POPs: pre solve vs post Solve

[Ronald Ruiz]

Hi! This is me trying to understand the popSolver in houdini. What is the difference between this two inputs and why they are both wired like that by the shelf (Check the attached image) The particles seems to do the same thing if I rewired as I showed in the second network and they don't do anything at all with the 3rd one. Also why is the wire dotted between the popsource and the popstream, They only work together is both inputs (pre and post) are wired like that. Thanks and pardon my English :$

[Krion]

Did you find an answer?

[char]

Pre-Solve

Microsolvers attached to this input will run before the main solve step. For example, a SOP Solver can be used to modfiy the object’s geometry, and POP force nodes can be used to apply forces to specific objects.
A good example for pre solve use is the Gas Resize Fluid Dynamics in Pyro Solver, where it resizes the container at each step.

Post-Solve

Microsolvers attached to this input will run after the main solve step.

 

[Krion]

What does this main solve step imply?

I see on this example that the only difference is that the smoke is a little thicker with the 'gas dissapate' in the pre-solve input:

 

Does this main solve step happen every substep? What are some basic, general rules for deciding when to put something in either the pre or post solve input?

 

Edited November 26, 2019 by Krion

[char]

You generally don't put microsolvers like Gas Dissipate into the post solve or pre solve in your case, as that's where the source comes in. Everything solves in a general direction which goes from LEFT to RIGHT. First a smoke object (container is created), that's your container which you will later populate with density, temperature, vel and other attributes, then you usually have a gas resize fluid dynamics which will check if density is there and how much it has to cut the bounding box to speed up the simulation (depends on how you set it up but that's the idea), third is usually velocity changes, so any microsolvers changing velocity overtime and after that you source in the volumes (that's my understanding of the Solve in DOPs).

I'm not sure why in your case that is happening but I would but that dissipate in the Velocity update (always use this input for modifying things using Gas nodes) input and use Pre Solve input for the Gas Resize Fluid Dynamics (that's always been the practice, I never questioned why that is but it has to do with the order of the solve. I think best example would be if your smoke is advected, you wouldn't resize your bounding box with gas resize after being advected but before, as it wouldn't update the size accordingly I believe).

If you want a really detailed explanation I'd recommend reading the documentation or wait a bit for a senior artist who knows the solver more in depth to step in and either correct me or explain further down how it works node by node.