Houdini 17.0 Python Scripting

Define a geometry node (SOP) using Python

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Overview

  • If you want to define a new, reusable geometry node using Python, see "creating a new node type" below.

  • If you just want to script a one-off change, you can use the Python SOP to run a Python snippet on geometry without having to create a new node type.

Creating a new node type

  1. Choose File ▸ New Asset.

  2. Set the Operator Definition to python, then set Network type to Geometry.

  3. Use the Save to library option to set an OTL library file to save the new node type into.

  4. Click Accept.

    The type properties window appears.

  5. Use the options in the type properties window to define the interface for your new node type.

  6. Click the Code tab to view and edit the Python script that defines the SOP’s behavior.

Tip

If you need to edit the script after closing the type properties window, right-click an instance of the node and choose Type properties.

Writing the code

To get the node’s incoming geometry, use

geo = hou.pwd().geometry()

The hou.pwd() function returns the currently cooking Node, and the geometry() method returns a hou.Geometry object. If an input SOP is connected to the first input of the Python SOP, Houdini copies the input SOP’s geometry into the Python SOP’s geometry before running the Python code.

Tip

To get the geometry from an input other than the first, use the inputs() method of Node to get the input nodes, and then the geometry() method of one of those nodes to get its geometry.

this_node = hou.pwd()
inputs = this_node.inputs()

# Get the geometry from the second input
# (first input=0, second input=1, third=2, etc.)
second_input_geo = inputs[1].geometry()

You can then call methods on the geometry object (geo in this example) to modify the outgoing geometry. See the documentation for the hou.Geometry object for how to manipulate the geometry.

For example, to add a polygon to the geometry:

poly = geo.createPolygon()
for position in (0,0,0), (1,0,0), (0,1,0):
    point = geo.createPoint()
    point.setPosition(position)
    poly.addVertex(point)

If you want to create a "source" node rather than a "filter" node, simply set the node’s number of inputs to 0 in the type properties. Calling hou.pwd().geometry() will return an empty geometry you can add to.

Making the SOP interruptible

If the node takes a long time to cook when given large input geometry, you may want to be able to interrupt its cooking by pressing Escape. To make your SOP interruptible, periodically call hou.updateProgressAndCheckForInterrupt(). For example, you can press Escape to stop this SOP from cooking further:

geo = hou.pwd().geometry()

# Evaluate the "t" parameter to see how much to translate each point.
translation = hou.Vector3(hou.parmTuple("t").eval())

for point in geo.points():
    # Set the new position for each point.
    point.setPosition(point.position() + translation)

    # Check if the user pressed Escape.
    if hou.updateProgressAndCheckForInterrupt():
        break

SOP Errors and Warnings

If your Python surface node generates an exception, the node will turn red with an error and you can view the stack trace of the error by middle-clicking on it.

If you would like to generate an error message to the user that doesn’t contain a Python stack trace, raise a hou.NodeError exception. For example, running

raise hou.NodeError("Invalid parameter settings")

will turn the node red with an error message of "Invalid parameter settings". Similarly, you can add node warnings by raising instances of hou.NodeWarning.

Creating local variables for new attributes

hou.Geometry.addAttrib() contains a parameter to control whether Houdini creates a local variable for newly added attributes. There is no need to modify the varmap attribute directly.

Profiling the SOP code

You can profile your Python surface node with Python’s cProfile module.

Tip

If you are on Ubuntu, use aptitude to install python-profiler since the standard library is missing pstats in Ubuntu.

You should write your surface node’s script so the work is done inside a function, instead of a bunch of statements at the top level, for example:

def cook():
    poly = geo.createPolygon()
    for position in (0,0,0), (1,0,0), (0,1,0):
        point = geo.createPoint()
        point.setPosition(position)
        poly.addVertex(point)

cook()

Import cProfile and use cProfile.runctx to call your function instead of calling it directly:

def cook():
    poly = geo.createPolygon()
    for position in (0,0,0), (1,0,0), (0,1,0):
        point = geo.createPoint()
        point.setPosition(position)
        poly.addVertex(point)

cProfile.runctx('cook()', globals(), locals())

This will print a summary of your script’s run time to the Python shell when the node cooks.

Writing Part of the SOP in C++

See Extending HOM with C++ to see how to easily write a portion of your SOP in C++.

Storing your Code Outside a Digital Asset

You may want to store your source code outside of your digital asset in order to manage it under a version control system. It is possible to use the Type Properties dialog to set up your asset’s parameters and then inject the Python source code into the otl file.

You can use the following function to update your digital asset to use the Python source code from a file:

def loadPythonSourceIntoAsset(otl_file_path, node_type_name, source_file_path):
    # Load the Python source code.
    source_file = open(source_file_path, "rb")
    source = source_file.read()
    source_file.close()

    # Find the asset definition in the otl file.
    definitions = [definition
        for definition in hou.hda.definitionsInFile(otl_file_path)
        if definition.nodeTypeName() == node_type_name]
    assert(len(definitions) == 1)
    definition = definitions[0]

    # Store the source code into the PythonCook section of the asset.
    definition.addSection("PythonCook", source)

TODO: Document how to use geo.globPoints(), geo.globPrims(), etc. to implement

Python Scripting

Getting started

Next steps

Python viewer states

You can write viewer states in Python that let you customize user interaction in the viewport for your node.

Guru level

Reference

  • hou

    Module containing all the sub-modules, classes, and functions to access Houdini.

  • Alembic extension functions

    Utility functions for extracting information from Alembic files.