V-Ray Compositing Tag is a object tag that control of object different ways of visibility, just select the object and right click on it to add the V-Ray Compositing Tag.
Name – here you can enter a name for the object.
Layer – If an element was assigned to a layer its layer color will be displayed here. This field reflects the layer color in the Layer Palette. You can drag & drop layers from the Layer Browser or similar layer fields onto this field. You can also assign layers or remove elements from current layers using the menus located behind the small triangle.
Visible to camera – if this is on, object will be visible for camera and object will be rendered.
Cast Shadows – if this is on, object will have shadows.
Recieve Shadows – if this is on, object will have recieve shadows on it surface from other objects.
Generate GIs – if this is off, object will not be used in GI calculation.
Object visibility – with this option you can change visibility of the object, it also can be animated.
Object ID – ID of object for multipass rendering.
Generate GI – controls the GI generated by the material.
Recieve GI – controls the GI received by the material.
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 are also computed, however GI shadows on matte objects are not supported by the Photon map and the Light cache GIengines, when used as primary engines. You can safely use those with matte surfaces as secondary engines.
Color – an optional tint for the shadows on the matte surface.
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.
Reflection amount – shows the reflections from the material.
Refraction amount – shows the refractions from the material.
GI amount – determines the amount of GI shadows.
Matte for secondary rays – normally the diffuse layer will be used when an object with a vray material is seen through reflections/refractions. Turn this option on, if you want the vray material to show the environment when seen through reflections/refractions.
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.