Cross-polarisation

Cross-polarisation is a way of photographing a surface without its reflections. A polarising filter goes over the light and a second one on the lens, turned 90° to the first. Specular reflections are blocked; diffuse colour passes through. The result is close to a pure base colour, which cuts down de-lighting work later.

Light Collab's texture-photography guide sums it up: it is "a technique that removes specular reflectance from photographs" (Light Collab Help Documents: Cross-Polarization Photography for Textures).

#How it works

Light that passes through a linear polariser vibrates in one direction only. What happens when it hits a surface depends on the kind of reflection. Paul Bourke explains that "shiny surfaces (metals, specular materials) preserve polarisation of the light they reflect", while a surface that behaves as a diffuse reflector "will randomly depolarise incident polarised light" (paulbourke.net: Cross polarisation in photography).

So a lens filter turned 90° to the light's filter blocks the reflections, which are still polarised, and lets most of the scattered colour through, which is not (paulbourke.net: Cross polarisation in photography). See photo-sourced and scanned materials for a diagram of the two light paths.

#The kit

• A linearly polarised light. Bourke is specific: the light source must be polarised "linearly (not circular)" (paulbourke.net: Cross polarisation in photography). Light Collab uses laminated linear polarising film over an LED panel (Light Collab Help Documents: Cross-Polarization Photography for Textures); Polycount's almighty_gir used the lenses of passive 3D cinema glasses taped over a flash as a cheap alternative (Polycount: Super ghetto cross polarised photos for…).
• A polarising filter on the lens. Bourke uses a linear polariser (paulbourke.net: Cross polarisation in photography); Light Collab uses a circular polarising filter (Light Collab Help Documents: Cross-Polarization Photography for Textures).
• Darkness around you. Any light that does not pass through your filter is not polarised and brings reflections back. almighty_gir recommends shooting in the darkest room you can find (Polycount: Super ghetto cross polarised photos for…). Light Collab notes that outdoors, reflections of the sky prevent true cross-polarisation (Light Collab Help Documents: Cross-Polarization Photography for Textures).
• A tripod and RAW files, so paired shots line up and in-camera sharpening does not alter texture detail (Light Collab Help Documents: Cross-Polarization Photography for Textures).
• A colour chart. Without one, EarthQuake and marks on Polycount agree the photos are only loosely useful as physically based rendering values (Polycount: Super ghetto cross polarised photos for…).

#Setting it up

1. Place the light at about 45° to the camera's axis; Bourke uses this arrangement for his painting scans (paulbourke.net: Cross polarisation in photography).
2. Point the camera at something very shiny. Bourke photographs "a metallic spoon while in live view mode and rotating the polariser until the highlight is suppressed" (paulbourke.net: Cross polarisation in photography).
3. Compensate for the light you lose. Bourke measured a loss of at least 1.3 stops on diffuse reflections and notes a bright light source usually makes up for it (paulbourke.net: Cross polarisation in photography). Light Collab warns that too little light forces slow shutter speeds and blurred or grainy images, so use a tripod and a bright light source (Light Collab Help Documents: Cross-Polarization Photography for Textures).
4. Lock white balance, or process every RAW file with identical settings (Polycount: Super ghetto cross polarised photos for…).

#Limitations

• It does not remove shadows or occlusion. Only reflections are blocked. Cast shadows and crevice darkening still need flat lighting and de-lighting in software (Unity-Technologies/DeLightingTool on GitHub: De-Lighting in Unity).
• It does not work well in daylight. Sky light is not polarised by your filter (Light Collab Help Documents: Cross-Polarization Photography for Textures).
• It throws away the reflection information you will later need for roughness unless you shoot a parallel reference too (Light Collab Help Documents: Cross-Polarization Photography for Textures).

#Related terms

de-lighting · photo-sourced and scanned materials · photogrammetry · roughness map · base colour (albedo) map · photogrammetry and scanning tools

#Further reading and watching