Input, output, and caching


This page describes…

  • How to get information into a simulation, for example grabbing geometry from a geometry network to use as an RBD object.

  • How to cache simulation information to avoid recalculating the simulation.

  • How to get information out of a simulation, for example taking the volume data from a smoke simulation into a geometry network so you can shade and render it.

A note on the dynamics shelf tools

When you use the shelf tools to create simulation objects from existing Geometry container objects, Houdini does some interesting magic where it sets up an import from the Geometry container into a dynamics node (such as an RBD Object), and a corresponding DOP Import surface node inside the container to import the dynamics object’s transforms back onto the geometry.

So, in the case of RBDs, the Geometry container provides the shape of the object to the simulation, and the simulation then drives the transforms of the geometry.

This lets you quickly set up a Geometry object, make it a simulated object, and have the simulation drive the motion of the original object, without worrying about import and exporting information.

The information on this page describes how this import and export works so you can set it up manually if you need to.

Importing geometry into a simulation

These nodes let you grab geometry from a node in a geometry network and attach it as Geometry data to a dynamics object.

To import…Use this node
GeometrySOP Geometry
Scalar field (from a volume primitive)SOP Scalar Field
Vector field (from a volume primitive)SOP Vector Field
Object position and velocityObject Position

Importing particles

See the particle solver.

Caching/archiving simulation to disk


These files are independent of the memory cache Houdini automatically maintains of the entire simulation, and which Houdini invalidates and recreates when you change a parameter. Unlike the memory cache, you can keep .sim files between runs of Houdini, and move them between computers.

See also how to optimize a simulation.

File node

The File dynamics node can read or write a file containing all information about the objects and data in its inputs at the current timestep to/from a .sim format file. The node has three modes of operation:

  • Write data to disk. This could be used to archive simulation state so you can move the simulation between computers or onto a simulation farm.

  • Read data from disk. This could be used to read back state from an archives simulation.

  • Automatic writes data to disk if the file doesn’t exist, or read the file in if it does exist. This gives you a classic cache, where the simulation will save its state the first time it runs, and then on subsequent runs will use the cache files. To clear the cache, you would delete the saved .sim files.


Many high-level dynamics nodes have file caching built-in to their interface. For example, in the RBD Object node, the File mode and File parameters on the Collsions/Volumes tab lets you cache the object to disk to avoid re-computing its collision volume.

File Data node

This is very similar to the File node, but where the File node writes all objects and data from its inputs, the File Data node only writes a single piece of data. This can be useful when only a few pieces of data change between timesteps.

Dynamics render driver

You can also use the Dynamics render driver to save the entire simulation at the current timestep. This is useful if you need to generate simulation data as part of a rendering dependency path. Also, because the render driver is not part of the simulation itself, changing its parameters will not cause the simulation to recook.

Sim and simdata files

The File and File Data nodes write out individual files for each invocation at each timestep, given the normal filename which includes the $SF variable, which will be replaced with the current timestep number. See how to use expressions in filenames for more information.

(You may want to use $ST (simulation time) instead of $SF (simulation timestep number). $ST is affected by offset time and scale time options, while $SF is not.)

To resimulate cached sim files, you must either set the File nodes to “write” or, if the File nodes are in “automatic” mode, delete the files so they will be recreated.


When you file in multiple .sim files, Houdini uses unique IDs to skip loading objects and data it already has in memory.

Exporting simulation data to other networks

Data reference syntax

You can use the following syntax to reference dynamics data in various import nodes discussed below:

network path[:name[/data name]]

…where name can be an object name, object ID, or group name, and can be a wildcard.

For example…

NetworkObjectData name

If you set the DOP Network as the node to import, you will import the geometry of every object in the simulation. If you include an object name or ID (or object name wildcard, such as brick*), you will import the Geometry data of that object. If you include a data name, you will import that specific piece of data.


You can pull out any type of geometry data from a simulation, including not just the final simulated geometry, but also collision fields, visualizations, and guide geometry. Look in the details view to see what data is available on a given object.

Geometry and transforms

To... Do this

Import geometry from a simulation into a geometry network

  • Use the Object Merge surface node in a geometry network to pull Geometry data out of a simulation.

  • The DOP Import surface node can also import geometry from a simulation.

    DOP Import includes an option to undo the world transformation so you can work on the object in local space instead of at its final world position. You can put another DOP Import node with another transformation inversion at the end of the surface node chain to move the geometry back to world space position.

Use position data from a simulation as the transform of an object

The Fetch object is a geometry container that gets its transform from the position data of a dynamics object.

Use position data from a simulation as the transform for geometry

The DOP Import surface node can take input geometry and then use the Position data from a referenced simulation object to transform the input geometry.

Fluid fields

Use the Object Merge or DOP Import surface nodes to import scalar or vector field data (as a volume primitive) or geometry from a fluid object for shading and rendering.

A dynamics Fluid Object has several useful pieces of data to import, including the surface and visualization fields, as well as fields like velocity (vel). You can use the details view to see what data is available on a fluid object.

For example:

  • Surface sign distance field: /obj/AutoDopNetwork:fluid/surface

  • Visualization geometry: /obj/AutoDopNetwork:fluid/surface/Visualization

  • Velocity vector field: /obj/AutoDopNetwork:fluid/vel

The visualization field may be more useful to extract the surface than the actual surface field so you don’t have to surface it yourself (see below), but you may want to turn off the bounding box first on the fluid object’s Guides tab.

Reversing and manipulating a field

The sign distance field you get by importing the surface data from a fluid object is reversed from what the volume primitive expects, so that the “fluid” and “absence” areas are reversed when you import a fluid into a volume primitive.

You can reverse reverse the field by appending a Volume Mix surface node. On the Mix Method node, set the Mix method parameter to User and the Value parameter to -$V ($V is the current value of a point, so -$V just reverses the value).

Surfacing volumes

You can use the Iso surface node to create a surface along the fluid boundary in a sign distance volume like you get from a fluid object. Append the IsoSurface node and set the Implicit function parameter to $V.

Display and render geometry

Like surface nodes, dynamics objects can have separate display and render geometry. In dynamics, this is accomplished by attaching two different pieces of geometry data to the object representing display and render geometry, and then attaching rendering sub-data to each piece of geometry.

You can use the Rendering Parameters node to attach sub-data to geometry data indicating whether the geometry should be used for display or render or both. To attach the sub-data, connect the Rendering Parameters node to the object with an Apply Data node, and set the Rendering Parameter’s Data name parameter to a path under the geometry where the sub-data should be attached, for example Geometry/RenderParms.

Once you've attached the sub-data, you can use the Rendering Parameter node’s Display and Render parameters to control whether the geometry data it’s attached to should be visible in the viewer and/or when the dynamics network is rendered directly.

(The Rendering Parameters node also lets you attach a material for test rendering.)