Multi-Channel Compositing

Here I've got a bunch of passes from my Robot for comp. Compositing is the process of taking all the elements that are going into a shot or sequence and essentially tying them together into one coherent image. This process quite often entails colour matching, focus matching and lighting adjustment. Green screen or chroma keying are also part of the compositing process as the need to "pull mattes" is central to a compositors roll.

For this latest exercise I wanted to do some touch ups of my robo-man to a level of accuracy that can't easily be done in a 3D package. As the compositing process is generally, but not exclusively, done in a 2D environment it is a lot faster and economical to do these touch ups as opposed to the 3D world. This however is a trend that is being challenged with ever more complex 3D systems being built into compositing software.

After my render was completed in Maya I exported my Robot via the .exr file type. .exr's are a very very handy file format. Developed by Industrial Light and Magic, .exr's enable a 3D artist to embed as many passes into the one file as necessary. Having this level of control makes the compositors job much easier to tweak any aspect of a particular sequence.

Below is the process of my quick comp job on my robot and outlines the amount of data that can be packed into and .exr file.

This Image is my straight rendered .exr file directly from Maya into Nuke. The image is displaying all layers concurrently and looks blown out due to incorrect layer blend modes. However once the .exr is unpacked and manipulated this ceases to be a problem.
Most Image formats that people are familiar with such as JPG and GIF files generall carry 3 or 4 channels, and RGB and an A, or, Alpha channel for transparency information. As you will see with the next series of Pictures, one .exr file can carry infinitely more than this.

When rendering out multiple passes in Maya, it is generally done in a lighting type basis. That is, all the separate aspects of the lighting state are broken down into their constituent parts to allow greater control over the fnial output.

This first cell is the diffuse pass. This is a lighting pass that will only render light that is reflected from a surface in a diffuse, or Matte way. 

The next pass I rendered out is the Specualr pass. This deals with the highlight of the objects materials and generally appears on shiny materials. The blend mode for this was screened over the top of the diffuse pass removing the blacks and just leaving the highlights.

The next pass is the reflection pass. This, as the name suggests, has simply picked out all the reflections from shiny objects. This is very handy as the over look of the material can be altered by adjust this pass.

This next pass is the shadow pass. The blend mode for this worked on a subtract basis, that is all the pixels in this pass are used to subtract, or add darkness to the composite image.

This final pass is whats known as a depth map. It is used as a matte to control a z-depth blur. When attaching a depth map to a z-depth blur, the darker areas are uneffected by the blur, where the white area's are completely blurred. This is used to give the render depth of field, thus the name z-depth blur.

After tweaking the final comp came out like this.

As you can see when compared to the top image, the straight Maya render, multi-pass compositing adds the final sheen to a CG render. The process is extremely fast and is a great way to get results that are rarely seen with a 3D package alone.