Grading AOVs - Guillermo Algora - Visual Effects Compositor

Guillermo Algora
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Table of Contents:
0. Introduction

How to keep shaders stable by keeping the maths consistent inside Nuke and how it has been treated in lighting and the shading applications.
One way to respect much of the shaders and at least certain advantages to that. I way to start and then working it to final.

- Contrast Pivot: when grading CG often the pivot point is not 0.5.
- Albedo is contained in the image and we divide the albedo (sometimes called diffuse) by the image to remove it and grade it (min 05:48). But according to Rafal this breaks the image really, really fast.

- Why is grading a single shader AOV more likely to break the look of the CG?
We get individual passes and when we grade something we often pick one of the AOVs (the strongest one) but by doing that we are unbalancing our shaders. Particularly when starting a shot and doing broad strockes, one needs to gang two together, one for the direct, one for the indirect. Otherwise, it might look fine on one shot but break on multiples. At least, to begin with, if you want to grade a component, grade all the components together. What if what I want to grade is the Indirect (i.e. fill up all my shadows in a clever way)? then we would gang all the Indirects together (if we just grade one indirect, we start to break the balance of the indirects again). That is all to keep the shaders stable.

- Why using a gamma or add on a light select (LS) pass break the CG?
Because you would be grading the diffuse contributions, you would be grading the texture. A light select is a combination of all the component together seen through a selection of lights. In grading the texture, you start lifting the blacks and making it look glowy (because when you grade texture, you are no longer grading light). A multiply does not break the maths of the shader, Rafal says so. Better to use multiply on a component that has diffuse contribution.
In light selects you can use multiplies? Because that would not affect the albedo according to Rafal.

- How do you grade and AOV Shader pass that contains only Diffuse contribution?
Divide the pass by the Albedo. Little tip, put and Add, very small value to avoid division by 0 (just in case). With the divide you separate the texture from the light. Unless you want to change the color of your texture, always remove the albedo before grading those components that have the albedo in it.

- How do you grade the Albedo?
You need to add all the passes that have Diffuse in it together, because either need to be all done individually (substract albedo, grade, put back, on each of them) or add them together and one in one go. You need to know what you are doing.

- What about SSS and Shaders not building to match the beauty?
SSS have albedo in it, but doesn't work exactly like the others, because the maths is 3 layers of albedo that have been filtered according to the properties of the material. But at least removing the albedo can also be effective.

- PBR Shading (RenderMan/Arnold:
Albedo, SSS Direct, SSS Direct, Specular Direct, Specular Indirect, Diffuse Direct, Diffuse Indirect, Emission, Volume, Refraction Direct, Refraction Indirect.

- Most renders call it differently, some places is called Albedo, Diffuse, ColorMap, is the color texture unlit. Some passes contain the albedo, the texture in it, and some not (Diffuse Direct, Diffuse Indirect, SSS Direct, SSS Direct).

- Shader Cookbok (will need to check it again):
When grading AOVs is easier to make look worse than make it look better. This covers the basics to not break what you have been given.

All you need is Add (plus them together)?:
- Can't add shaders and light selects.
Options are:
Grade substractively:
Shaders cookbook minute 13, explains more.

- Easy to break other channels. i.e: premults, alphas, data channels.
Grading needs to be done on unpremultiplied, then premultiply.
Data AOV or alpha need to not be graded or premultiplied.
Filtering of data needs impulse.
- Not efficient.
- Ignores left overs. i.e. full beauty.
Substract the total of all the channels from the beauty and add that.
- Grades texture color?
- Breaks integrity/balance shaders.
- Not good for sharing / template.

Maybe talk a bit about what each light is?
Direct is the light while indirect is the bounce.

Vray AOV Contributions:

- He combines them in Direct:
Specular: all with the word spec on it.
Diffuse: everything with the word diffuse in it.

and Indirect:
Everything with the word indirect in it (refraction, self illumination, sss, GI, reflection).

Those together is the Beauty. Basic additive beauty build.

- It's often if you have SSS that the beauty will not build.
- Most companies now use single substraction or maybe an additive setup but inside a group with lots of slider.
- Reflection and refraction is based upon a filter: allows you to change the reflected angle / strength.
- Shadows: separate it and minus or plus it (that is vRay), but Arnold is multiplication? or use a Grade with the mask?

Two approaches:
- Additive: is useful when you do heavy grading (more than 2/3?) but it might not build up to the beauty. For that you might want to do a difference.
- Substractive:

Most common passes:
1. Beauty.
Direct lighting, indirect lighting.
2. Light Selects.
How to work with them.
3. Utility.
Require impulse filter.

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