Groups#

Create, combine and convert groups.

11 nodes.

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Contents#

• Edge Group Promote (PCG) — Converts an edge group to a point or primitive group, or a point or primitive group to an edge group.
• Group Combine (PCG) — Combines several groups into one using a set-algebra expression.
• Group Create (PCG) — Creates a group by stacking independent filters (bounding box, normal, edges, random) on a base selection.
• Group Delete (PCG) — Deletes groups by name, with wildcards, from points, vertices, primitives and edges.
• Group Edges (PCG) — Creates a real edge group from edge angle, length, boundary and random rules.
• Group Expand (PCG) — Grows or shrinks a point or primitive group along the topology.
• Group Find Path (PCG) — Finds shortest paths or boundary loops between points and writes them as an edge group.
• Group from Attribute Boundary (PCG) — Creates a group at the places where an attribute value changes.
• Group Inside (PCG) — Groups the points or primitives that lie inside a second piece of geometry.
• Group Promote (PCG) — Converts a group between point, primitive, edge and vertex classes.
• Partition (PCG) — Creates one group for every distinct value of an attribute.

Nodes#

Edge Group Promote (PCG)#

PCG_EdgeGroupPromote · category PCG/group

Converts an edge group to a point or primitive group, or a point or primitive group to an edge group.

Four conversions: edge to point, edge to primitive, point to edge and primitive to edge. The result is written to a group of the target class. If no new name is given the source name is reused, and if that is empty the group is called promoted.

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Group Combine (PCG)#

PCG_GroupCombine · category PCG/group · Houdini: Group Combine (SOP)

Combines several groups into one using a set-algebra expression.

The expression uses | (union), & (intersection), - (difference), ^ (symmetric difference) and ! (complement), with arbitrarily nested parentheses. Precedence follows Python sets: ! binds tightest, then -, &, ^, |. Group names may contain wildcards, and * stands for the whole set.

Example: (top & selected) - border. The group type can be point, primitive, vertex or edge.

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Group Create (PCG)#

PCG_GroupCreate · category PCG/group · Houdini: Group (SOP), Group Expression (SOP), Group Range (SOP)

Creates a group by stacking independent filters (bounding box, normal, edges, random) on a base selection.

A base set, empty meaning everything, starts the selection. It accepts group names, indices, ranges and @ expressions. The Bounding Box, Normal, Edges and Random sections are each switched on separately. The first, second and fourth intersect with the running selection, and the edge rule can either union into it or intersect with it. The result is finally merged with any existing group of the same name according to the initial merge mode.

When only Include by Edges is enabled and the base group is empty, the selection starts from the empty set instead of everything.

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Base Group

Keep in Bounding Regions › Bounding Region

Keep by Normals › Normals

Include by Edges › Edges

Keep by Random Chance › Random

Group Delete (PCG)#

PCG_GroupDelete · category PCG/group · Houdini: Group Delete (SOP)

Deletes groups by name, with wildcards, from points, vertices, primitives and edges.

Each of the four fields takes a list of group names for its element class. Wildcards are supported: extrude* matches every group starting with extrude, and * ^keep means all groups except keep. Edge groups are handled too.

A field left empty leaves that class untouched, also when Delete Unselected is on, where an empty field does not mean everything.

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Group Edges (PCG)#

PCG_GroupEdges · category PCG/group

Creates a real edge group from edge angle, length, boundary and random rules.

Corresponds to the Edges section of Houdini's Group Create, but outputs an actual edge group. The group is stored as pairs of points, not edge indices, because edge indices are derived from the topology and change after delete, merge or fuse, so a group based on indices would silently point to different edges.

All enabled rules (angle, length, unshared, random) must hold at the same time for an edge to be selected. Without any rule enabled, every edge of the base set is selected.

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Parameters

Angle

Length

Random

Group Expand (PCG)#

PCG_GroupExpand · category PCG/group · Houdini: Group Expand (SOP)

Grows or shrinks a point or primitive group along the topology.

