Houdini 20.0 Nodes Dynamics nodes

ROP Output Driver dynamics node

Saves the state of a DOP network simulation into files.

The Dynamics output driver generates simulation state (.sim) files from a dynamics (DOP) network. Each file contains the complete state of the simulation for a certain frame. You can then read the state back into the network using a File DOP. You can also directly point to a sequence of these files in the Playback Simulation parameter of the DOP Network Object or DOP Network SOP.

Saved files are a persistent cache of a simulation. You can move these files around to run the simulation in one place and use the results in another. For example, you can use the HScript standalone utility to run a simulation on a renderfarm and save out state files, then play the simulation back locally by loading the state files with a File DOP. This would let you run multiple versions of a simulation on the renderfarm overnight and compare the results the next day.



Saves the simulation to disk as a sequence of .sim files.

Background Render

Starts another copy of Houdini in the background and instructs that copy to save out the simulation as a sequence of .sim files. This allows one to continue working and load the .sim files as they complete.

Render Control

Opens the render control dialog to allow adjustments to the render parameters before rendering.

Valid Frame Range

Limits the rendering of frames, when set to Render Frame Range or Render Frame Range (Strict).

Render Current Frame

Allow the renderer to render only the current frame.

Render Frame Range

Only render the frames in the range set below, but allow the renderer to render other frames if they are referenced by in-frame frames.

Render Frame Range (Strict)

Only render the frames in the range set below, and don’t allow the renderer to render other frames, even if they are referenced by in-range frames.


Specifies the range of frames to render (start frame, end frame, and increment). All values may be floating point values. The range is inclusive.

These parameters determine the values of the local variables for the output driver.

Render with Take

Uses the settings in a particular take while rendering. Choose Current to use the current take when rendering.

DOP Path

The DOP network to render from. You can point to a DOP within a DOP, but Houdini saves the state of the entire network.

Output File

The file to save the simulation state to. Make sure to include $F in the filename to write out separate files for each frame.

Initialize Simulation OPs

Force all simulation OPs to be reset. This includes DOP Networks, POP SOPs, and other OPs that cache their results.

This is the safest way to render out a simulation, because it starts the simulation from scratch and discards any partial simulations you might have done with different parameters. However, throwing away and already-cooked simulation can be expensive, especially for slow solvers such as fluids.

Compress .sim Files

Determines if .sim files, written to disk for caching, will be compressed. Compression can reduce the size of the disk cache significantly, but can also increase the time to cache significantly. If loading or writing to cache seems slow, try using uncompressed .sim files, especially with fluids sims.

Alfred Style Progress

A percentage complete value is printed out as files are written. This is in the style expected by Pixar’s Alfred render queue.

Pre-Render Script

Pre-Frame Script

Run this HScript before each frame.

Post-Frame Script

Run this HScript after each frame.

Post-Render Script

Run this HScript after all rendering.


To access the current simulation frame $SF from the DOP simulation, the Output File should be:


and the frame range Inc should be:


Dynamics nodes