[novice] understanding fluids

[art3mis]

Please fee free to correct any of the following assumptions. Just trying to get a grasp on the entire workflow in creating 'simple' Flip fluid sims. Have gone thru most of the documentation, webinars, the detailed Fxguide article on fluid sims, but still some gaps in my understanding.

1st assumption >  in Houdini 15 there are essentially 3 ways of creating fluids, SPH, Flip and Volume based.

2nd > Flip is the recommended option, although for smaller and slow moving sims , SPH might be considered

3rd > A typical workflow can very roughly be broken down into 2 steps, the creation of the geometry from particles if is a particle based sim, and then the shading-rendering of this geometry

 

So my first question, what exactly is meant by 'surfacing'?

Does the term'rendering' also refer to the creation of the geometry?

Which part of the simulation typically takes longer, the geometry generation from particles, or final render shading?

[Solitude]

Correct - Flip is definitely the recommended option, SPH in Houdini hasn't been updated in years, and probably isn't worth using for many cases. Volume based also is a bit older. Flip holds it's volume and detail much better.

Surfacing (or meshing) - turn the particles into a mesh. You have to create some geometry from the particles to actually render it looking like water. in some cases you could render as points and such, but meshing is what makes it look like a fluid.

Rendering - sometimes we'll say 'render' or 'bake' the sim or mesh to disk, but really rendering is the term for the final lighting and shading calculations that produce the pretty pictures. (mantra / arnold / vray are renderers)

Generalizing a lot here: Sim usually takes a few minutes a frame for something with a lot of detail --- and it has to happen in sequence (each frame depends on the last), so that can take a few hours for sequence. 
The surfacing/meshing will usually be a bit faster, but it might take a few minutes a frame for something highly detailed, but that can be sent to a render farm in parallel (each frame can be calculated independently), so you will get it back a lot faster.  
Rendering usually takes the longest because most fluids have a lot of reflection and refraction and often volumes inside the fluid or sss, and motion blur ...then you add in particles and volumes for spray/mist... but basically there is going to be a lot of ray tracing going on. This can take hours per frame (dependent upon a lot of things), but this is usually the slowest part.

 

[art3mis]

Thanks for the clarification.

[Atom]

VFX Pipeline has a nice step-by-step walkthough for all the major tasks involved in creating a FLIP simulation.

If you have not watched the YouTube Flooded Hallway, give it a view. It was created for Houdini 14 but a lot of it is still relevant and you can pick the sections that interest you. The meshing step has been improved  for Houdini 15+ but the video demonstrates how you can achieve meshing with raw nodes.

[art3mis]

Thanks Atom. yes you read my mind, am going thru the Flooded hallway tutorial.