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Cross Polarization Photography
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:
Reflection and Polarization of light in machine vision – Toshiba Teli Corporation
FilmicWorlds – How to Split Specular and Diffuse in Real Images
Polarization Explained: The Sony Polarized Sensor
Youtube – Cross Polarization Tutorial: Removing Specular Highlights and Reflections – Classy Dog Studios
Youtube – Cross Polarisation Explained by Grzegorz Baran
Youtube – The Key to Cleaner 3D Scans: Cross-Polarization – William Faucher
PetaPixel – Cross Polarization: What It Is and Why It Matters
optometryzone – How do Polarised glasses work?
Youtube – ScholarSwing – 16 – Class 12 – Physics – Wave Optics – Polarisation
Youtube – Khan Academy Polarization of light, linear and circular | Light waves | Physics
Youtube – xmtutor – What is Polarisation?
Youtube – xmtutor – Third Polariser
The Light Stages and Their Applications to Photoreal Digital Actors – PDF
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