Layers

SCREENSHOT DESCRIPTION
Coating layer: The coating layer uses a special reflection model with only the specular component. It is useful for simulating varnishes and paints on a layered material. You can use several coating layers to simulate multiple varnishes and paints. The coating itself reflects some light, while any layers of material underneath absorb the rest of the light. You can change 'extinction coefficient' to modify the reflectance (based on fresnel equations) and define the absorption density of the layers of material underneath. Both the 'extinction coefficient' and the 'thickness' of the layered material are used to calculate absorption at a microscopic level.
Basic layer: The basic layer consists of a diffuse, translucent, and fresnel based specular component. It is a highly energy efficient material designed to be used mostly for matte and plastic materials. You might also use basic layers to create metals and translucent materials. Metals, in most cases, have a non-zero extinction coefficient, which corresponds to a high fresnel coefficient under any viewing angle.
Metal layer: You can create a metal with perfect reflection (roughness = 0), a very rough metal (roughness = 100), or something in between. The bidirectional scattering distribution function (BSDF) uses fresnel equations for reflections, which is controlled by the index of refraction and the extinction coefficient. Set the 'index of refraction' to around 1 to make the material less reflective. As you increase the value, reflection becomes stronger and stronger; at very high values, the reflected light takes on the color of the selected color. Use a non-zero value for the 'extinction coefficient' to amplify reflection.
Glass layer: You can create a glass with perfect reflection and refraction (roughness = 0), a very rough glass (roughness = 100), or something in between. The bidirectional scattering distribution function (BSDF) uses fresnel equations to balance reflection and refraction, which is controlled by the index of refraction and the extinction coefficient. Set the index of refraction to around 1 to make the material less reflective and more refractive. Set the value to exactly 1 to make the glass perfectly transparent. As you increase the value, reflection becomes stronger and stronger; at very high values, the reflected light takes on the color of the selected color. Tip: To achieve perfect transparency, we recommend that you create a thin glass layer instead of a glass layer with transmittance enabled and the index of refraction set to 1. Important: Fresnel coefficients are based on both the index of refraction and the angle of incidence. Even with a very small index of refraction, the BSDF will be quite reflective for grazing angles. A real world example is a swimming pool. When you look straight into the pool, the water is transparent; however, when you look at the pool from afar, the water reflects the environment.
Thin glass layer: This glass model describes thin glass materials that show perfect (mirror) reflection and transparency. Thin glass models are very accurate models and are great for assigning to thin surfaces, such as windows and thin transparent plastics. Although you could also use a glass material with transmittance enabled and index of refraction set to 1, it is recommended to use the glass model whenever you want to achieve transparency. Another way to achieve transparency is to actually model a surface, such as a window, as a thin double interface where refraction takes place at both sides. Using the glass model though is optimal in terms of visual accuracy and additionally, it can be traced during shadow evaluation (something like this cannot be done with the double interface model which will create shadows). The glass model does not assume the model to be closed as it does not define an interior/exterior volume. The index of refraction is used as if the model was a double interface, in order to compute the overall transmittance due to double refraction.
SSS layer: The bidirectional subsurface scattering distribution function (BSSDF) is a generalization of the bidirectional scattering distribution function (BSDF); however, unlike BSDF, the entry and exit points for BSSDF may differ instead of coinciding. Therefore the evaluation of BSSDF is far more difficult, as it involves the interaction of surface reflectance - transmittance along with scattering through participating media. Besides the surface reflectance entries, there are also parameters describing absorption and scattering inside the object. In order for the SSS material to be evaluated correctly, the object should be closed (without holes). Participating media with high albedo (i.e., when the scatter density is much higher than the absorption density) are particularly difficult to render. To accelerate rendering with minimum loss of accuracy, usually you can turn an asymmetric medium into an anisotropic medium with a synchronous decrease of its scatter density. Assuming that the asymmetry of the medium is g > 0, you can set asymmetry to the isotropic value of 0 and decrease the scatter density to a value that is equal to the old scatter density multiplied by 1-g. The new medium will have lower albedo, and it will render faster with minimum loss of accuracy.