What is a FLIP Fluid?

A FLIP fluid is a mix between a particle, and volume simulation. 

How to Build a Successful FLIP Fluid Simulation 

Collisions When building a FLIP simulation of any kind most of the issues that will occur will be with the in the scene. Some tips to look for are:

  • Make sure your collision object isn't too thin. If it is, the particles may "stick", stack, compress, or disappear. Sometimes, after increasing the thickness you may also want to reduce the scale of your particles just a tiny bit as well. This will also reduce the pressure step, and make sure the collision is small enough to fit inside the container.

 

  • FLIP solver If your colliding object is animated and colliding with your fluid. You need to tell your simulation what the velocities are. In the 's collision tab, set the velocity type to point. Therefore, it will then start to read the point velocities of your colliding object. As well as doing this, sometimes you need to add a Trail SOP set to compute point velocities onto your colliding object. This will help create velocity trails that the FLIP solver can recognize more easily.

  • Collision Velocity Visualization If your collisions are not reacting the way you expected, turn on on the FLIP object. This will help you see what the problem is.

  • FLIP Solver If you would like your fluid to stick to your colliding object, you can enter the , and turn on Stick on collision. This is located in the volume motion tab. By default, the created fluid will slide across the collider, or surfaces you place in your scene. You can also make the fluid cling to the collision by turning on Sticky Fluid in the FLIP Solver. It will be under Projection.

  • Feedback Scale If you would like your fluid to affect the motion of your colliding object, then you need to turn up the. This parameter is once again located in the FLIP Solver under the Volume Motion/Projection Tabs. In order to get the correct feedback onto your object, you'll need to scale the feedback correctly. Keeping the value for the feedback parameter in the 100s is preferred.  

  • $FF or $T over $F If you are animating your colliding object, it is preferred if you use either . This is because $FF and $T will relay the movement of the object over the time of the animation. If you use #F, Houdini will read that as you animating the object on a certain frame number.

Here are some other tips that might help you out:

  • point velocities If you would like whitewater to appear when something splashes or moves your fluid, you need to once again focus on the .  Take the velocity from your fluid, and advect it against the whitewater in your simulation. Just make sure that the advection taking place is within the tolerance of the SDF of the simulation.

  • If you are emitting a fluid from an object, try and make the object more abstract. This will not only make the simulation more interesting, but it will also allow more noise to be included into the simulation.

  • (V, a Vector Attribute.)Every particle fluid simulation moves on the velocity attribute of the point on the object it is birthed from. This is their natural initial velocity. Therefore, you can enter the velocity attribute into the geometry sheet to control the emission rate/velocity. Then you can create a more abstract flow.

  • By default, particles will be emitted from the points of an object. To create a solid particle stream, scatter points in a volume/object/etc, then use your new points as the start area for your fluid. Play around with this method. Based on where the point clusters are in your object, the more interesting the fluid will be.

  • Stream Emission.To have your fluid particles emit as a regular burst of particles instead of a stream, you'll need to turn off You'll find it under the Particle Fluid Emitter.

  • Smoothing parameterPlaying with the in the FLIP Solver is also recommended. If you would like your particles to each move in their own direction, change the smoothing value to 0. If you would like a smooth particle stream, change smoothing to 1.

  • FLIP SolverUsing Dynamic force nodes in the will also allow you to move your simulation in different ways. The POP Axis Force, or the Fan Node are two of these many nodes.

Rendering Tips

Rendering FLIP Fluids can be a bit difficult as depending on type of fluid you are creating, you may need different render settings for each one. (water, jelly, toothpaste, etc) Here are some rendering tips that might work for your simulation.

  • When you create a fluid from one of the shelf tools, Houdini automatically creates the surface attributes and information set for particle visualization for Mantra. If you are creating a custom particle fluid simulation, you'll need to set up attributes to tell mantra what you would like rendered.

  • If the surface of your liquid looks too rough, you have the option of adding a Peak Surface Node to your Particle Fluid Surface, as well as a Smooth Node. 

  • A good part of understanding what is happening while you are rendering, is understanding what is Houdini being told to visualize. If your visualization for something is not turned on, your simulation will never appear how you intended it to be.

 

  • If you are rendering water, make sure you have something in the scene that the water is reflecting. An environment light works great for this! However, a colored ground plane, or other objects and light sources will also do the trick.

  • Using a Sunlight is also recommended with some water/liquid scenes. This will help give your fluid simulation a sunlight based glare.

  • Sometimes bounce lighting can interfere with your water simulations. This might cause indirect diffuse noise in your simulation. In this case, keeping the diffuse limit at 2 or below will help reduce this. 

  Another huge part for rendering water or FLIP fluids is building a shader that works. Here's some tips on that.

  In Houdini, almost every shelf standard FLIP fluid loads with a default liquid shader, ocean shader, or ocean volume shader. Here are some good go-to settings for the basic liquid shader.

 

  • Try setting the Diffuse Intensity to 0.05 or less. 

  • If you would like sharp reflections on your liquid, then set the reflection angle to 0, if  you like , you can also increase the reflection angle just slightly higher than zero to increase the sharpness.

  • Reduce the Specular Intensity to 0.95 or less.

  • Have the object reflections in the scene match the light reflections for the best results. To do this Turn off the Separate Object Reflections parameter.

  • If you would like your liquid to be either dark or murky, then focus on changing the refraction color until it matches what you would like.

  • If you have a volume shader attached to your simulation, this shader controls how the light travels under the water/liquid. Depending on what you are building, these changes will be up to you.

 Here are some tips if you are using a custom built liquid shader in the material network. 

  • Try controlling the reflection and refraction with PBRSpecular Nodes. Then enable the Fresnel blending on them. as well as setting the eta on them for 0.6.

  • To color the Vorticity and Velocity consider using the RGB Color Ramp, and then multiplying it with a PBRSpecular Node. You can use the same color ramp here to control the shadow color of your liquid.

  • Keep the roughness of the shader to zero.

  • Using Phong for the Specular of your PBRSpecular Refraction or Reflection Nodes is also a good way to start. Sometime you might have to change these to GGX for better results

Houdini FLIP Fluids