The File Cache SOP lets you assemble a path, where the output files are stored. Before you cache a simulation, the project should be saved with ⌃ Ctrl + S. By default, the file path is constructed from a series of variables in the Base Name and Base Folder input fields.

You can also choose an extension. For geometry, we recommend bgeo.sc and vdb for voxel data. If you don’t want to save your files to a different location, click the Open floating file chooser icon.

In the node’s Caching tab, go to the Sequence subpane, where you can adjust Start and End frames. By default, both fields are green, indicating that they're driven through expressions. These expressions read out the project’s start and end frames. For a custom range, right click Start/End/Inc and choose Delete Channels. Now you can enter your own values.

When you're ready to cache your simulation, press Save to Disk or Save to Disk in Background. For the latter option you must save the project. The button opens another instance of Houdini to cache the files through a the render manager. Houdini remains responsive and you can load already cached files to evaluate them. However, Save to Disk is faster.

The Load From Disk option is turned on automatically for your convenience. To change this, open the Advanced tab and expand the Save subpane. Then, turn off Enable Load from Disk on Save.

The Caching tab’s Load subpane might be of particular interest for you, because you can choose what you want to load: All Geometry, Point Cloud, Packed Geometry, etc. For heavy scene, please also consider turning on Delay Load Geometry to reduce load times.

To play back the files, press the Play button in the timeline.

When working with simulations, the timeline (at the bottom of the main Houdini window) uses background colors to indicate the cached state of each frame.

By default, when Allow Caching To Disk is on, Houdini saves on-disk cache files to the temporary directory. If these cached frames are invalidated, Houdini automatically deletes them from disk.

You can turn on Save Checkpoints to save the on-disk cache to a known location instead, with explicit control over how often and how many frames to cache. These files are not deleted if they're invalidated. However, checkpoints are useful for large simulations running on render farms. If the render fails for some reason at frame 50, you can change the start frame to 50, and Houdini will re-start the simulation from the previous checkpoint (for example, frame 45) instead of having to re-simulate from frame 1.

1. Select the DOP Network OBJ.

2. In the parameter editor, click the Cache tab and turn on Save Checkpoints.

3. Check the Checkpoint File parameter, which controls where the sim files are written and the filename pattern of the .sim files. You must use $SF (internal simulation framestep number) instead of $F (global frame number) in the filename pattern.

4. Set the Checkpoint Trail Length to control how many checkpoints Houdini saves at a time. If there are more checkpoints than this, Houdini will automatically delete the oldest one. This may be necessary for large simulations to prevent checkpoints from filling up the disk.

5. Set the Checkpoint Interval to control how often Houdini saves checkpoint files. For example, setting this to 5 will save every fifth frame.

The Output DOP at the end of the simulation network inside the DOP Network OBJ has controls for writing out simulation files. These files are independent of the cache. You can keep .sim files between runs of Houdini, and move them between computers.

1. On the Network Editor’s obj level, double click the DOP Network node to dive into it. There, select the Output node.

2. In the parameter editor, you set the Start and End frames to simulate. By default, the parameters are controlled through expressions to automatically use the project’s start and end frames. If you want to change these values, right click the Start/End/Inc parameter and choose Delete Channels from the menu.

3. Check the Output file parameter, which controls where the sim files are written and the filename pattern of the .sim files.

   You can use a .sim.gz (GZIP) or .sim.sc (Blosc) extension to have Houdini compress the files. This is slower but saves disk space.

4. Click either Save to Disk or Save to Disk in Background.

   Saving in the background launches a new copy of Houdini in the background to run the simulation. You must save the .hip file before you use this option. You can monitor the background task using the render manager.

You can also use the Dynamics OUT render node to write out sim files. The render node 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.

Once you've baked out simulation files, you can set up a DOP network to never cook and only play back the contents of the simulation files.

1. Select the DOP Network OBJ.

2. In the parameter editor, turn on Playback Simulation and set the filename pattern for the .sim files to use.

   You may want to make this a channel reference to the filename parameter on the node saving out the files, so you don’t have to keep the two parameters in sync manually. Right click the filename parameter on the output node and choose Copy Parameter. Then, on the DOP network node, right click the playback filename field and choose Paste copied relative reference.

When working with simulations, the timeline (at the bottom of the main Houdini window) uses background colors to indicate the cached state of each frame.