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Overview ¶
Groups are named collections of points or faces.
The 
Group SOP node lets you sort points or
faces in its input geometry into groups based on various criteria.
Other operators may also create groups as a side effect.
Most nodes that operate on points or primitives give you the option of applying the node’s effects to certain groups in the input stream, instead of to every point/primitive. In this way, you can identify groups of points based on certain properties, and apply nodes to those groups and not to other points.
When a point is deleted, Houdini automatically removes the point from all the point groups it belongs to.
Ordered and unordered groups ¶
A point group can be ordered or unordered. Unordered groups store their points in creation order. Ordered groups store points in selection order.
When you select points in the viewer pane, a single click performs an ordered selection, while a marquee box generates an unordered group. The only time bulk selections generate or maintain an ordered selection is when only one point is selected in the marquee box.
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Create a point or primitive group |
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Use existing groups when selecting points/primitives for a new group |
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Select on the left toolbar.
Geometry
Selection Mode and choose Points, or press Combine and filter groups ¶
The 
Group Combine node
lets you create a group that is a combination of existing groups.
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Create a group. This creates a
Group node with the geometry connected to it. -
Create a
Group Combine node, and connect the
Group node output to the Group Combine input. -
In the Group Combine parameter editor, specify the starting group of component types (primitives, points, edges, vertices) with the Group Name parameter:
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In the left-hand field, enter the name of the new group to create, or choose an existing group to start with from the drop-down box.
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Include the components of the group in the right-hand field by choosing Equals from the middle drop-down box. Include the components that are not in the right-hand field group by choosing the Equals All But option.
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Further combine and filter the group components in the options below Group Name by choosing to perform a union, intersection, exclusive or, or subtraction with additional groups.
Group syntax ¶
Many geometry operators (SOPs) work on a specific subset of points or primitives. When you use the operator in the viewer, you select the points/primitives you want to use. In the parameter editor, this list of points/primitives is specified in the Group parameter using the following patterns.
You can use multiple patterns separated by spaces. The matches for each pattern are combined to form the group.
Pattern |
Meaning |
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All points, primitives, vertices, or edges. To match no elements,
use |
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Match anything in a named group. |
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Component number ‹n›. |
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In SOPs other than the Group SOP, you can specify a named group of points/primitives (created with the Group SOP). You can use pattern matching ( |
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Components numbered from ‹n› to ‹m› (inclusive). |
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Components numbered from ‹n› to ‹m› (inclusive),
skipping every ‹step›. For example, |
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Components numbered from ‹n› to ‹m› (inclusive). Use the first ‹keep› numbers and then skip every ‹step› after that. |
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Every component except the ones matching the pattern.
For example, ‹pattern› may be a numeric pattern, attribute pattern, or group name pattern. |
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Remove components matching the pattern from the
results of the preceding pattern. For example, ‹pattern› may be a numeric pattern, attribute pattern, or group name pattern. |
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Components where ‹attribute› matches
‹value›, based on the ‹op›, which can be one of
For example, You can specify components using As a convenience, you can also use (If you don’t specify a component for a multi-component attribute type, the comparison will use the first component.) For example, For string attributes, you must put quotation marks around the
‹value› if it contains spaces, for example
You can use wildcards ( You can combine the attribute syntax and the range syntax: @id=5-10 You can also use the attribute syntax with an integer pattern, but you need to enclose the pattern in quotes: @id="5 8 10 15" @id="0-4 78" You can use this to compare against intrinsic attributes as well: @intrinsic:indexorder<100 |
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Components with the given attribute containing one of the listed values. For example: @id=1,2,90,120 For string attributes, you must enclose values containing spaces in double quotes, for example: @path=/foo/*,/bar,"some string" |
Edge syntax ¶
Some nodes deal with edges (the line connecting two points) rather than points or polygons, for example, the 
Dissolve node. In these nodes (or a node set to a mode that affects edges), you can use the following syntax in a group field to select edges.
Pattern |
Meaning |
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All edges in primitive number ‹n›. |
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Point number ‹n› / edges with point number ‹n›. |
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Edge number ‹m› of primitive ‹n›. |
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Edges between point number ‹a› and point number ‹b›. |