Mesh Extra Attributes
Subdivision and Displacement Quality
The V-Ray Additional Attributes can be added to any polygonal object by selecting its shape node and choosing the desired attributes from the Attributes>VRay Menu. Currently there are five different sets of extra attributes that you can add: Subdivision , Animation, Subdivision and Displacement Quality, Displacement control and Round edges. Adding these attributes will add new settings in the Extra VRay Attributes roll up.
Render as subdivision surface - this option appears when you add the Subdivision extra attributes. When this option is checked the object will be subdivided during rendering. The settings for this subdivision can be controled globally from the Default Displacement roll up in the Settings tab of the Render Settings dialog. You can also control the subdivison quality per object by adding th Subdivision and Displacement Quality extra attribute.
Subdivide UVs at borders - allows you to choose whether or not the UVs of the object will be subdivided at the borders.
Subdivision and Displacement Quality
Override Global Settings - when this option is checked the settings in the Default Displacement roll up will be overriden for the current object.
View-dependent - when this is on, Edge length determines the maximum length of a subtriangle edge, in pixels. A value of 1.0 means that the longest edge of each subtriangle will be about one pixel long when projected on the screen. When View-dependent is off, Edge length is the maximum subtriangle edge length in world units.
Edge length - this determines the quality of the displacement or subdivison. Each triangle of the original mesh is subdivided into a number of subtriangles. More subtriangles means more detail in the displacement, slower rendering times and more RAM usage. Less subtriangles mean less detail, faster rendering and less RAM. The meaning of Edge length depends on the View-dependent parameter above.
Max. subdivs - this controls the maximum subtriangles generated from any triangle of the original mesh. The value is in fact the square root of the maximum number of subtriangles. For example, a value of 256 means that at most 256 x 256 = 65536 subtriangles will be generated for any given original triangle. It is not a good idea to keep this value very high. If you need to use higher values, it will be better to tesselate the original mesh itself into smaller triangles instead.
Displacement Type - specifies the mode in which the displacement will be rendered
2D Displacement - this method bases the displacement on a texture map that is known in advance. The displaced surface is rendered as a warped height-field based on that texture map. The actual raytracing of the displaced surface is done in texture space, and the result is mapped back into 3d space. The advantage of this method is that it preserves all the details in the displacement map. However, it requires that the object has valid texture coordinates. You cannot use this method for 3d procedural textures or other textures that use object or world coordinates. The displacement map can take any values (as opposed to Normal Displacement, which will ignore values outside the 0.0-1.0 or black to white range).
Normal Displacement - this is a general method which takes the original surface geometry and subdivides its triangles into smaller sub-triangles which are then displaced. It can be applied for arbitrary displacement maps with any kind of mapping. Note that with Normal Displacement the displacement map's range of values must be within the 0.0-1.0 range (black to white). Values outside of this range will be clipped.
Vector Displacement - If you have a displacement texture that is not grayscale V-Ray will convert it to grayscale before rendering the displaced geometry.This mode allows V-Ray to use the Red Green and Blue channels of the displacement texture to displace the geometry in the U and V directions in addition to the normal of the face
Displacement Amount - the amount of displacement. A value of 0.0 means the object will apear unchanged. Higher values produce a greater displacement effect. This can also be negative, in which case the displacement will push geometry inside the object.
Displacement Shfit - this specifies a constant, which will be added to the displacement map values, effectively shifting the displaced surface up and down along the normals. This can be either positive or negative.
Keep continuity - when this option is enabled V-Ray will try to produce a connected surface. Use it when you get splits (usually around sharp edges) in the displaced geometry.
Enable Water Level - when enabled this will clip the surface geometry in places where the displacement map value is below the threshold specified with the Water Level field. This can be used for clip mapping a displacement map value below which geometry will be clipped.
The following options affect the only the 2D Displacement.
Texture resolution - this determines the resolution of the displacement texture used by V-Ray. If the texture map is a bitmap, it would be best to match this resolution to the size of the bitmap. For procedural 2d maps, the resolution is determined by the desired quality and detail in the displacement. Note that V-Ray will also automatically generate a normals map based on the displacement map, to compensate for details not captured by the actual displaced surface.
Precision - this parameter is related to the curvature of the displaced surface; flat surfaces can do with a lower precision (for a perfectly flat plane you can use 1), more curved surfaces require higher values. If the precision is not high enough you can get dark spots ("surface acne") on the displacement. Lower values compute faster.
Tight bounds - this parameter will cause V-Ray to compute more precise bounding volumes for the displaced triangles, leading to slightly better rendering times.
Filter texture - when this box is checked the texture map will be filtered before the actual displacement takes place.
Filter blur - specifies the amount of blur that will be applied to the texture before the displacement takes place
Enable Round Edges - enables the round edges effect which uses bump mapping to smooth out the edges of the geometry during render time.
Radius - specify a radius (in world units) for the round edges effect. Since the actual geometry is not being changed and only the normals of the faces are affected large values here may produce undesirable effects.
Allows you to give unique object IDs to your objects so that you can create selection masks using the "Multimatte" render element
Allows the user to specify their own attributes, which can then be used with the VRayUserColor and VRayUserScalar textures to provide per-object values for shading attributes. They can also be used in the file name of File nodes to resolve the bitmap file name on a per-object basis. Several attributes can be specified in the string, separated with a semicolon ';' character. User attributes can also be specified on transform nodes. The final string of user attributes for a given object is a concatenation of the user attributes specified on the shape of the object and the user attributes specified on its parent nodes along the DAG path. This allows to conveniently add attributes to a large part of the DAG hierarchy.