Light and Prisms: Uncovering the Nature of Color - prism of light
When done correctly, cross-polarised light can strip out all the reflection and the result can then be graded, projected, cleaned up, and then plugged into the ‘base_colour’ or ‘diffuse’ component of your shader, resulting in a more physically accurate result.
P-polarizedlight
Before we dive in, I just want to give a huge shout out to our collaborative partner, Clear Angle Studios, for supporting CAVE Academy and allowing us to capture high-quality datasets, and share the results on our Wiki.
Here at CAVE Academy the beauty of giving and sharing is very close to our hearts. With that spirit, we gladly provide Masterclasses, Dailies, the Wiki, and many high-quality assets free of charge. To enable the team to create and release more free content, you can support us here: Support CAVE Academy
Unpolarizedlight
Elliptical polarization
To capture cross-polarised shots, we’ll need to cover all the light sources with linear polarising filters. If you are using multiple lights, the polarising filter must be going in the same direction (for example, vertical). You DO NOT want one light to have a filter running horizontally and another light to have a filter running vertically. If you then apply a circular polarising filter (CPF) onto your camera lens, and then rotate the CPF to be perpendicular (in this example, horizontal) to the light filters, you should then be able to remove all the highlights and the glare, and be left with wondrous colour. Easy peasy.
Circular polarising filters (CPF) are made of special materials that are able to block one of the two planes of vibrations depending on how you rotate the filter. Do note, however, that by using a CPF, you will be reducing the amount of light transmission, and therefore, you will need to shoot a Macbeth chart with and without the CPF. This will allow you to properly grade your cross-polarised images later on for texture projection.
Photon polarization
The cross-polarised and polarised images will need to line up perfectly in order to subtract the specular component (which can then be used as visual reference to compare against the specular AOV).
You reached this page when trying to access https://iopscience.iop.org/article/10.1088/0022-3727/37/23/E01 from 38.187.249.183 on December 16 2024, 23:30:30 UTC
And here is an example of a prop shot under cross polarised and polarised light, and then the subtraction (and desaturation) process.
This will not be an in-depth walkthrough of setting up a look dev and texture shoot, but below, you’ll be able to watch a short timelapse video of how I go about setting up a shoot (although this is generally configured depending on the type of asset I am shooting).
Unpolarised light is a light wave that vibrates in a variety of directions. For simplification, let’s say it vibrates in a vertical and horizontal plane. Examples of unpolarised light include light emitted from the sun, a candle, a lightbulb or an LED panel.
Circularly polarizedlight
Shooting under cross-polarised light is extremely useful for texture artist and a fantastic way to gain a truer reference of the surface colour and the type of material itself. For metallic materials for example, we’ll just be left with black as true metals do not have any diffuse colour. So not only is it a great way to find the diffuse colour of non-metals but also a great way of figuring out what is a metallic object and what is non-metal.
When the light wave hits a surface, the reflected component will still maintain its polarisation. However, the diffuse component will be unpolarised. This is a good thing as it will allow us to have reference of the specular side of things, but by polarising just the lights, we are not getting reference of the diffuse only. To do this, we’ll need to look at cross-polarisation.
Do note, however, that by using filters on the lights and the lens, you’ll be reducing the amount of light transmission. Therefore, you’ll want to double-check your exposure as you shoot and make sure to shoot the Macbeth chart for each of the setups (unpolarised, polarised and cross-polarised). This will allow you to grade and expose your images later on.
So hopefully at this stage you can see the benefits from shooting assets under UNPOLARISED (look dev), POLARISED (spec ref) and CROSS POLARISED (diffuse ref) light. If you have any thoughts or other ideas, it would be great to hear from you.
Polarization oflight
There are many ways in which unpolarised light can be polarised. For example, through reflection, refraction and scattering but for now, we’ll stick to using a filter.
In this document, we’ll dive into how we can control light in order to support us in the capture of texture and look development reference. And just a quick note, I am not a scientist (although a big, big fan of science), therefore, if I use the wrong terminology, feel free to let me know and I can make the corrections as required 😉
Linear polarization
Here is an example shot at Clear Angle Studios using cross polarised light, (left) polarised light (middle), and what happens when we subtract one from the other, which reveals the spec (of which I have desaturated).
We should already know at this stage that a light wave is an electromagnetic wave. For a deeper breakdown of light, make sure to check out this page:
Cross polarisation is the process of polarising the incident light and the reflected light using two polarisers with perpendicular orientation. By doing this, we can effectively remove the specular reflectance and be left with just the diffuse component.
So that means the lights and the camera should not move between the images. Because of my shoot setup, removing the polarising filters from the lights usually results in some movement to the lights themselves, so the last thing I’ll shoot is the actual look dev (unpolarised) reference. I don’t mind if the look dev images are not perfectly aligned to the cross-polarised/polarised images. If you can, ideally you would have a separate set of lights that are polarised and unpolarised, and you would fire off the specific lights for the specific shot.
We can transform unpolarised light into polarised light by using a polarising filter that sits in front of the emitting light, or by fitting a circular polarising filter on the lens. The chemical composition of the filter can restrict the vibrations of the electromagnetic wave to a single plane (either vertical or horizontal). By restricting one half of the vibrations, we’ll also be reducing one-half of the intensity of the light, so we’ll need to factor this in when shooting reference. Again, this is where the Macbeth chart comes in, so make sure to shoot it under polarised light.
Do note, however, that some sources will emit polarised light, for example, the light from your computer screen will more than likely emit polarised light.
Take any object and shoot under cross-polarised light and you’ll see that it is probably darker than you expected it to be, and you might even spot a slight shift in hue and saturation. So, as you explore materials in the real-world, do observe and think about how the colour might not quite be what you think it is.
For look development, shooting reference with unpolarised light is a good idea as on most occasions, that’s how we’ll be seeing the assets in real-life. Therefore, when shooting look development reference, you should shoot with no polarising filters are on the lights and no cross polarising filter is on the lens. Don’t forget to shoot the Macbeth chart reference, so you have a ground truth of how colour is captured with your selected camera, and from which you’ll be able to grade and expose your images later on.