In this Bonus video on Material AOVs, I cover Cross Polarization photography, which is a technique that allows us to separate diffuse and specular components of everyday objects. I go into detail about the lighting concepts that allows this separation to occur, and how it’s used to gather reference and textures to recreate objects in 3D.
Electromagnetic Spectrum
Visible Light is a section of the Electromagnetic Spectrum
Light / Color is represented in 2D as a Sine Wave with a specific frequency
3D Light Wave Representation
The 2D representation looks a bit different in 3D space, since the light waves could be oriented in any and all directions along it’s forward axis
A light beam with randomly oriented Light Waves is referred to as an Unpolarized Light
Linear Polarization of Light
Linear Polarization isolates one specific angle of the light wavelength, only allowing a portion of the light waves that were oriented in the that direction, through the filter
Cross Polarization of Light
Cross Polarization uses 2 Polarizers that are perpendicular to each other, effectively eliminating the light wave passing through.
The first polarizer isolates the light wave to only one orientation
The second polarizer, if parallel to the first, continues to allow the polarized light through, but as it becomes more perpendicular, the light gets dimmer, and eventually blocked entirely
Polarization Upon Reflection
When unpolarized light hits a reflective surface (with a refractive index different than the surrounding medium, such as glass, snow, or water) the specular reflection is polarized or partially polarized to the angle perpendicular to the plane of incidence. (along the surface)
How polarized the Reflection depends on many factors; angle of incidence, material type, etc.
Brewster’s Angle
At a specific angle, the specular reflection is completely polarized to the angle perpendicular to the plane of incidence.
This angle is known as Brewster’s Angle.
Unpolarized Diffuse Component
Only the Specular Reflection has the effect of the Brewster’s Angle Polarization
The Diffuse Component is Unpolarized, because they are newly emitted photons from excited atoms
This phenomenon only happens when the light is reflected off dielectric materials such as water or glass.
When reflection occurs on a metallic surface, no Brewster Angle nor refracted light exist
Polarized Specular Reflections
Placing a Linear Polarizer filter in front of the observer will Cross Polarize some Specular Reflections if angled correctly. It blocks the polarized reflection light wave from shining through it
This is how Polarized Sunglasses are able to eliminate harsh glares and reflections from dielectric surfaces such as glass, water, snow, etc.
Cross Polarized Photography
If you polarize the light source, the Specular Reflection is also polarized (because it’s a mirror reflection of the light wave).
The Diffuse Component is unpolarized light because it is newly created lightwaves oriented randomly. Adding a second polarizer on the Camera, means we can block the Specular Component entirely depending on the angle of the Polarizers. When the 2 polarizers are parallel, we see Specular + Diffuse , and when they are perpendicular we will see only Diffuse.
The Parallel Polarized image gives use the Specular and Partial Diffuse (only Diffuse Component of that orientation)
The Cross Polarized image, negates the Specular, and only shows the other half of the Diffuse Component
To isolate the Specular Component, take Parallel Polarized image (Specular + Partial Diffuse) and minus the Cross Polarized image (Partial Diffuse). The Diffuse Components cancel out, and all that is left is the Specular Component
This Cross Polarization Photography allows CG Artists to collect photogrammetry data of everyday objects, and allows them to recreate these objects in 3D with accurate Diffuse and Specular Maps for Physically Based Rendering
What seems just like theoretical Diffuse/Specular Render Pass separation in CG is actually a lighting phenomenon that can be separated into Diffuse and Specular Components in the real world
Notice that Metallic Materials have no real Diffuse Color to them, They show up as completely black in the Cross Polarized result. Metals are entirely surface level Specular Reflections
Occasionally, the Diffuse Components of the Parallel Polarized and Cross Polarized Images are slightly different, (brighter or a shift in color for example)
In this case, when we minus the Cross Polarized result from the Parallel Polarized result, we are left with leftover color information or artifacts. The Specular Component can be desaturated to compensate for those color artifacts
Remember that in Dielectric Materials the Specular Component is the same color as the light source, but Metals can sometimes tint the Specular color depending on the type of Metal
Light Stage: Cross Polarization
The light stage used in films is capturing evenly lit, cross polarized textures of various facial expressions.
