Scripting in Houdini - Talk Notes

Last updated Aug 15, 2022 Edit Source

• Houdini
• Python

These are my personal notes about Paul Ambrosiussen’s Talk Python Scripting - Houdini.

use an external editor for anything longer than a couple of lines

• worst way
• limited space
• copy pasting to external editors for improved python features is prone to errors

• okayish
• more space
• limited editor features
• same copy pasting problem
• useful in PDG » displays most common pdg python functions in a column

• install SideFX Labs to be able to link an external editor (Sublime, VS Code etc.) to write code
• VS Code has a VEX language extension to enable auto complete

Errors and Warnings are displayed in the node’s info section while Messages trigger a popup window

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error = "Error!"

if not something:
	raise hou.NodeError(error)

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warning = "Warning?"

if not something:
	raise hou.NodeWarning(warning)

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message = "Message."

if not something:
	raise hou.ui.displayMessage(message)

library that ships with Python and lets you run other programs

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import subprocess

To run another program you have to build a command (cmd) string. Essentially a single commandline snippet that runs some executable with all the necessary parameters.

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executablepath = "path/to/program.exe"
value = 5 # some value from Houdini

cmd = [executablepath]

if something:
	cmd.append("-p") # append a parameter
	cmd.append(value) #parameter value

This results in the following cmd string: path/to/program.exe -p 5

to run the command you can use the call() function

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# disable IM dialog box (windows specific)
si = None
if os.name == 'nt':
	si = subprocess.STARTUPINFO()
	if hide_dialog:
		si.dwFlags != subprocess.STARTF_USESHOWWINDOW

subprocess.call(cmd, startupinfo=si)

In case you want to get some geometry back this function overwrites any input geo and returns the new one instead.

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geo.loadFromFile(path)

Relevant Doc Pages

When building HDAs it’s possible to use python callbacks to trigger scripts with custom buttons or on events such as OnCreated() or OnInputChanged(). For a full list of events have a look at the DOCs.

Easiest way is to select a node tree and drag and drop the nodes on an empty shelf spot. this will automatically create all the necessary script to recreate the setup on the push of the shelf button. Unfortunately this creates quite messy and bloated code.

To write it yourself you can start by creating a ‘New Tool’ on a shelf and importing the hou module. To easily find all the necessary functions, and python objects you can drag and drop nodes inside the scripting panel which will give you the correct syntax to access the data.

you can trigger scripts before and after a hip file is saved adding code to HOUDINIPATH/scripts/beforescenesave.py & afterscenesave.py -> DOCS

Counting how often each node gets used (useful to figure out support and update importance in bigger studios):

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import hou
from collections import Counter

parentnode = hou.node("/obj/geo1")
allnodes = parentnode.children()

nodenames = [x.type().name() for x in allnodes]

print(Counter(nodenames))

Every Houdini Menu is described by a .xml file that describes how it’s constructed and what items it consists of. Take a look at SideFX Labs MainMenuCommon.xml.

Python Scripts can be added in an xml in the following way:

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<submenu id="name_menu">
	<scriptItem id="itemid">
		<label>LABEL</label>
		<insertAtIndex>10</insertAtIndex>
		<scriptCode>
			<![CDATA[
			PYTHONSCRIPT
			]]>
		</scriptCode>
	</scriptItem>
</submenu>

can be used to sanity check or load default scenes etc. on startup

pythonrc.py runs on startup (as many as found in $HOUDINIPATH)

123.py runs when Houdini is started without a scene (only first found)

456.py runs after a scenefile is loaded (only first found)

sources / further reading:

• Python Scripting - Houdini - Paul Ambrosiussen