Working with HDA

HDA (Houdini Digital Asset), which used to be name OTL (Orbolt Tool Library), are usually tools built to streamline workflows. They allow to wrap up large network into a single node with an artist-friendly interface.

As TD, HDA are one of the main way to provide tools for the Artist to interact with as they can integrate smoothly in their work.

HDA Creation is covered in many tutorial such as SideFX Introduction to Digital Assets.

Special HDA

It is most common to create HDA from a subnetwork and click Create Digital Assets... This mechanic is great for creating tools that are based on Houdini nodes, but other types of HDA can be created.

In the Main menu, go in File > New Asset... , a new dialog will open asking you the same basic questions about a name and label for your HDA and where to save it. But you can notice you have the ability to select a different "Definition" type. The most common type of definition is "Subnetwork", but other most useful one could be Python and Output Driver.

Those HDA behavior can be customized by editing the Code Section when editing them.

You might also notice that those nodes cannot be "Unlocked and edit..."

HDA versioning

Node can only have one active NodeType and NodeType can only have one active HDA Definition.

Following the above graph, there is usually two way of versionning your HDA, with both pro and cons. Either by switching:

• NodeType
• HDA Definition

hou.Node.changeNodeType(new_node_type)

hou.preferredNodeType("Category/basename")

hou.HDADefinition.setPreferred(True)

Embedded HDA

Often overlooked, Embedded HDA is a useful alternative to storing files on disk as it will store your HDA in the hipfile. Storing an HDA as Embedded can be useful in some cases.

If you are planning to share your file with someone who is not the same network as you or won't have access to all your HDA files, embedding the HDA is a really nice and conveniant way to package your scene.

HDA files when saved, depending on your configuration, should create backup file, though, those changes are only stored when the HDA is being written to disk. In development stages, by storing an HDA in your scene, you can use the scene file save mechanic to create clever backup of your HDA development.

Note: When you open a hipfile but the HDA source file is unavailable, Houdini will automaticly create a dummy embedded definition. This way, if you accidently save your scene, Houdini will still be aware of the original NodeType the Node was.

HDA Expanded structure

When you save your HDA, they are by default saved into a binary file which is easy for houdini to open. Though, if the HDA is part of a package it can be really hard to track changes on it. An option is to expand the HDA using the following commands:

# To expand an HDA into a VCS friendly folder
hotl -t expanded.hda original.hda

# To collapse a VCS expanded HDA into a binary HDA
hotl -l expanded.hda collapsed.hda

Once expanded a HDA should be looking like this with more or less files depending on the number of Sections and scripts that you may have defined in your HDA:

• expanded.hda - This is the main folder under which your HDA is expanded. If it uses the .hda extension, you can load it straight away in Houdini

  • houdini.hdalibrary - Just here to say this is a hdalibrary
  • INDEX_SECTION - Specify some basic information regarding the declared operator such it operator name, operator label, context, number of input and output, etc...
  • Sections.list - List files in this folder and how they should be evaluated by houdini

  • Sop_1hda_name - This is the folder under which you HDADefinition is expanded

    • Contents.dir - (Optional) This folder represent the content inside your HDA

      • Contents.createtimes - When were each children nodes created
      • Contents.houdini_versions - Which version of houdini was used to create each children nodes
      • Contents.mime - This file define most properties of the children nodes such as how they are connected, the values on the parameters, placement in the HDA.
      • Contents.modtimes - When were each children nodes last modified
      • Sections.list - List the files in this folder and how they should be evaluated by houdini

    • CreateScript - Automaticly created script defining how to create the HDA
    • DialogScript - This file define the ParmTemplateGroup on the HDA, with all the information relative to the list of parameters, default values and parameter behavior, etc...
    • ExtraFileOptions - Houdini internal file used to remember where was your cusor when editing files within the HDA Editor dialog
    • Help - (Optional) Help file for your HDA
    • InternalFileOptions
    • PythonCook - (Optional) This is the content to be evaluated when the node is used to programaticly generate contents.
    • PythonModule - (Optional) Whenever you are creating a PythonModule, the content is saved in this file.
    • Sections.list - List the files in this folder and how they should be evaluated by houdini
    • Tools.shelf - A file very similar to a "Shelf Tool", because it is one! This is the tool that is called when you ask to create the node
    • TypePropertyOptions - Various properties about the HDA

Expanded HDA are slower for Houdini to load as it need to compile it. Though, for development, it might be interresting to work with the expanded version as you can edit those files from an external editor, and changes will be reflected in Houdini.

Python Panel instead of Parameters

It is now possible to interact with an operator by using a PythonPanel (Qt UI) instead of the regular Parameter view. For doing so, you first need to create a new Panel.

<xml version="1.0" encoding="UTF-8">
<pythonPanelDocument>
    <interface name="my_ui" label="My UI" icon="MISC_python" shownetworkNavigationBar="true" help_url="">
        <script><![CDATA[
from hutil.Qt import QtWidgets


def onCreateInterface():
    widget = QtWidgets.QLabel("Hello World")
    return widget
        ]]></script>
        <help><![CDATA[]]></help>

        <!-- This is the part where you define all the operators that your UI can be used from. -->
        <showInParametersPane optype="Sop/hda_name"/>

    </interface>
</pythonPanelDocument>

Then when you select your operator, you should have an addional icon on the left of gear parm icon (looking like 2 slider). Click on it to enable/disable the python panel view.

Advanced HDA Functionality

Managing operators

Over a HDA lifecycle, it's integration with Houdini might require you to define how it integrate with existing tools. For example, you might want to give preference to your newly created caching HDA over the default Houdini filecache.

Here are couple way to help integrate your tools with existing HDA:

• Hide an operator
• Uninstall an operator
• Create operator aliases

hou.NodeType.setHidden(True)

# Finally uninstall all definition
for definition in node_type.allAvailableDefinition():
    # Handle the cases where multiple definition might be defined in a file
    for file_definition in hou.hda.definitionsInFile():
        # No problem if current definition is not current
        if not file_definition.isCurrent():
            continue

        # Check if the definition is being used by any node in the scene
        elif file_definition.nodeType().instances():
            raise RuntimeError

    # Finally uninstall the file
    hou.hda.uninstallFile(definition.libraryFilePath())

node_type = hou.nodeType("Sop/box")
node_type.addAlias("cube")

hou.node("/obj").createNode("geo").createNode("cube")

node_type.removeAlias("cube")

HDA & Startup time

When working with Houdini Digital Assets (HDAs), you might consider how they impact loading time.