Houdini 17.5 Dependency networks

File paths

Best practices for input/output file paths in TOP networks.

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Most TOP nodes that do work let you specify input and/or output file paths. In TOPs, each scheduler node can specify a working directory (this is because different render farm software may use different shared network filesystems). You should make all the file paths you output to relative to this PDG_DIR directory.

How to

  • Set the base working directory on the scheduler node. This directory is available to work scripts as the PDG_DIR environment variable.

    • Use separate working directories for each HIP file (that is, avoid having two HIP files writing to the same PDG_DIR). Many of the default generated filenames used in parameter defaults are only unique within the HIP file.

    • For render farm schedulers, make sure the directory inside the network filesystem (for example, NFS mount of SMB share) shared with the render farm client machines.

  • When you specify filenames (for example, images to render or geometry files to generate), make sure the path is relative to PDG_DIR.

  • When you use environment variables such as PDG_DIR in filenames, use the form __PDG_DIR__ instead of ${PDG_DIR}. If you use ${PDG_DIR}, Houdini will try and fail to expand the variable itself before the dependency graph gets it. Houdini will ignore __PDG_DIR__ syntax, but the dependency graph knows to expand the text between double-underscores as an environment variable.

  • Put intermediate files under __PDG_TEMP__ and "final output" files under __PDG_DIR__.

    You probably want to categorize output files using subdirectories, for example __PDG_TEMP__/geo for intermediate geometry files and __PDG_DIR__/geo for final geometry output.

Environment variables


The TOP network’s working directory, as specified on the Scheduler node.


A shared temporary file directory for the current session, inside the working directory. The default is $PDG_DIR/pdgtemp/houdini_process_id.


A shared script directory, inside the temp directory. Script files are copied into this directory if they are listed as file dependencies. The default is $PDG_TEMP/scripts.

Alternatively, you can simply put scripts in known locations in the shared network filesystem.

Dependency networks


Next steps

  • Running external programs

    How to wrap external functionality in a TOP node.

  • File tags

    Work items track the "results" created by their work. Each result is tagged with a type.

  • Feedback loops

    You can use for-each blocks to process looping, sequential chains of operations on work items.

  • Command servers

    Command blocks let you start up remote processes (such as Houdini or Maya instances), send the server commands, and shut down the server.

  • Integrating PDG and render farm software

    How to use different schedulers to schedule and execute work.

  • Visualizing work item performance

    How to visualize the relative cook times (or file output sizes) of work items in the network.

  • Tips and tricks

    Useful general information and best practices for working with TOPs.


  • All TOPs nodes

    TOP nodes live inside TOP objects and do work.

  • Python API

    The classes and functions in the Python pdg package for working with dependency graphs.