The FLIP Boundary SOP converts its input geometry into a volume that can be used to control simulations. It’s also possible to directly connect a volume. You can also combine multiple objects through a Merge SOP to get more complex shapes.
The FLIP Boundary SOP node is only needed if you want to add or remove particles from a source or sink geometry to/from the simulation.
The FLIP Boundary node provides two modes: Source and Sink:
When used as a source, the connected object acts like an emitter and creates particles. The fluid particles can be sourced through velocity and/or pressure. With an Additonal Velocity it’s possible to give the particles a certain direction. Pressure-based sourcing can be used to fill glasses or pools by defining a Hydrostatic Pressure. Sourcing will stop automatically once the associated Waterlevel is reached. With velocity-based sourcing, such a container overflows and particles are created during the entire simulation.
A sink is a volume where particles are deleted. In simulations with a sink and a waterline, new particles are sourced at the domain’s boundaries to maintain the overall number of particles. Like sources, sinks can have any closed shape and you can use both geometry and volumes. Sinks can also have negative velocity or pressure values to create a suction effect where the particles are drawn towards the sink.
Sources and sinks can be animated or deforming. With Compute Velocity From Deformation and Scale Velocity you're able to take the velocity from animated and deforming objects into account and scale it.
In scenes, where you fill objects, the source object should be located at the object’s bottom. If you use pressure-based sourcing, the object should be below the adjusted Water Line.
For detailed information on how to use the available nodes, please visit the Fluid sourcing page.
Houdini provides several preconfigured source and sink nodes for your convenience, e.g. FLIP Source: Velocity or FLIP Sink: Pressure. You can find them in the ⇥ Tab menu.
Enter the frame when the source/sink should be active and contribute to the simulation. You can animate this parameter to turn a source/sink on or off on demand.
You can choose whether the connected object should be a source to create particles or a sink to remove particles.
New particles are created and added to the simulation based on the chosen Boundary method.
Particle are removed from the simulation at the boundaries of the connected object based on the chosen Boundary method.
You can choose from three methods. Each method has its own set of parameters to control the sourcing or sinking process.
New particles are sourced using velocity. This mode works similar to DOP-based FLIP fluids.
New particles are sourced using velocity.
New particles are sourced using pressure and an optional velocity.
Particles are emitted in direction of the source object’s normals with the given pressure. If you fill an object with Uniform Pressure, sourcing stops once the given pressure value is reached. Pressure-based sourcing often creates very fast particles. Animate the parameter to tame the particles. When in sink mode, Uniform Pressure is set to
-1 to create a suction effect towards the connected geometry. The higher the value, the stronger the suction effect. With positve values, you can observe a repulsion effect before the particles are finally deleted.
Particles are emitted in direction of the source object’s normals with the given pressure. If you fill an object with Hydrostatic Pressure, sourcing stops once the adjusted Water Level is reached. Pressure-based sourcing often creates very fast particles. Animate the parameter to tame the particles. Although the parameter accepts any positive value, we recommend using values between
1. Hydrostatic Pressure is currently limited. Imagine a water hose with a nozzle. In the real world you can control the pressure inside the hose by closing or opening the nozzle. As a result, the velocity of water, leaving the hose, changes accordingly. This dynamic interplay between pressure and velocity is not possible with SOP particle fluids. Hydrostatic Pressure can also be used in sink mode and negative values create a suction effect. In sink mode there is hardly any difference between Hydrostatic Pressure and Uniform Pressure.
This parameter is only avaiable in conjunction with Hydrostatic Pressure and defines a threshold. Once the water level has reached the given value, sourcing stops. This method can be used to fill a glass or a pool, for example.
By default, the water’s baseline is at
0,0,0. Water Origin shifts this baseline by adding an offset. In most cases you only change the Y direction. Imagine you fill a pool to a Water Level of
1, but the pool’s bottom is at
0.2. In this case, the effective fill level is
1.0 - 0.2 = 0.8. To compensate for this difference, set the Y value of Water Origin to
Pressure Voxel Band
The default value is good for most simulations. Higher values shrink the source volume and decrease the amount of sourced water.
You can give the fluid particles an initial velocity and direction by defining a vector. If other velocities are invloved, e.g. Normal Velocity, then Additional Velocity is added. In sink mode, Aditional Velocity can be used to define a direction in which the particles are attracted, e.g. only along the Y or the negative Z axis. With negative values it’s possible to flip directions.
When Compute Velocity From Deformation is turned on, the velocity of animated and deforming objects can be scaled with this factor.
When turned on, particles are emitted in direction of the source object’s normals with the given speed. In sink mode, negative values create a suction effect towards the connected sink geometry. Higher values create a stronger suction. With positive settings you can observe a repulsion effect before the particles are finally deleted.
Compute Velocity From Deformation
You can turn on this option to take the velocity of animated and deformed objects into account. Increase or decrease this velocity with the Scale Velocity factor.