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Reflection – Enables reflection of the material. 

Reflection Color – Specifies the reflection color. Note that the reflection color dims the diffuse surface color based on the Energy preservation option. For more information, please see the Reflection Color Parameter example below

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BRDF – Determines the type of BRDF (the shape of the highlight). The use of GGX model is recommended for metals and other highly reflective materials There are 4 types available - PhongBlinnWardMicroface GTR (GGX)For more information, see the BRDF Type example below. 

Phong
Blinn
Ward
Microfacet GTR (GGX)
 

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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.

 Back Side Reflect – When disabled, reflections are calculated for the front side of the objects only. When enabled back-side reflections will also be calculated.  

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Diffuse Texture Alpha – The diffuse texture alpha channel controls the opacity. Diffuse Texture Alpha source works the same way as Diffuse Map Alpha as Transparency legacy option from the V-Ray versions before 3.60. 
Opacity Texture Alpha – The opacity texture alpha channel controls the opacity, instead of the default texture intensity. If there is no texture in the source slot, the option will be ignored. 

 

Advanced Options

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Double-sided – When enabled, V-Ray will flip the normal for back-facing surfaces with this material. Otherwise, the lighting on the "outer" side of the material will be computed always. This can be used to achieve a fake translucent effect for thin objects like paper.

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