# Difference between revisions of "Skin SOP"

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## Revision as of 17:22, 29 May 2018

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## Summary[edit]

The Skin SOP takes any number of faces and builds a skin surface over them. If given two or more surfaces, however, the SOP builds four skins, one for each set of boundary curves.

All face and surface types are valid as long as the input(s) contain only faces or only surfaces. Different face types can be skinned together into one surface. For example, it is possible to skin a cubic open NURBS curve with a polygon and a quintic closed Bzier curve even if the three faces have a different number of control vertices. Similarly, this SOP can skin the boundary curves of surfaces of different types, number of rows, columns, etc.

When face types are input, the number of input SOPs and the number of faces in each input establish the skinning method. If only one input exists, a "linear-skinning" operation is performed by running a skin across the cross-sections. The result is the classic ruled or skinned surface. If a second input exists, a "bi-linear skinning" is performed which computes a cross-skin between the faces in the first input (U cross-sections) and the faces in the second input (V cross-sections). The result is a surface whose name derives from the number of cross-sections in each direction: triangular, square, or multiple boundary surface, as well as a special case of swept surfaces and N-rails. When possible, cross-sections are interpolated as isoparms.

If you need more control over tangency in the skin, try using the Bridge SOP.

**Tip:** If you have problems with the results being skinned in the wrong order, try inserting a Sort SOP ahead of the Skin SOP, and Sort by Normals.

## Contents

## Types of Surfaces

Single Boundary Surface - One face, open or closed, is converted into a surface whose boundaries match the shape of the face exactly. Basically, this operation builds an interior area for the face. The surface type will be similar to the type of the face. For example, a NURBS curve yields a NURBS surface. If the curve is highly concave, the result may look less satisfactory than expected.

Patch - Two boundary faces define a ruled surface. The arrows on the two faces indicate the required parametric direction, which must be the same for both faces to avoid a bad twist in the surface. Use the Primitive SOP or the modeler to correct the problem. The surface type will be similar to the most complex type between the two cross-sections. For example, if a polygon and a NURBS curve are skinned together, the surface type will be NURBS. The surface always contains the two faces as two of its boundaries.

Linear Ruled/Skinned Surface - Two or more faces are skinned linearly into a single surface. The arrows on the faces indicate the required parametric direction of each face, which must be the same for all faces to avoid bad twists or flips in the surface. Use the Primitive SOP or the modeler to correct the problem. The surface type will be similar to the most complex type among all cross-sections. For example, if a polygon, a Bzier and a NURBS curve are skinned together, the surface type will be NURBS. The surface goes through each cross-section unless "Preserve Shape" if OFF (see parameters below). If the cross-sections have repeated points, or share points between them, the result might not look good when shape preservation is enabled.

A Special Swept Surface - This case does a bilinear skin and requires two inputs. The U face (1st input) is swept along the V face (second input). The two faces do not need to touch at their endpoints. If their endpoints coincide, though, the two of the surface's boundaries will match the two faces exactly. The surface type will be similar to the most complex type of the two faces. For example, if a polygon and a Bzier curve are skinned together, the surface type will be Bzier.

Triangular Surface - This case requires two inputs for the bilinear skin. One input has two faces; the other input, just one. The endpoints of the faces need not coincide, but if they do, the surface boundaries will match the face shapes exactly. Basically, the three faces define an interior area to be filled by a surface. The surface type will be similar to the most complex type among the three boundary faces. For example, if the faces are Bzier and NURBS curves, the surface will be a NURBS primitive.

Square Surface - Four faces define the outer boundaries of a surface. This case requires two inputs for the bilinear skin: the two U boundaries (1st input) are cross-skinned with the V boundaries (the 2nd input). The endpoints of the faces need not coincide, but if they do, the surface boundaries will match the face shapes exactly. Basically, the four faces define an interior area to be filled by a surface. The surface type will be similar to the most complex type among the four boundary faces. For example, if the faces are polygons and NURBS curves, the surface will be a NURBS primitive.

A Special Case of M-rails - One input contains the rails, and the other input the cross-section. The cross-section is swept along the rails to form a surface. The arrows on the faces indicate the required parametric direction of each face, which must be the same for all faces to avoid bad twists or flips in the surface. Use the Primitive SOP or the modeler to correct the problem. The surface type will be similar to the most complex type among both rails and cross-section. For example, if the faces are polygons and NURBS curves, the surface will be a NURBS primitive.

Multiple-Boundary Surface - Not to be confused with N-ary patches. This case generalizes the square surface concept by allowing more interior cross-sections both in U and V. If no interior cross-sections exist, this case reduces to a square surface. The surface interpolates all the boundaries and the interior cross-sections. The result improves when the faces intersect. The arrows on the faces indicate the required parametric direction of each face, which must be the same for all faces to avoid bad twists or flips in the surface. Use the Primitive SOP or the modeler to correct the problem. The surface type will be similar to the most complex type among all faces. For example, if the faces are polygons and NURBS curves, the surface will be a NURBS primitive.

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An Operator Family that reads, creates and modifies 3D polygons, curves, NURBS surfaces, spheres, meatballs and other 3D surface data.

A surface type in SOPs including polygon, curve (NURBS and Bezier), patch (NURBS and Bezier) and other shapes like sphere, tube, and metaball. Points and Primitives are part of the Geometry Detail, which is a part of a SOP.