Houdini 19.5 Fluids

DOP and SOP workflows

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In Houdini 19.5 we introduced a new, SOP-based workflow for FLIP fluids with many improvements. For beginners, the main questions are:

  1. What are the general differences between DOP and SOP networks?

  2. Why are there two workflows, obviously doing the same thing?

  3. Are there any features in SOP which aren’t available in DOP and vice versa?

  4. Why should I work with SOP FLIP fluids?

Differences between DOP and SOP networks

Houdini works with different types of network folders. Those folders are associated with certain “tasks”, for example for simulation or rendering. To differentiate the types of network, Houdini uses acronyms. For FLIP fluids, there are two relevant network types:

  • SOP stands for Surface OPerator. Inside a SOP network you find everything that’s required for constructing and modifying geometry and volumes.

  • DOP is the abbreviation of Dynamic OPerators and DOPs are used to construct simulations. Simulations read in geometry from SOP networks and transfer the data to the DOP network, for example collision or source geometry for rigid body and pyro simulations.

The exchange of data between DOP and SOP networks is often an obstacle for new users, because it can be difficult to keep track of all nodes inside the networks. The advantage, on the other hand, is that things are clearly separated.

Two FLIP fluid workflows

Moving simulations from DOP to SOP follows a long-term approach in Houdini to make things more user-friendly and streamline workflows. Having all relevant nodes inside one network can help to speed up workflows, minimize the number of nodes, and have a better overview of nodes, working together. The SOP-based approach has been introduced for pyro and Vellum already.

The new SOP-based FLIP fluids also differ from their DOP FLIP fluid counterpart: SOP FLIP fluids provide new ways of sourcing fluid particles, handling boundaries, and transferring data between simulations.

Despite all these changes, the standard DOP-based workflow is still valid and experienced users can open older files and access the networks.

Features in DOP and SOP FLIP fluids

Houdini’s SOP FLIP fluids are a new technology, introduced in version 19.5 and there are differences, of course. Right now it’s not possible to create whitewater from SOP FLIP fluids.

DOP FLIP fluids have the advantage that they're production-proven and fully-featured. Over the years, many users and studios created their own tools, scripts, and networks around DOP FLIP fluids. It’s very likely that you have to - at least - adapt your tools to the new workflow.

With SOP FLIP fluids you have additional features, for example:

  • Use almost any shape for the domain, not just boxes or cubes.

  • Greatly improved boundaries.

  • Choose from pressure and velocity boundaries.

  • Emit fluid particles with pressure and/or velocity.

  • Inject pressure and/or velocity from other sources, e.g. a low-res simulation.

  • Add custom fields like varying viscosity and mass-density.

  • Better accessibility of physical parameters.

  • Reduced solver parameter set.

  • Extra node for collision objects.

Both, DOP and SOP FLIP fluids, provide surface tension, a robust viscosity solver, air incompressibility, waterline, and the narrow band technology.

SOP or DOP FLIP fluids?

This question can’t be answered directly, because both methods have their pros and cons:

  • The entire simulation setup can be done inside one SOP network.

  • For new users, SOP-based workflows might appear less technical.

  • Houdini aims to streamline workflows: if you're familiar with SOP FLIP fluids you’ll find very similar workflows in other environments like Vellum or the pyro tools.

  • SOP technologies are based on the latest improvements and features.

  • Future development is focused on SOP technologies.

  • Interesting possibilities like up-res simulations, attribute-field pairs or simplified guided oceans.

DOP technologies also have advantages:

  • Production proven.

  • Large library of tutorials and training materials.

  • Existing scripts and tools work inside DOPs.

  • Complete feature sets.

  • More users can share tips and provide help on specific questions.


For beginners


Particle Fluids (SOP)

Viscous fluids (SOP)

FLIP Configure tools (SOP)

Optimization (SOP)

Particle Fluids (DOP)

Viscous Fluids (DOP)

Oceans (DOP)