Houdini 19.5 Fluids

Caching, compressing, surfacing, previews

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Caching

You can simulate the fluid and temporarily store the results to the computer’s memory. This workflow can become problematic with heavy simulations, because you might run out of memory. The result will also be lost when you close the project and you have to re-simulate. To save simulation data permanently, you can write them to disk. This process is called caching. Depending on factors like Particle Separation, Voxel Scale or domain size, the amount of FLIP fluid data can become very large. Houdini provides the Fluid Compress SOP node to compress the data and save disk space. Note that compression is lossy, so you will lose quality. However, SOPs such as Particle Fluid Surface are designed to detect compressed fluid input and regenerate any required missing data.

  1. Add a Fluid Compress node to the network. The node provides three inputs for Particles and Volumes, the Container, and Collisions. These inputs match the outputs of the FLIP Solver, but in normally you probably only use the first input to compress the fluid data.

  2. Connect the Fluid Compress node’s first input with the solver’s first output.

  3. The Particle Separation parameter should match the FLIP Container node’s equivalent.

  4. If you have configured custom attribute-field pairs, append them to Keep Attributes.

Now, the data can be cached. In Houdini 19.5, all cache nodes are streamlined and provide the same base functionality and layout. In many cases you can proceed with the default settings.

Note

If you want to cache not only fluid data, but also the Container and Collisions information, you need one File Cache SOP per data stream.

  1. Add a File Cache and connect it to the Fluid Compress node’s first output.

  2. Base Name contains two tokens:

    • $HIPNAME is the name of the Houdini project.

    • $OS stands for the node’s own name, e.g. filecache1.

  3. If you keep the entry under Base Folder, you can find the cache files in the project directory under geo.

  4. To save the data, click Save to Disk (the UI will be locked) or Save to Disk in Background (the UI remains responsive).

  5. To interrupt the simulation, press Cancel Cook. Depending of the simulation’s complexity, it might take a moment until cooking stops.

Note

You might see a warning in File Cache, telling you that the simulation data couldn’t be loaded. The Load from Disk option is turned on automatically when you hit Save to Disk (in Background). The node tries to load the first cache file, but this file is not present when you cache the simulation for the first time.

Surfacing

To render the fluid as a solid object, you have to create a polygon surface around the particles through a Particle Fluid Surface SOP node. Connect the first input with the first output of the File Cache node (or the FLIP Solver, if you don’t want to cache the simulation).

It’s difficult to provide common settings for this process, because the quality of the mesh depends on many factors. For FLIP fluids there are a few rules of thumb.

Surfacing tab

  • By default, Method is set to Average Position. If you want to use Spherical, you should go to the Filtering tab and turn on Dilate, Erode and Smooth.

  • Under Particle Separation, use the same value as in the FLIP Container node.

  • Influence Scale should be changed in small steps, because it affects cooking time and memory usage.

  • Droplet Scale has to be smaller than Influence Scale.

  • Adaptivity helps to avoid inconsistencies and flickering. With 0, the node creates a regular mesh with equally-sized polygons.

  • Don’t forget to append your custom attributes to Transfer Attributes.

  • If you want to smoothen the attributes, use an upstream Attribute Blur node.

Filtering tab

Filters are a fast and effective way to change the mesh’s shape and look. Filters can be used to smooth the surface, blow up the mesh (Dilate) or shrink it (Erode). Especially smoothing has a strong influence and you might lose surface detail with high values.

As mentioned above, Filters should always be used, when Method is set to Spherical. The filters remove microbumps from the mesh surface and care for a better look.

Regions tab

SOP FLIP fluid simulations with water tanks and ocean surfaces show a particle band around the domain’s outside. This is the area where new particles are created. When you create a surface, the particle band is taken into account and expands the mesh. You can cut away the band, but please note that this method only works with box-shaped domains.

  1. Turn on Bounding Box.

  2. For Size and Center use the values from the FLIP Container or the connected object, serving as the domain.

  3. To represent the entire water volume, not just the surface, turn on Closed Boundaries.

  4. For better edges, also try Flatten outside Boundary Box.

The image shows a simulation without (left) and with (right) optimized surface parameters. A color Cd attribute was used here to separate fluids with different densities.

Note

If you want to know how to create the multi-density simulation from above, you can find a complete workflow guide under Point variable density: multiple densities.

Tip

If the bounding box is smaller than the container, you can use it to reveal a fluid’s inside, for example to see the interaction between differently colored fluids below a narrow band surface.

Previews

On the lower left side of the viewport you can see two stacked icons.

  1. Click the icon in the UI’s lower left corner to open the Render Flipbook settings.

  2. You can leave the default settings and click Start to record an image sequence.

  3. When ready, click File… in the preview window and then Save Sequence As…

  4. A new dialogue appears where you can specify file name, directory, and quality.

Fluids

For beginners

SOP WORKFLOW

Particle Fluids (SOP)

Viscous fluids (SOP)

FLIP Configure tools (SOP)

Optimization (SOP)

Particle Fluids (DOP)

Viscous Fluids (DOP)

Oceans (DOP)

Optimization