Procedural modelling in Houdini based on Function Representation

3.2 Houd<strong>in</strong>i Development Kit (HDK)
Figure 3.6: Basic class diagram for custom Houd<strong>in</strong>i nodes (adapted from SideFX (2011a))
3.2.3 The SOP Node
SOP nodes are dist<strong>in</strong>guished between generators and filters. Generators do not require any
<strong>in</strong>put, <strong>in</strong>stead they generate geometry purely <strong>based</strong> on their parameters. Filters on the
other hand alter geometry and apply a set of operations to it. In the case of this project,
nodes always except at least one <strong>in</strong>put. Even the FRep node itself that outputs the mesh
after polygonisation is retriev<strong>in</strong>g his attributes from connected nodes dur<strong>in</strong>g traversal (see
Section 4.4.2).
As shown <strong>in</strong> the previous section, the first step <strong>in</strong> creat<strong>in</strong>g a custom SOP is to sub-class
SOP_Node, register the operator and start def<strong>in</strong><strong>in</strong>g attributes. The method which conta<strong>in</strong>s
the actual code to manipulate the geometry is called cookMySop and is called automatically
as soon as the node is evaluated or refreshed.
Another useful concept which helps users understand what the SOP is work<strong>in</strong>g with, is the
option of add<strong>in</strong>g guide geometry. This geometry is not part of the operator and is only
displayed as guid<strong>in</strong>g <strong>in</strong>formation, as used to display the bound<strong>in</strong>g box of the polygonisation
(see Figure 3.7).
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