After Effects), or create another node and set it to "multiply".

Render Passes Can Become A Handful

The matte and depth passes are used to enhance the detail and blend the 3D assets with the scene. The matte pass is used to blend/feather the composited piece with the background. Without the matte pass, the scene would look flat and pieced together. The depth pass builds upon what the matte pass provides by creating depth to the 3D asset. Maya or any 3D software can apply depth of field to the renderable camera, but it is time consuming and resource heavy. By rendering out a depth pass, time is saved and the quality is rarely affected. Each software is different, but the general idea is to set the depth path to an alpha and then applying a camera/depth blur. Once completed, color adjustments and other edits should be made to build upon the composition.

Render passes are a strong and necessary tool in a compositor's toolbox. Experiment with different passes and shader materials. Look into different rendering software. There may be more efficient ways to render pass a scene. Autodesk offers Composite  and MatchMover a free compositing and match moving bundle. This set of free software is an opportunity to learn more about the computer graphics and how to apply those skills in a practical manner.