NURBS Surfaces

NURBS mesh is constructed from a two dimensional array of control points. Correspondingly, there are two sets of Count, Order, and Closed attributes: one set for each dimension.

NURBS Mesh Property Gadget

Attribute: Selects a channel for pointwise value control. The value can be set for the selected points using the value gadget below the channel selector.

Clear: Removes the pointwise values of the channel in the Attribute field from all points.

Wrap U, Wrap V: Activates modulo wrapping into the unit interval for the selected channel component. This option is useful for UV coords channel and other texture parameter channels. Thanks to it, texture parameter can be interpolated correctly over the mapping seam on closed surfaces.

Count U: The amount of mesh control points in U direction. Reducing the point count usually changes the shape.

Count V: The amount of control points in V direction.

Selected: The index of the selected control point.

Position: The position of the selected control point.

Weight: The weight of the selected control point(s). Increasing the weight pulls the mesh towards the selected control points.

Constructor: Selects how child NURBS curves rebuild the mesh.

Selection: Selects geometry editing by single points or by complete row/column curves.

U Parametrization, V parametrization: Parametrization affects mesh shape significantly, and is relevant also in path animations. Uniform parametrization means that all mesh patches represent an equal parameter square in parameter space. The chord length parametrization computes the parameter division from the patch size.

Order U, Order V: The order of the mesh in U and V directions. The higher the order, the smoother the mesh. Order = 2 produces a polygonal mesh. The most usual smooth NURBS mesh type (Cubic mesh) has order 4.

Boolean: The trim curve operation. This becomes relevant when the mesh has several trim curves. By default, the exclusive OR operation is used.

Invert Boolean: Inverts trimming. If set, trim curves cut holes. If not set, trim curves leave their interior left.

Quality: The quality in photorealistic rendering. NURBS mesh rendering is based on a numerical algorithm. The quality value controls the amount of algorithm iterations. The higher the value, the more accurately the 'ideal' surface shape is computed.

Max. Displacement: If the value of this field is greater than zero, and if surface geometry shaders are mapped to the surface, the surface is displaced according to the bump height channel in ray trace rendering. 'Max. Displacement' defines the maximal amount of displacement.

The displacement value is a real distance value (the unit can be selected from 'Options/Window' pull down menu, 'Metrics' tab), so usually quite small values are sufficient (such as 0.01 meters=1 centimeter). The amount of displacement has a great impact on rendering speed, so it is wise to use a minimal 'Max. Displacement' limit value here. Note also that displacement is symmetric to both sides of the original surface. Therefore, a displacement map that is centered around zero and whose amplitude varies between -1 and 1 centimeters renders faster than a bump map whose amplitude range is from 0 to 2 cm. In former case, the 'Max Displacement' value can be 1 cm, whereas the latter case requires a limit value of 2 cm.

Note: Use of displacement mapping does not automatically increase the rendering accuracy to meet the new surface detail requirements. If the displacement map includes lots of details, the original surface must be reasonably dense and the rendering Quality value described above must be high enough. If rendering artifacts appear, you can increase the mesh density using the CountU and CountV values described above.