Entry 7: Intro to Maya Render Layers and AOVs

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 As a 3D Artist I was familiar with the modelling and texturing side of Maya, but I was never exposed to the VFX side of it until now. This week we looked into how to add textures to our assets and how to use the node graphs for the same. Once that is set we learnt about render layers, overriding textures and AOVs.

All of the assets in the scene will have aiStandardSurface and the surfaces that receive shadows and reflections will have aiShadowmatte as their materials. 

Tips for Materials :

1. Always set color space as RAW for everything except base color.

2. Enable "Alpha is luminance" for everything except base color and normal map.

3. If you have a very shiny CG asset in the scene, add real textures from the footage to the ground and wall planes for the asset to pick up and reflect them as this would help it blend  with the scene.

What are Render Layers and why do we use them?

Render Layers allows us to separate each element of the render into their own exr image, so in the end we'll a separate sequence for the main CG asset's geometry, one for the shadows and one for the reflections. This is done so that we can manipulate each of the 3 layers separately, such as blurring and darkening the shadows without affecting the reflections and so on.


Render layers and shader overrides

First we put each of the CG asset's geometry in a collection and a second collection for the ground plane that receives shadow and reflection. This is where things get interesting, we don't want to see the ground plane itself, but the presence of the plane will contribute to some indirect light that bounces off of it and lights up the CG assets, which we need. So we add a shader override to this collection that overrides the existing material with an aiStandardSurface, then we add an absolute override to the Primary visibility, which makes the actual plane invisible in the render but all of its contributions such as indirect diffuse and specular etc will have effect on the render. 

These properties only have effect within this Render layer, then we create another layer for capturing shadows and another for reflections.


Geo Layer- Only the 3D asset is visible in render

Shadow Layer- Only the shadows are visible

AOVs and their Magnificence

Arbitrary output variables (AOVs) allows us to individually extract data from a render to be output to provide additional options during compositing. These AOVs are by default factored into the beauty pass render where all of it layered and the final render is produced, but by adding them to the active AOVs tab. Once set and rendered as an EXR file, these AOVs can be individually manipulated in Nuke.

The main reason we do this is to have greater flexibility in the final stages of compositing in Nuke to go in and individually tweak all the different components such as grading them, color correction and affecting the amount of influence each AOV has on the final composite.

Renders of different AOVs - (from top left) Beauty, Direct Diffuse, Indirect Diffuse,
Direct Specular and Indirect Specular


Special AOVs :

MotionVector: To have much better control of motion blur we can add it to the AOV list instead of actually rendering it in every frame. So we create a custom AOV in the list add "aimotionVector" as its shader input. The data is stored in the motion channel which can be manipulated in Nuke using the VectorBlur node, here we can control the amount of blur, its falloff etc.Enable MotionBlur in the Render settings and enable "Instantaneous Shutter". Increase key value for complex animations especially for rigged humans. 

Z-Depth: This AOV calculates and separates out the depth values of each asset in the scene from the camera, this can be used in Nuke along with the ZDefocus node to setup focal planes and apply blur accordingly. Green areas are in full focus, red areas in front of the focal point and blue areas are behind it.

SubsurfaceScattering: As a gamer, this term is not alien to me. Though very subtle in these renders, its effect on the overall composite can't be ignored if we are using human assets especially. Add the "sss" AOV to the active list and we can color grade it in Nuke to bring out the Subsurface Scattering even more.

Tips of the Week:

1. Increase sampling values for Camera(AA), Diffuse, Specular and the Skydome light incrementally, render and snapshot each sampling level and stop at a point where improvements out weigh the performance cost.

2. Disable transmission or sss if you don't have anything in the scene with those properties to cut down on render times.

3. Don't forget to enable "Merge AOVs" in the Arnold Driver properties.


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