In this tutorial I will try to explain the various render passes that I found useful in Maya and mental ray workflow during the production of my latest character. This tutorial will also cover some workarounds for mental ray shaders that won't render properly with render layers.
At the end of the production on my Asian Lightning Mage I decided to make a few very large renders of the character to show off all the detail. These renders were all about 8k tall; this resolution was needed for high fidelity printing on poster sizes.
Rendering at such high resolution can be very time-consuming. Tweaking small things can be really expensive because you have to wait the entire render time again to see the changes. To negate this problem a lot of render passes can be made, each with its own information. They can then be composited in a compositing package or Photoshop.
Before starting to work with render passes, I rendered a batch of all the available passes to see which ones were going to useful for the creation of the final image. Below is a list of all the useable outputs. For maximum flexibility during the compositing stage, all passes were rendered out as 16 bit uncompressed TIFF images to avoid banding during compositing. Below I would like to share my findings.
This is the image you normally get when rendering (Fig.01). Without the use of render passes you would be stuck with this image, only being able to do simple adjustments in Photoshop.
When working with render passes you can either neglect this pass (since you can recreate it with the other passes), or use it as a starting point and comparison image for you final image.
Fig.01 - Master Beauty
Diffuse/Diffuse Material Color/Diffuse no Shadow
These passes only render the diffuse component, giving a very matte look. Diffuse Material Color is an unlit version that can be multiplied with lighting. However some problems occur with rendering some of the mental ray shaders. The Architectural Mia Shader and any SSS shader will appear black in the Diffuse Material Color and Diffuse no Shadow passes.
There is a workaround for this by placing an ambient light with a value of 1 in your scene, but the Diffuse pass renders correctly and so this might be enough in most cases (Fig.02 - 04).
Fig.03 - Diffuse Material Colour
Fig.04 - Diffuse no Shadow
Specular/Specular no Shadow/Reflection
The specular and reflection components can also be rendered out separately, just like the diffuse component. The way to add these in post is by using either Screen, Color Dodge or Linear Dodge. However some pretty dramatic reflections can be created by using Overlay, Soft Light or Hard Light (Fig.05 - 07).
Fig.06 - Specular no Shadow
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Fig.07 - Reflection