In addition to the options in the Render Settings, which are global for the scene, you can set different render settings on a per-object basis. Some of those properties (primary and secondary visibility, visibility to camera, etc.) are accessible through the V-RayObject Propertiesrollout of the Object tab.
Object ID – Specifies an ID for the object. This is used by V-Ray for the Multi Matte render channel. Note that when used with a Cloner - the object ID of the original geometry is always considered over the Cloner Object ID.
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A value of 0 does not create an Object ID. Values of 99 or higher render correctly but are not displayed in the options.
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Matte Surface – Enabling this option turns the object into a matte object. This means that the object is not directly visible in the scene; instead, the background color is shown in its place. However, the object appears normally in reflections/refractions and generates indirect illumination based on its actual material.
Alpha contribution – Controls how the object appears in the alpha channel. Note that an object doesn't have to be a matte object for this setting to work - the alpha contribution can be changed regardless of whether the object is matte or not. A value of 1.0 means that the object appears normally in the alpha channel. E.g. if the object is opaque, the alpha is white; if the object is fully transparent, it does not influence the alpha channel at all. A value of 0.0 means that the object, regardless of its actual opacity, is not present in the alpha channel, although it is present, as usual, in the RGB channel. A value of -1.0 means that the object's alpha is cut out from the alpha of the objects behind it. E.g., if the object is fully opaque, its alpha is black, and it blocks the alpha of the objects behind it, whereas if the object is fully transparent, it does not influence the alpha of the other objects at all. Note that turning an object into a matte object does not change its appearance in the alpha channel. You need to explicitly change its alpha contribution.This option can also be mapped by a texture.
Shadows – When enabled, the matte object receives shadows.
Affect alpha – When enabled, makes the shadows affect the alpha contribution of the matte surface. Areas in perfect shadow produce white alpha, while completely unoccluded areas produce black alpha. Note that GI shadows (from skylight) are also computed. However, GI shadows on matte objects are not supported by the Light CacheGI engine, when used as the primary engine. You can safely use Light Cache with matte surfaces as a secondary engine.
Shadow Tint Color – Specifies the color of the shadows.
Shadow Brightness – An optional brightness parameter for the shadows on the matte surface. A value of 0.0 makes the shadows completely invisible, while a value of 1.0 shows the full shadows.
Reflection Amount – If the material of the object is a reflective V-Ray material, this controls how much of the reflection is visible on the matte object.
Refraction Amount – If the material of the object is a refractive V-Ray material, this controls how much of the refraction is visible on the matte object.
GI Amount – Controls how much of the indirect illumination received by the object is visible on the matte.
No GI on Other Mattes – When enabled, the object appears as a matte object in reflections, refractions, GI, etc., for other matte objects. Note that if this is enabled, refractions for the matte object might not be calculated. (The object appears as a matte object to itself and is not able to "see" the refractions on the other side.)
Matte for Secondary Rays – Normally, the base material is used when an object with a V-Ray Override Material is seen through reflections/refractions. Enable this option, if you want the Override Material to show the environment when seen through reflections/refractions. V-Ray can also do projection mapping to increase the realism.
