Additional surface properties
The VRayMtlWrapper can be used to specify additional surface properties per material. These properties are also available in the Object settings dialog. However, the settings from the VRayMtlWrapper override those from the Object settings.
Base material - this is the actual surface material.
Additional surface properties
Generate GI - controls the GI generated by the material. The numerical value is a multiplier for the amount of GI generated by the object
Generate caustics - turn this off, if the material should not generate caustics.
Receive GI - controls the GI received by the material. The numerical value is a multiplier for the amount of GI received by the object
Receive caustics - turn this off if the material should not receive caustics. The numerical value is a multiplier for the amount of caustics received by the object
Matte surface - makes the material appear as a matte material, which shows the background, instead of the base material, when viewed directly. Note that the base material is still used for things like GI, caustics, reflections etc.
Alpha contribution - determines the appearance of the object in the alpha channel of the rendered image. A value of 1.0 means the alpha channel will be derived from the transparency of the base material. A value of 0.0 means the object will not appear in the alpha channel at all and will show the alpha of the objects behind it. A value of -1.0 means that the transparency of the base material will cut out from the alpha of the objects behind. Matte objects are typically given an alpha contribution of -1.0. Note that this option is independent of the Matte surface option (i.e. a surface can have an alpha contribution of -1.0 without being a matte surface).
Shadows - turn this on to make shadow visible on the matte surface.
Affect alpha - turn this on to make shadows affect the alpha contribution of the matte surface. Areas in perfect shadow will produce white alpha, while completely unoccluded areas will produce black alpha. Note that GI shadows (from skylight) are also computed, however GI shadows on matte objects are not supported by the photon map and the light map GI engines, when used as primary engines. You can safely use those with matte surfaces as secondary engines.
Reflection amount - a multiplier for the reflections from the base material. This only works if the base material is directly a VRayMtl.
Refraction amount - a multiplier for the refractions from the base material. This only works if the base material is directly a VRayMtl.
Color - an optional tint for the shadows on the matte surface.
GI amount - determines the amount of GI shadows.
Brightness - an optional brightness parameter for the shadows on the matte surface. A value of 0.0 will make the shadows completely invisible, while a value of 1.0 will show the full shadows.
No GI on other mattes - this will cause the object to appear as a matte object in reflections, refractions, GI etc for other matte objects. Note that if this is on, refractions for the matte object might not be calculated (the object will appear a matte object to itself and will not be able to "see" the refractions on the other side).
GI surface ID - this number can be used to prevent the blending of light cache samples across different surfaces. If two objects have different GI surface IDs, the light cache samples of the two objects will not be blended. This can be useful to prevent light leaks between objects of vastly different illumination.
The Material ID group is described here