Hi all,

Bit of a junior question here.

I'm just wondering what the difference is between all these terms (FLIP, particles, Pyro, etc).

I am wondering this because I've seen many different Houdini artists that are using different methods for creating FX. For example: particles for smoke, or volumes for smoke, or pyro for smoke, particles for FLIP, FLIP for particle / smoke advection, etc. It's all a little confusing.

If I wanted to create say a smoke plume or cigarette smoke or something. I'm not sure how I would go about approaching it. If these terms were a little clearer to me I could attempt to tackle it.

I am aware that there are various shelf tools that do this stuff for us. However, I would first like to just get my head around these different techniques at a basic level.

i will take a stab at this:


1. Particles : this is one of the first methods for creating a dynamic system used in tv and film. they are controlled with forces to direct them as the artist sees fit. They can exhibit physical or non-physical behavior. Rules govern them, the artist sets the rules. Particles are as smart or as dumb as the creator wants them to be. https://en.wikipedia.org/wiki/Particle_system [en.wikipedia.org]

2. Volumes/Pyro : This came along much later as processing power was limited, and volumes take time to sim and render. It is typically used to simulate gaseous fluids. Volumes are grid(voxel) based, meaning there is a 3d cube in space to determine how the forces interact with the “density” field. The density field is simply a float value that fills the voxel. Other volumes, either float, or vector, control the behavior of the grids, and are usually very physically based , based on scientific studies of how these types of fluids behave. These typically include pressure, heat, temperature, velocity as well as various types of masks that influence the other fields. These fields can be used to drive particle simulations.

3. FLIP : this is one of the newer constructs and combines volumes and particles. It is generally used in the simulation of liquid based fluids. Particles are used to keep track of and be the ultimate result of a FLIP simulation. FLIP stands for fluid implicit particle. A set of volume grids are used to calculate the pressure and velocity of the particles. FLIP is a hybrid, implicitly driven particle system and is a PIC ( particle-in-cell ) method. https://en.wikipedia.org/wiki/Particle-in-cell [en.wikipedia.org]

It all comes down to the basic need of the look you are after, and the time you are given to achieve the look. in a nutshell particles give the most control and generally the least computationally expensive. Volumes/Pyro/Flip can also be given a great level of control, but require a bite more understanding in order to manipulate the simulation and computation time is generally longer.

This is by no means fully descriptive and highly subject to scrutiny, but is at least my basic understanding of them….hope it helps

Hello Sidenimjay,

Thank you for your informative reply! You broke these down in a way that is making more sense to me now.

If I understand correctly, the FLIP system is a voxel grid that is using vectors to determine the position of each particle at the next frame based on the previous frame's values?

Sounds like you understand it!

Glad I was able to shed some light!

hello sideinmijay

thanks for your valuable answer
I am a newcomer in Houdini and this question had kept me in trouble for a long time.
after all thanks a lot.