This helps separate Diffuse and Specular and aids in tracking features of the face
References:
Here are some great websites that go into more detail about polarizations:
In this post we are going to be focusing in on the Material AOVs Category.
Levels of Complexity
There are different levels of complexity to rebuilding Material AOVs into the beauty, and it all depends on how much flexibility and control you want with the cost of complexity and speed.
Simple
Diffuse
Specular
Emission
Other – Refraction / True Reflection
Intermediate
Diffuse
Direct Diffuse
Indirect Diffuse
Sub Surface Scattering
Specular
Direct Specular
Indirect Specular
Reflection
Coat
Sheen
Emission
Other – Refraction / True Reflection
Complex
Diffuse
Direct Diffuse
Indirect Diffuse
Sub Surface Scattering
Raw Diffuse
Albedo / Color / Texture
Specular
Direct Specular
Indirect Specular
Reflection
Coat
Sheen
Raw Specular
Albedo / Filter / Texture
Emission
Other – Refraction / True Reflection
Diffuse, Specular, and Emission are the Foundational Categories, and the complexities are subdivisions of the Diffuse and Specular Categories
So let’s first focus on the Simple category of Diffuse, Specular, and Emission and really break those down and understand them fully. This will make the future subdivisions easier, familiarise us with terms and concepts, and help us have a grounded foundation of knowledge for what we are adjusting when using these passes.
The full presentation from the video can be downloaded here in pdf format, for those who want to keep or study it offline:
Emission is any object, material, or texture that is actively emitting light into the scene
This includes any Lights, Super-heated metals, or Elemental FX like fire/ sparks / lightning / magic etc
Neon Lights, Screens, Monitors are all examples of real life Emission objects
Diffuse vs Specular
Specular – Surface Level Reflections
Diffuse – Light passes through surface and interacts with the material at a molecular level, Scattering and Absorption allow certain colors to re-exit and scatter into scene
Let’s talk about Specular first andSurface level Reflections
Specular
Law of Reflection
The angles of incidence is equal to the angle of reflection
Smooth Surface – Specular Reflections
Light Beam = a bundle of parallel light rays
Light Beam remains parallel on incidence and parallel on reflection
Planar Mirror and Virtual Image
An Image created by planar specular reflection that does not actually exist as a physical object is referred to as a Virtual Image.
The Virtual Image appears to be located “behind” the mirror
Virtual Image distance = Object to Mirror + Mirror to Observer.
Speculum is the Latin word for “mirror”, which is where “Specular” derives from
The people are witnessing a virtual image of themselves looking back, that is double the distance from them to the mirror. The light travels from them -> to the mirror, and then from the mirror -> back to their eye
Notice the reflected virtual image of the chess piece is in focus, even though the real piece (in the foreground) is out of focus. The camera lens is respecting the mirror’s virtual image distance, even though the mirror itself is out of focus.
Here you can see a ground plane mirror appearing to invert the tree in it’s virtual image
Rough Surface – Diffused Reflection
The uneven surface causes the Incidence Rays to hit at different angles
The outgoing reflection rays scatter in different directions
Here you see some examples of different CG materials along the Roughness / Glossiness spectrum
Wet Surface Reflections
When a surface is wet, the water fills the gaps and flattens the surface and causes more a specular reflection
Microscopic Surface Details
In these slides and examples we are discussing surfaces at a microscopic level. You might think a piece of paper looks smooth, but under a microscope it has quite a bit of roughness to it, which is what makes it so evenly lit and diffuse.
Metallic vs Dielectric Surfaces
The diffuse and specular terms describe two distinct effects going on. The Light interacts with materials differently depending on if the material is a metal, or a non-metal (Dielectric)
Dielectric – Absorbs and Scatters light
Metallic – Does not Absorb light. Only Reflects
Dielectric (Non-Metal)
Light penetrates the surface level and the molecules of the material absorb and scatter the light within
The light photons excite the atoms they hit below the surface. Some of the light is absorbed, and this energy is converted to heat. Then new light rays (photons) are emitted from the excited atoms. Those might excite nearby atoms or exit the surface as new photons. These new photons are same color as our material.
