VRayMtl

A special material, the V-Ray Material, is provided with the V-Ray renderer. It allows for better physically correct illumination (energy distribution) in the scene, faster rendering, more convenient reflection and refraction parameters. Within the V-Ray Material you can apply different texture maps, control the reflections and refractions, add bump and displacement maps, force direct GI calculations, and choose the BRDF for the material. 

BRDF Type - Determines the type of BRDF (the shape of the highlight). See the BRDF Example for illustration of the types.

Phong - Phong highlight/reflections
Blinn - Blinn highlight/reflections
Ward - Ward highlight/reflections 
GGX - GGX Microfacet highlight/reflections

Opacity - Here you can assign opacity to the material where white is completely opaque and black is completely transparent. You can also assign a map by clicking the check board button. This way you can create a material that has a non-uniform opacity.

Diffuse - The diffuse color of the material. Note the actual diffuse color of the surface also depends on the reflection and refraction colors. See the  Energy preservation  parameter below.

Roughness - Can be used to simulate rough surfaces or surfaces covered with dust (for example, skin, or the surface of the moon).

Self-Illumination - The self-illumination color of the material. A texture map can be used for the self-illumination color by clicking on the check board box next to the color slider.

Reflect - The reflection color. Note that the reflection color dims the diffuse surface color. See the Reflection Color example below for illustration.

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.

Reflect 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. See the Reflection Glossiness example below for illustration.

Highlight Glossiness - Determines the shape of the highlight on the material. Normally this parameter is locked to the Reflection glossiness value in order to produce physically accurate results.

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.

Anisotropy - Determines the shape of the highlight. A value of 0.0 means isotropic highlights. Negative and positive values simulate "brushed" surfaces. For more information, see the Anisotropy example below.

Anisotropy Rotation - Determines the orientation of the anisotropic effect in degrees (rotation in degrees). For more information, see Anisotropy Rotation example below.

Anisotropy Uvwgen - Allows you to assign a placement Texture node and change its UV coordinates to control the direction of stretching of the highlights.

Refract - Refraction color. Note that the actual refraction color depends on the reflection color as well. See the Refraction Color example below for illustration.

Refract IOR - Index of refraction for the material, which describes the way light bends when crossing the material surface. A value of 1.0 means the light will not change direction. See the Refraction IOR example below for illustration.

Refract Glossiness - Controls the sharpness of refractions. A value of 1.0 means perfect glass-like refraction; lower values produce blurry or glossy refractions. Use the Subdivs parameter below to control the quality of glossy refractions. See the Refraction Glossiness example below for illustration.

Fog Color Tex - The attenuation of light as it passes through the material. This option allows to simulate the fact that thick objects look less transparent than thin objects. Note that the effect of the fog color depends on the absolute size of the objects and is therefore scene-dependent. See the Fog Color example below for illustration.

Translucency Color - Normally the color of the sub-surface scattering effect depends on the Fog color; this parameter allows you to additionally tint the SSS effect.

Environment Override - Allows you to connect an environment texture to override the environment for that particular material.

Reflections

Brdf Type - Determines the type of BRDF (the shape of the highlight):

Phong - Phong highlight/reflections
Blinn - Blinn highlight/reflections
Ward - Ward highlight/reflections
GGX - GGX Microfacet highlight/reflections

Subdivs - Controls the quality of glossy reflections. Lower values will render faster, but the result will be more noisy. Higher values take longer, but produce smoother results. This parameter has effect only when the Use Local Subdivs option is enabled from the Global DMC Sampler Settings.

Depth - The number of times a ray can be reflected. Scenes with lots of reflective and refractive surfaces may require higher values to look correct.

 Hilight Gloss. Lock - Enable to use the reflection glossiness for hilights.

Opacity Mode - Controls how opacity is sampled.

Normal -  (Legacy) The opacity map is evaluated as normal: the surface lighting is computed and the ray is continued for the transparent effect. The opacity texture is filtered as normal.
Clip - (Very fast) The opacity texture is not filtered and it is clipped to either fully opaque or fully transparent based on the mid-point value. Useful when there are many transparent surfaces one behind the other like leaves.
Stochastic - (Optimal) The opacity texture is filtered and the surface is randomly shaded as either fully opaque or fully transparent for a correct average appearance.

Fresnel reflections - Enabling this option 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 as well. See the Fresnel example below for illustration.

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.

Lock fresnel IOR- Allows you to unlock the Fresnel IOR parameter for finer control over the reflections.

Affect Channels - Allows you to specify which channels are going to be affected by the reflectivity of the material

Color Only - The reflectivity will affect only the RGB channel of the final render
Color+alpha - This will cause the material to transmit the alpha of the reflected objects, instead of displaying an opaque alpha.
All channels  - All channels and render elements will be affected by the reflectivity of the material

UV Vectors Derivation - Specifies the method for deriving anisotropy axes:

• UVW Generator - Allows the user to assign a UVW Generator for the anisotropy effect.
• Local Axis - Uses a local axis for the anisotropy effect.
  

Anisotropy Axis - Determines how the anisotropy axis is derived.

Refractions

Subdivs - Controls the quality of glossy refractions. Lower values will render faster, but the result will be more noisy. Higher values take longer, but produce smoother results.

