Creases control the strength of pull of the polygon faces on the subdivision surfaces, much like a magnet drawing the surface towards the reference polygon. They can be applied selectively using the Creases field to specify an input group to use.

Creases work by controlling the strength of the pull of the polygon faces on the subdivision surfaces, like a magnet drawing the surface towards the reference polygon. The figure below shows the result of setting the Crease Weight to 0, 1, 2 reading from left to right. As the weight increases so the pull effect strengthens and the shape approaches the reference polygon.

Crease weight attributes which are added, but not yet Subdivided are preserved when a rib file is generated. They are then used internally by RenderMan to implement subdivision surfaces during the rendering process.

If there is a second input:

• If the Override button is enabled, each edge defined in the second input will have its edge crease weight set to the value of the override.

• If the vertex attribute creaseweight exists on the second input, each matching edge in the input will have its crease weight set to the maximum of the vertex attributes for any shared edges.

• If the primitive attribute creaseweight exists on the second input, each polygon in the second input will set matching edges to the appropriate weights.

If there is no second input:

• If an override is specified, the value of the override is used for all edges in the sub-divided surface.

• If the vertex attribute creaseweight exists, this attribute will be used to define the crease weights on the edges of the surface.

• If the primitive attribute creaseweight exists, this attribute will be used to define the crease weights for the subdivision surface.

When defining crease weights on shared edges, the maximum of the weights of the shared edges is used. For example, if two polygons share an edge and the primitive attribute is used, the maximum of the crease weight will be used for the shared edge.

When OpenSubdivision is used for refinement, the following are also considered:

• If the point attribute cornerweight exists, this attribute will be used to define corner weights on vertices of the surface.

• If the group subdivision_hole exists, any polygons in that group will be considered holes in the subdivision process. The polygons will contribute to the subdivision, but will not appear in the final surface.

• If a detail attribute osd_scheme exists, it will override the subdivision algorithm specified on the SOP. When this attribute specifies Loop subdivision, the incoming geometry must be triangulated.

• If a detail attribute osd_vtxboundaryinterpolation exists, it will override the value of the Point Attributes parameter.

• If a detail attribute osd_fvarlinearinterpolation exists, it will override the value of the Vertex Attributes parameter.

• If a detail attribute osd_creasingmethod exists, it will override the value of the Crease Method parameter.

• If a detail attribute osd_trianglesubdiv exists, it will override the value of the Triangle Subdivision parameter.

Please see the OpenSubdiv documentation for further details.

Cracks can be closed by either Pulling or Stitching, so only one of these two options can be chosen at a time from the Surrounding Faces parameter. Bias applies only to Pulling, and is disabled when Stitching is chosen.

A crack is formed by a single edge on the non-subdivided area and multiple edges on the subdivided area. The 'Surrounding Faces' menu determines what will will happen to the single edge on the non-subdivided area, and in more generally, to the polygon containing this edge.

If No Edge Division is chosen and cracks are pulled closed, all the points on the subdivided edges are pulled (i.e. moved) to the closest points on the non-subdivided edges. Bias is disabled, when No Edge Division is specified.

If cracks are pulled with the Divide Edges option, the non-subdivided edge is split into many sections, so that each each point on the non-subdivided edge now corresponds to a new point on the subdivided edge. Then, points on the newly divided edge are joined with the points on the subdivided boundary. A Bias of 1 will place these joined points along the subdivided boundary. A bias of 0 will place them along the non- subdivided boundary (and values between 0 and 1 will place them somewhere in between).

Pulling cracks with the Triangulate option will do exactly the same thing as Divide Edges, except it will also triangulate the non-subdivided polygon. This is desirable because pulling the non-subdivided boundary towards the curved subdivided boundary will likely generate a non-planar polygon, so Triangulate will divide this polygon into smaller (planar) ones. Pulling cracks with a Bias of 1 and triangulating usually produces the nicest results.

The Triangulate option is necessary because the Divide op is not designed to handle (very) non-planar polygons.

The Stitch Cracks Together option, on the other hand, inserts new polygons (triangles) to close up the cracks. When No Edge Division is chosen, Many triangles are created, each having one vertex on one point of the non-subdivided edge.

When Divide Boundary Edges is chosen, the non-subdivided edge is divided, so there are more points available to be used for triangles. The resulting triangles are more regularly shaped (not as long and skinny). The triangulate option will again triangulate the non-subdivided polygon, although this option is less likely to be used because this polygon should remain planar during stitching.