4-sided surface creation

The 4-sided surface has been substituted by the NURBS surface (see NURBS surface ). This new entity has all the functionality of the old one.

A 4-sided surface is an entity formed by a closed set of four lines in the space. Its mathematical definition is a bilinear Coon's surface. The surface is totally defined by the shape of the lines, with no information about the interior. This means that it will sometimes be necessary to use more surfaces to obtain a good shape definition.
To create a 4-sided surface, several lines must be selected (see Entity selection). For the creation of a 4-sided surface defined by three lines, it is necessary to di

vide one of the lines into two pieces (see Divide). The order of selection is not important, but all of them must join each other by sharing common points and they must form a closed contour. If it cannot be created, information about the endpoints is displayed in a window.

In order to make one or more lines form parts of a polyline (see Polyline), select the entire polyline as one of the lines and GiD will automatically select the piece or pieces of the polyline that are required. Using this facility, non-conforming surfaces can be created. This means creating a surface by using the entire line on one side of the polyline, and creating more than one 4-sided surface by using parts of it on the other side.

When selecting more than four lines, GiD will automatically search for all the possible 4-sided surfaces that can be created with these lines. This allows the creation of many surfaces at the same time.

The Automatic button is equivalent to automatic 4-sided surface creation (see Automatic 4-sided surface creation).

If the surfaces lie on the z=0 plane, the orientation of the surfaces will be anti-clockwise in this plane (normal vector points towards z positive). Otherwise, the orientation will be arbitrary. This can be checked with the DrawNormals command (see Normals (only Preprocessing)).

The Number option lets you choose the label that will be assigned to the next created surface. If a surface with this number already exists, the old line changes its number.

Caution: When creating more than one surface at a time, some undesired surfaces may also be created. It is necessary to check the surfaces after creation and erase the undesired ones.