Two measures are available. Topology Steps hops across a number of edges and is tied to mesh density: on a subdivided mesh, expanding by 3 steps covers half the distance it did before. Edge Distance accumulates edge length, giving a geodesic distance that is independent of density. This is not Euclidean distance: two pieces of geometry that touch in space but are not connected are infinitely far apart, so the selection on either side of a wall never bleeds across.

Shrinking is an expansion of the complement followed by an inversion (morphological erosion). The outer boundary of the geometry is not eroded, because the complement only contains elements that really are not in the group and there is nothing outside the geometry. This matches Houdini but differs from image erosion.

Output Falloff also writes a 0 to 1 distance falloff attribute, so no separate Attribute Fade is needed.

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Parameters

Falloff

Group Find Path (PCG)#

PCG_GroupFindPath · category PCG/group · Houdini: Group Find Path (SOP)

Finds shortest paths or boundary loops between points and writes them as an edge group.

Writes an edge group, plus a point group of the same name, on the source geometry; no new geometry is created.

In Shortest Path mode the points are visited in the order they are written (for example 12 40 7), with a shortest path between each consecutive pair. With Close Path the path also returns to the first point. Boundary Loop writes, for each given point, the entire boundary loop it sits on, such as the rim of an opening or a hole. Start/End Pairs pairs the points two by two (1st with 2nd, 3rd with 4th and so on) and builds one path per pair; the number of points must be even.

The cost of an edge is its length by default, optionally scaled by a point cost attribute. Collision groups block points from being used by the path (end points excepted).

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Group from Attribute Boundary (PCG)#

PCG_GroupFromAttribBoundary · category PCG/group · Houdini: Group from Attribute Boundary (SOP)

Creates a group at the places where an attribute value changes.

The attribute class decides what counts as a change. For a primitive attribute, edges between neighbouring faces with different values. For a vertex attribute, edges where the corners on the two sides differ, which finds UV seams. For a point attribute, edges whose two end points differ.

The output group can be an edge group, the points of those edges, or the faces next to them. Unshared (open boundary) edges can be included as well.

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Group Inside (PCG)#

PCG_GroupInside · category PCG/group

Groups the points or primitives that lie inside a second piece of geometry.

Equivalent to the Bounding Object option of Houdini's Group SOP, but using the actual closed surface of the second input. The test is ray parity: a ray is cast from the point, the surface crossings are counted, and an odd count means inside. Since the ray caster only returns the nearest hit, the ray is restarted from just past each hit until nothing is hit.

A ray that grazes an edge or vertex may count 0 or 2 crossings and flip the parity, which is common on axis-aligned meshes. Several unrelated oblique directions therefore vote on the result; with three directions, grazing all of them at once is negligible. The directions avoid small integer ratios so they do not hit cube vertices exactly.

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Tips

• The second input must be a closed surface; open meshes give unreliable parity.

Group Promote (PCG)#

PCG_GroupPromote · category PCG/group · Houdini: Group Promote (SOP)

Converts a group between point, primitive, edge and vertex classes.

One Convert From / To menu replaces Houdini's two separate dropdowns, so the available conversions are listed as pairs. The conversion rules differ per direction.

Two defaults are opposite to Houdini so that existing graphs keep their behaviour: Keep Original Group is on, and Include Only Elements Entirely Contained is on, which means a point group only produces the primitives whose points are all in the group.

The result can alternatively be written as a 0/1 integer attribute instead of a group.

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Partition (PCG)#

PCG_Partition · category PCG/group · Houdini: Partition (SOP)

Creates one group for every distinct value of an attribute.

Each different value of the attribute gets its own group. The group name is the rule with $ATTR replaced by the value. Characters that cannot appear in a group name (the decimal point and minus sign of floats, and other special characters in strings) are replaced, and a name that would start with a digit is prefixed with an underscore.

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