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This node takes an input polygon surface (which can be piped into one or both inputs), and divides each face to create a smoothed polygon surface using a Catmull-Clark subdivision algorithm. It is similar to the Paste op in that it divides up all or part of a surface allowing you to increase areas of local detail (especially useful for avoiding the angular appearance often associated with polygonal models) without adding lots of extra geometry to the entire object. While the topology of the input mesh can be arbitrary, for best results all polygons should be convex and relatively uniform in distribution.
All polygons in the left input which are specified by the "Group" field are used to determine the polygon mesh to subdivide. For polygon edges to be classified as the same edge, they must share the same points. Merely being physically close is not sufficient.
The elements of the right input specified by the "Creases" field are used as creases. Each edge in a crease polygon corresponds to the edge in the polygon mesh which has the same point numbers. Again, point position is irrelevant.
Vertex attributes are supported and preserved, however, instead of being properly subdivided they are merely linearly interpolated.
Subdivide a polygonal surface
Add local detail
Add local detail
Crease edges interactively
Crease edges by number
The RenderMan renderer will directly use the Crease attribute applied by the Crease Operation, but it will not be used by the Mantra renderer due to patent issues.
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 Weights and Renderman
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.
In order to output the crease weights in the rib file, your geo object must have the Render page > Geometry > Polygons as Subdivision Surfaces parameter set.
The Crease Algorithm
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.
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.
Subset to use as a polygonal mesh and subdivide
Subset (of right input) to use as creases
Choose the algorithm used to perform subdivision.
Wses Houdini’s native subdivision algorithm.
Mantra Compatible Catmull-Clark
Uses mantra-style subdivision.
Uses the Open Subdiv library to perform subdivision.
Uses the Open Subdiv library to perform Loop subdivision (more suitable for models made of triangles).
Uses the Open Subdiv library to perform bilinear refinement (with no blending of vertices).
How many iterations to subdivide, higher numbers give a smoother surface.
Override Crease Weight Attribute
Determine if the crease sharpness should be determined by the primitive or vertex
creaseweight attribute or by overridden by this operation.
If the crease weight is overridden, this is the weight used.
Generate Resulting Creases
If the creases are sharper than the depth, some will still be present in the final geometry. This option determines if they will be present in the resulting geometry.
Name of the group to place all generated creases into.
Do Not Close
Don’t close cracks.
Pull Cracks Closed
Move points on boundary of subdivided area in order to close cracks formed during the subdivision.
Stitch Cracks Together
Add polygons to close the cracks caused by subdividing.
No Edge Division
Do not divide edges.
Whether or not to divide edges surrounding subdivided area when pulling or stitching cracks.
Whether or not to triangulate the polygons on the boundary of the cracks.
Determines which points are moved when pulling cracks closed.
Means move points on subdivided area to meet boundary.
Means move points on boundary to meet subdivided area.
Recompute Point Normals
Re-evaluates point normals if they exist. If this is not set, the point normals will be interpolated in the same manner as the points.
Smooth Vertex Attributes
If on, vertex attributes will be smoothed in the same manner as the point attributes. However, vertex attributes at seams (places where points meet but the vertex attributes differ) will still be linearly interpolated. If off, the vertex attributes will be linearly interpolated. To match the Mantra subdivision rendering, this should be set to off.
Enforce Consistent Topology
If Pull Cracks Closed or Stitch Cracks Together is enabled, different topology might be generated depending on boundary edges between the subdivided primitives and their non-subdivided neighbors. Turning on this option will ensure that this behavior will always produce consistent topology by disregarding collinear edges.
If this option is turned on will switch from the standard (RenderMan) style creases to Mantra’s linear creases.
Build Polygon Soups
Creates polygon soup primitives instead of separate polygon primitives.
Treat All Curves as Independent
If you are subdividing polygon curves and this option is enabled, the possibility of sharing points between curves will be ignored. Each polygon will be subdivided as if each vertex has its own point. This is slightly faster, and since the viewport and Mantra make this assumption when rendering polygon curves as subdivision curves, the results will be consistent.
When using OpenSubdiv for refinement, this determines the
interpolateboundary setting for point attributes.
When using OpenSubdiv for refinement, this determines the
interpolateboundary setting for vertex attributes.