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NURBS are non-uniform rational B-splines. They are a type of surface that is defined by its control polygon (one set of points that the surface approximates smoothly), one set of knots for the two directions u and v (a non-decreasing list of real numbers between 0 and 1) and, optionally, one set of rational weights.

To draw the isoparametric lines in u,v=0.5, check the surface drawing type option in the preferences window.

• By contour: this creates a NURBS according to its contour lines. GiD automatically calculates the interior information of the surface so as to interpolate the boundaries smoothly. To create a NURBS surface, some lines must be selected (see Entity selection). The order of selection is not important but all of them must join each other by sharing common points and must form a closed contour. The number of lines must be equal to or greater than one and their shape must be topologically similar to a triangle or a quadrilateral in the space if the algorithm is to work correctly. This last argument is not necessary if all the lines lie in one plane. In this case, the surface is created as a trimmed one and any problems with the shape are avoided. It is possible to select the boundary lines and the boundary lines of interior holes at the same time, if all the lines belong to a plane.

Note: The No try planar option (found in the Contextual mouse menu) avoids the creation of a trimmed NURBS surface when lines are coplanar.

Note: To enter rational weights for the surface, use the Edit NURBS surface command (see Edit NURBS).

• Automatic: this automatically creates all possible surfaces with the number of sides given by the user. Every new surface will be created in the current layer.

Caution: When creating more than one surface in one go, it is possible that some undesired surfaces may also be created. It is necessary to check the surfaces after creation and erase the undesired ones.

• Trimmed: this option lets you select one existing NURBS surface and a set of closed lines that are inside the surface. Some of these lines may already belong to the contour of the existing surface. Some other lines may be created with an intersection with another surface. Another new surface will be created without changing the old one. It is possible to select the boundary lines and the boundary lines of interior holes at the same time, if all the lines belong to the surface:

• Untrimmed: this constructs one new surface with the selected surface as its base and with the natural contours of the NURBS surface as its contours. The resulting surface is not trimmed.
• Parallel lines: this lets you create one surface given a set of parallel lines in the space. The new surface will interpolate all the selected lines.
• By points/By line points: these two options are available in the Contextual mouse menu after the NURBS surface creation tool is selected. By points creates a NURBS surface from a cloud of points, and By line points creates a NURBS surface from level curves. These two functions are very useful for creating relief and terrains. In the image below there is a NURBS surface created from level curves:

Note: This surface is an approximation to the selected points/lines, but there is no interpolation.

• Search: this lets you select one line and then creates one surface that contains that line.

NURBS surface creation

Create NURBS surface

Automatic NURBS surface

Parallel lines. NURBS surface

Trimmed NURBS surface