Houdini 20.0 Nodes Geometry nodes

FLIP Collide geometry node

Converts connected geometry and volumes into collision object for SOP FLIP fluids.

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Since 19.5

This node accepts 2D and 3D geometry, and volumes as collision objects for SOP-based FLIP fluids. To turn an object into a collision object, connect the object with the 4th input of the node. The connected geometry is converted to a volume, and you can see this volume in the viewport when you turn on the FLIP Collide node’s Display/Render flag.

The only way to change the collision volume’s resolution is to change the FLIP Container SOP node’s Particle Separation. Smaller values create better volumes, but also increase the number of fluid particles, voxels and simulation time. You can also decrease Grid Scale, but be careful, because the number of voxels increases drastically. Good Grid Scale values range between 2 and 1.2 depending on the scene’s overall dimensions and complexity.

Accurate velocities for moving collision objects are also very important for fluid simulations in general, but particularly for FLIP. Incorrect collision velocities can lead to less dynamic splashes or fluid leaking through geometry.

You can combine multiple objects through a Merge SOP or using multiple FLIP Collide nodes. The FLIP Collide node provides Volume Collide and Surface Collide mode to handle both open and closed 3D objects as well as 2D geometry like grids or planes.


To make objects interact correctly, you have to adjust the FLIP Solver SOP node’s Collision Settings. There you can find the Collision Detection dropdown menu with appropriate modes.

Collision objects can be moving or deforming and their velocity can be taken into account by default to create accurate splashes. You can turn off Compute Velocity on demand. For stronger splashes and more turbulence, increase Velocity Scale.


For more information see FLIP fluid collision.



Enter the frame when the collision geometry should be active and contribute to the simulation. You can animate this parameter to turn a source/sink on or off on demand.

Compute Velocity

When turned on, velocity is calculated from moving or deforming objects and then multiplied with Velocity Scale.

Velocity Substeps

When computing velocity the collision is evaluated on multiple frames to find the deformation. If it is fast moving, it may vary considerably within a frame requiring the samples to be closer together. Usually this should match the Global Substeps of the FLIP Solver.

Velocity Scale

Scale the velocity from moving or deforming objects to create stronger (>1) or weaker (<1) splashes.

Volume Collide

Turn on this mode for solid 3D objects and volumes. Volume Collide creates a volume from the input geometry. Quality and resolution of this volume is controlled through the FLIP Container SOP node’s Particle Separation. For low resolution simulations it is often useful to use both the volume and surface collisions (= default state) for the same geometry. The volume collision will handle the bulk fluid, and the surface collision will avoid any particles leaking. If you use a volume as collider, then Surface Collide is not available. Only volume collisions are able to support a column of fluid. If your fluid collapses in on itself, it may be that the volume collision isn’t thick enough to be seen by the solver.

For solid objects, merged with open 3D or 2D geometry, Surface Collide has to be turned on as well. In this case, go to the FLIP Solver’s Collision tab. From the Particle Collisions menu, choose Particle.

Surface Collide

Turn on this mode for 2D objects or open 3D geometry. Surface Collide is a 1:1 representation of the input object’s mesh. For correct collisions, go to the FLIP Solver node’s Collision tab. From the Particle Collisions menu, choose Particle. The Particle mode is also required for 2D or open 3D geometry, merged with solid 3D objects. In this case you have to turn on Volume Collide as well.

Surface colliders must have the same topology every frame - continuous collision detection is done between corresponding points, so if they do not match up, the sub-frames will be a tangled mesh. Use a Time Blend SOP and switch the playbar to sub-frames to verify the geometry interpolates properly.

  1. Press Alt + ⇧ Shift + G to open the Global Animation Options.

  2. Turn off Integer Frame Values to switch to sub-frames.

  3. If necessary, adjust Step.

Object Name

The $OS token represents the node’s name, displayed in the Network Editor. The object name allows multiple FLIP Collide nodes to generate different collisions geometry.



This output expects the fluid particles.


This input expects the FLIP Container domain geometry.


This input expects a collision object or volume.

New Collision Geometry

Any object or volume, connected to this input, can act as a collider. Multiple objects can be combined through a Merge node, and then connected.



This output contains the fluid particles.


This output is a passthrough of the FLIP Container’s domain geometry.


This output contains collision geometry/volume from the New Collision Geometry input. If there is another FLIP Collide node connected already, all objects and volumes are merged.

See also

Geometry nodes