HtoA dynamically adds object (OBJ) nodes for all procedurals loaded at startup. For example, with the default HtoA installation, you get the Arnold Alembic, Arnold Usd, Arnold Procedural, and Arnold Mandelbulb object nodes.

Arnold Procedurals can be created by tab-typing Arnold Procedural or Tab > Arnold > Arnold Procedural at the OBJ level. The procedural node can be used to load an external geometry file or a compiled DSO (Dynamic Shared Object).

Geometry File 

The path to the geometry file (obj, ply or ass) or program to be loaded.

Auto Instancing (Arnold)


The .asstoc file is a cache that describes the bounding box of the procedural. It allows an optimization of the scene since the plugin does not have to read the referenced ASS file to detect the bounds. However, there are cases when no asstoc files exist (e.g. the ASS files are coming from a different source (RealFlow, etc.)). If HtoA cannot detect the bounding box (e.g. no asstoc file is given and the

Arnold Procedural

The following parameter are Arnold procedural specific parameters.



Override Nodes 

Viewport Proxy


Reads the bounding box from the Geometry File. This will only work for .obj and .ply files. Bounds for .ass files will be displayed if an .asstoc file is found alongside the .ass file. 

Target SOP

Use an object in the scene to attach as a bounding box preview. 

Display As

The assigned file can be displayed as Global Bounding Box, Bounding Box per Node, Points, and Polygons. 

Proxy SOP

Provides a bounding box representation for visualizing the contents of a procedural when importing geometry into Houdini using a SOP.

Operator Graph

You can choose to connect any existing operator graph to a procedural.

To connect a Procedural Operator to an Arnold Procedural, you have to pick the operator ROP in the Arnold properties of the arnold_procedural OBJ node. Use the Arnold Properties top-level Arnold | Operator Graph parameter to pick the operator ROP, which can be anywhere in your scene.

Procedural Operator parameter in Arnold properties of Arnold Procedural