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tagosaku

advect by volumes node limitation

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Hi, I am trying to understand advectByVolumes node. Basically I create custom vel field and animate density by advection.

When increasing timestep, density moves more, but there looks a certain distance limitation, and density does not move further anymore. I wonder what's a logic and a solution for that, instead of using sop-solver, in order to continue to proceed density animation.

image.png.1fd16bc61773790644939e058c3c63e1.png

 

advectTest.hip

Edited by tagosaku

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Advect By Volumes VOP is not made for long advections, it's usually used for a single frame advection within a solver, it has a hardcoded maximum limit of 32 steps depending on cfl condition

if you really don't want to use Solver or DOPs have a look at VDB Advect SOP, that can advect over long distances as you can specify number of substeps and doesn't have such low limit for your purposes 

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uh, that's good to know there are step limitation!

Then, I tried VDB advect. It's working well. but calculation gets heavier and heavier when setting time step more and/or faster vel.

I wanted to try this method because I heard one of VFX companies have a VEX/VOP based fog animation, instead of using sop-solver or dopnetwork ;) The system is not for replacement with regular fog sim, but it works for a simple Background-quality fog. And I assume that should create unique fog shape and unique motion, unlike filling out an entire box boundary volume, unlike simple linear straight offset animation.

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I doubt any VFX house would advect from 0 every frame, since as you said it get's slower and slower the more steps you take and it's counter-intuitive to keep redoing the work per frame instead of doing one step at a time in a Solver SOP

so I'd assume what you heard is that companies advect the fog in the solver using simple vector field, so without an actual smoke sim

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