The Base Color Texture (Albedo Map) – determines the color of the diffusely scattered photons from excited atoms. It’s the color that is scattered back out and not absorbed by the material
Metallic
Does not Allow light to penetrate the surface and does not Absorb light. They only Reflect light on the surface
Metals can be thought of as positively charged ions suspended in a “sea of electrons” or “electron gas”. Attractions hold electrons near the ions, but not so tightly as to impede the electrons flow. This explains many of the properties of metals, like conductivity of heat and electricity
The incoming photon does not excite the atoms, but bounces directly off the electron gas
The Base Color (Albedo) is used to describe the color tint of the specular reflection
“Electron Gas” Model
Notice the Specular Reflections are tinted a certain color depending on the metal type:
On Dielectric Plastic balls, the material color changes, but notice the specular highlights are the same color, maintaining the color of the light or surrounding environment.
Comparison of a Metallic vs Dielectric Material in CG
Chrome Sphere and Diffuse Ball
Used as a reference to see what something 100% Smooth and Metal (Specular) and 100% Rough and Dielectric (Diffuse) looks like in the scene.
The diffuse component includes light that penetrates the surface and interacts with the materials molecules. This happens in different ways in the real world
Transmission
Light passing through the material / surface
Can be thought of as “transparency”
Refraction
when light changes angles as it goes through different materials or mediums
Absorption
When certain wavelength colors of light get absorbed by the material
Scattering
when light is dispersed in many directions when it comes into contact with small particles or structures in the material
Simplified Diffuse Calculation
When the distance that light travels beneath the surface is insignificant and negligible, the calculation can be simplified by the renderer and just calculated at the surface point where the light hits. It uses the Base Color Texture (Albedo) as the Diffuse Color that will scatter.
Sub Surface Scattering
When the distance the light travels beneath the surface of the material is significant, the interior scattering must be calculated. This is referred to as Sub Surface Scattering (SSS)
Physically Based Rendering Terminology
Albedo
Base Color Texture Map
On Dielectrics (non-metal) refers to color of material
On Metals, refers to the color tint of the specular reflection
Texture map is without highlights, shadows, or ambient occlusion
Metalness Map
What area is metallic or not. (will use Albedo Color differently). Usually Black or White
Roughness (Glossiness) Map
How blurry or how sharp the reflection will be
Real life objects often have a diffuse and a specular component
Diffuse describes the color of the billard balls, but the specular highlights are all the same color (reflecting the color of the light above the table)
Iridescence
There is also Iridescent materials that change specular color depending on viewing angle.
Iridescence is a kind of structural coloration due to wave interference of light in microstructures or thin films.
Nuke – Simple Material AOV setup
We can break our fruit bowl render into the 3 simple components, Diffuse, Specular, and Emission. They layers look like this:
You can download the nuke script shown in the Tutorial. I created the mini setups for the 3 different types of renderers, Arnold, RedShift, and Octane. Dividing the Beauty render up into their 3 Diffuse, Specular, Emission Components, and Recombining them.
The project files and the Renders are separate downloads, so if you have already downloaded 1.1 What and Why files or the Fruitbowl Renders, there are a couple ways to combine them to work.
Either add the .nk script to the previous package (in the folder above SourceImages, with the other .nk scripts)
Or simply drop the Render files into the SourceImages folder of the new 1.2 project folder
This will help the Read nodes auto-reconnect to the sourceImages for you.
Recap
Emission / Illumination materials emit light
Specular and Diffuse can be separated by Surface Level Reflections and below surface Material Interactions
Each individual light ray follows the Law of Reflection.
The smoother a surface is, the more mirror-like the specular reflection will be.
The roughness of a surface will cause the reflected rays to scatter, and reflection to be blurred.
Metallic materials do not allow light to enter the surface. They only reflect light
Dielectric materials allow light to enter the surface. Light rays are refracted, absorbed, scattered by the materials molecules. Certain color wavelengths re-exit the surface in random directions, which is what we perceive as the materials color
Albedo – Base Color Texture. On Dielectrics – color of material | On Metals – color tint of the specular reflection.
Sub Surface Scattering is when light below the surface travels a significant distance before re-exiting
Iridescent materials tint the color of the specular reflection depending on viewing angle.
References, Resources, Credits
Firstly, Thanks to Pexels for providing such a good resource for stock reference images
I did a hell of a lot of research on this topic before creating the video, I really encourage you to dig a little further and explore the topics more using these great resources:
Compositing Mentor 
