Model – Determines the type of BRDF (the shape of the highlight and glossy reflections). This parameter has an effect only if the Reflection Color is different from black and Reflection Glossiness is different from 1.0.
GGX is the most modern and flexible BRDF (Bidirectional reflectance distribution function) type and is able to better represent a broad range of materials thanks to its ability to control the shape of the specular lobe.
There currently isn't any particular performance difference between models and there is little reason to choose any of the other types.
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Historically, the Phong, Blinn, Ward and GGX are successive reflectance models developed over the years in computer graphics where each model aimed to improve on the limitations of the previous ones. For example, the specular highlights with the Phong model have a very narrow and bright center with no falloff, but it doesn't work well with anisotropic reflections. The Blinn model has broader highlight center with a tight falloff. The Ward model has an even broader center and falloff. The GGX model has a bright center and an even longer falloff (at default settings). In the past, each model's characteristics resembled more closely a certain type of material, for example Phong could be used for plastics, Ward for cloth and metals, and Blinn for other common surfaces. However with the introduction of the GGX model, all of these surfaces can be approximated well, thus reducing the need for using the other models.
It should be noted that no principled model is able to represent all possible materials entirely accurately, and where those models fail - for example when the material isn’t viewed frontally - only approaches such as that of VRscans are able to capture the correct material representation.
GTR Tail Falloff – Controls the transition from highlighted areas to non-highlighted areas when the Model (BRDF Type)is set to GGX.
Color – Reflection color. Note that the reflection color dims the Diffuse Color.
Glossiness – Controls the sharpness of reflections. A value of 1.0 means perfect mirror-like reflection; lower values produce blurry or glossy reflections. Use the Reflection Subdivs parameter below to control the quality of glossy reflections.
Unlock Fresnel IOR – The IOR to use when calculating Fresnel reflections. Normally this is locked to the Refraction IOR parameter, but you can unlock it for finer control.
Use Fresnel – When enabled, makes the reflection strength dependent on the viewing angle of the surface. Some materials in nature (glass, etc.) reflect light in this manner. Note that the Fresnel effect depends on the index of refraction (IOR) as well.
Show Advanced Options – When enabled, exposes some advanced options in this tab.
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Reflection
Metalness – Controls the reflection model of the material from dielectric (metalness 0.0) to metallic (metalness 1.0). Note that intermediate values between 0.0 and 1.0 do not correspond to any physical material. This parameter can be used with PBR setups coming from other applications. The reflection color should typically be set to white for real world materials.
Glossy Fresnel – When enabled, uses glossy fresnel to interpolate glossy reflections and refractions. It takes the Fresnel equation into account for each "microfacet" of the glossy reflections, rather than just the angle between the viewing ray and the surface normal. The most apparent effect is less brightening of the grazing edges as the glossiness is decreased. With the regular Fresnel, objects with low glossiness may appear to be unnaturally bright and "glowing" at the edges. The Glossy Fresnel calculations make this effect more natural.
Anisotropy
Strength – Determines the shape of the highlight. A value of 0.0 means isotropic highlights. Negative and positive values simulate "brushed" surfaces.
Rotation – Determines the orientation of the anisotropic effect in a float value between 0 and 1 (where 0 is 0 degrees and 1 is 360 degrees).
Axis– Specifies a local object axis for the anisotropy effect.
Advanced Options
Show Advanced Options – When enabled, exposes some advanced options in this tab.
Dive to Surface Node – Press this button to dive into the Complex Material network of nodes and select the Surface Node.
