BRDF parameters determine the type of the highlights and glossy reflections for the material. The parameters have an effect only if the reflection color is different from black and reflection glossiness is different than 1.0.
Type – – Determines the type of BRDF (the shape of the highlight). For more information, see the BRDF Type Example below.
Phong – Phong highlight/reflections. Specular highlights have a bright center with no falloff. Blinn – Blinn highlight/reflections. Specular highlights have a bright center with a tight falloff. Ward – Ward highlight/reflections. Specular highlights have a bright center with a falloff broader than Blinn, but tighter than Microfacet GTR (GGX). Microfacet GTR (GGX) – GGX highlight/reflections. Specular highlights have a bright center with a longer falloff.
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GGX is the most modern and flexible BRDF 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 broaded 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.
Use glossiness / Use roughness – These options control how Reflection Glossiness is interpreted. When Use glossiness is selected, the Glossiness value is used as is, and a high Glossiness value (such as 1.0) results in sharp reflection highlights. When Use roughness is selected, the Reflection Glossiness inverse value is used. For example, if Reflection Glossiness is set to 1.0 and Use roughness is selected, this results in diffuse shading. Conversely, if Glossiness is set to 0.0 and Use roughness is selected, this results in sharp reflection highlights. Note that the Roughness parameter itself has no bearing on the results of this option.
GTR tail falloff – Controls the transition from highlighted areas to non-highlighted areas when the BRDF type is set to Microfacet GTR (GGX).
Anisotropy – Determines the shape of the highlight. A value of 0.0 means isotropic highlights. Negative and positive values simulate 'brushed' surfaces. The accepted values are in the range from 0.99 to -0.99. For more information, see the Anisotropy example below.
Rotation – Determines the orientation of the anisotropic effect in degrees. For more information, see the Anisotropy example below.
Local axis – When enabled, the orientation of the anisotropic highlight is based on the object's local X, Y, or Z axis.
Map channel – When enabled, the orientation of the anisotropic highlight is based on the specified map channel.
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Use the VRayMtl whenever possible in your scenes. This material is specifically optimized for V-Ray and often GI and lighting is computed much faster for V-Ray materials than for standard 3ds Max materials. Many V-Ray features (e.g. Light Cache, render elements) are guaranteed to work properly only with VRayMtl and other V-Ray compliant materials.
VRayMtl can produce reflections/refractions for matte objects – see VRayMtlWrapper.
There's currently a known issue with setting the Opacity mode to Clip and bump mapping not rendering properly on a material. A workaround is to load the opacity map through a VRayBitmap texture map.
Note that Sheen, Coat and Reflection components are affected by the following parameters: Use roughness, Trace Reflections, Max Depth, Dim Distance, Dim Falloff, Reflect on Back Side, Affect Channels.
Since V-Ray 6 Fresnel reflections always include glossy Fresnel algorithm. Therefore, the Glossy Fresnel option is hidden from UI for VRayMtl and VRayToon materials.