Depth - The number of times a ray can be refracted. Scenes with lots of refractive and reflective surfaces may require higher values to look correct.

Enable dispersion - This option enables the calculation of true light wavelength dispersion.

Aberration - This option allows you to increase or decrease the dispersion effect. Lowering it widens the dispersion and vice versa.   

Fog multiplier - The strength of the fog effect. Smaller values reduce the effect of the fog, making the material more transparent. Larger values increase the fog effect, making the material more opaque. See the Fog Multiplier example below for illustration.

Fog bias - Allows to change the way the fog color is applied; by adjusting this parameter you can make thin parts of the object to appear more transparent than normal, or less transparent than normal.

Affect Channels - Allows you to specify which channels are going to be affected by the transparency of the material

Color Only - The transparency will affect only the RGB channel of the final render.
Color+alpha - This will cause the material to transmit the alpha of the refracted objects, instead of displaying an opaque alpha. Note that currently this works only with clear (non-glossy) refractions.
All channels - All channels and render elements will be affected by the transparency of the material.

Affect shadows - This will cause the material to cast transparent shadows, depending on the refraction color and the fog color.

Translucency

Translucency type - Selects the algorithm for calculating translucency (also called sub-surface scattering). Note that refraction must be enabled for this effect to be visible. Currently only single-bounce scattering is supported. See the Translucency type example below for illustration.

None - No translucency is calculated for the material;
Hard (wax) model - This model is specifically suited for hard materials like marble;
Soft (water) model - This model is mostly for compatibility with older V-Ray versions (1.09.x);
Hybrid model - This is the most realistic SSS model and is suitable for simulating skin, milk, fruit juice and other translucent materials.

Back-side color - Normally the color of the sub-surface scattering effect depends on the Fog color; this parameter allows you to additionally tint the SSS effect. 

Thickness - This limits the rays that will be traced below the surface. This is useful if you do not want or do not need to trace the whole sub-surface volume. 

Scatter Coeff - The amount of scattering inside the object. 0.0 means rays will be scattered in all directions; 1.0 means a ray cannot change its direction inside the sub-surface volume. 

Fwd/Bck Coeff - Controls the direction of scattering for a ray. 0.0 means a ray can only go forward (away from the surface, inside the object); 0.5 means that a ray has an equal chance of going forward or backward; 1.0 means a ray will be scattered backward (towards the surface, to the outside of the object).

Light Multiplier - A multiplier for the translucent effect. 

Trace reflections - If this is off, reflections will not be traced, even if the reflection color is greater than black. You can turn this off to produce only highlights. Note that when this is off, the diffuse color will not be dimmed by the reflection color, as would happen normally.

Trace refractions - If this is off, refractions will not traced, even if the refraction color is greater than black.

Cutoff - This is a threshold below which reflections/refractions will not be traced. V-Ray tries to estimate the contribution of reflections/refractions to the image, and if it is below this threshold, these effects are not computed. Do not set this to 0.0 as it may cause excessively long render times in some cases.

Double-sided - If this is checked, 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. You can use this to achieve a fake translucent effect for thin objects like paper.

Reflect on back side - If this is checked, reflections will be computed for back-facing surfaces too. Note that this affects total internal reflections too (when refractions are computed).

Use irradiance map - If this is checked, the irradiance map will be used to approximate diffuse indirect illumination for the material. If this is off, brute force GI will be used. You can use this for objects in the scene which have small details and are not approximated very well by the irradiance map.

Dim Reflect Ray Distance - Allows you to stop tracing reflections after a certain distance.

Distance - Specifies a distance after which the reflection rays will not be traced. 

Falloff - A fall off radius for the dim distance. 

Reflect Exit color - If a ray has reached its maximum reflection depth, this color will be returned without tracing the ray further.

Refract Exit color - If a ray has reached the maximum refraction depth, the ray will be terminated and the exit color returned.

Glossy Rays As GI - This specifies on what occasions glossy rays will be treated as GI rays:

Never - Glossy rays are never treated as GI rays.
GI rays - Glossy rays will be treated as GI rays only when GI is being evaluated. This can speed up rendering of scenes with glossy reflections and is the default.
Always - Glossy rays are always treated as GI rays. A side effect is that the Secondary GI engine will be used for glossy rays. For example, if the primary engine is irradiance map, and the secondary is light cache, the glossy rays will use the light cache (which is a lot faster).

Energy mode - Determines how the diffuse, reflection, and refraction color affect each other. V-Ray tries to keep the total amount of light reflected off a surface to be less than or equal to the light falling on the surface (as this happens in the real life). For this purpose, the following rule is applied: the reflection level dims the diffuse and refraction levels (a pure white reflection will remove any diffuse and refraction effects), and the refraction level dims the diffuse level (a pure white refraction color will remove any diffuse effects). This parameter determines whether the dimming happens separately for the RGB components or is based on the intensity:

RGB - This mode causes dimming to be performed separately on the RGB components. For example, a pure white diffuse color and pure red reflection color will give a surface with cyan diffuse color (because the red component is already taken by the reflection).
Monochrome - This mode causes dimming to be performed based on the intensity of the diffuse/reflection/refraction levels.

Environment Priority - Specifies how to determine the environment to use if a reflected or refracted ray goes through several materials each of which has an environment override.