Issue when retiming a Stoke simulation


#1

Hello guys,
I have simulation that was done in FumeFX and then used inside Stoke as distribution and velocity source to create PRT particles. I then used the ‘Retiming’ section to slow down the sim to about 1/6th of the original speed. Most of the end result looks good except for a few places where sections of the sim have some sort of stuttering effect, like when you time-stretch a video clip without any frame blending… I have simulated this many times with different settings but still have the problem (the ‘Use Playback Interpolation’ is on by the way), did one sim using 100 sub-steps in Stoke which help in some places but still not good enough.
Have also tried doing the retiming in FumeFX (in order to avoid retiming in Stoke) using the ‘Post-Processing’ option but for some reason Stoke is not reading correctly the FumeFX files (the ones with the “pp” on their file name), It will only work when FumeFX is set to the ‘Default Cache’… even tried renaming all the “pp” files to have the same name as the original *.fxd files and then setting the cache to default but get the same problem; Stoke will act as if there was no velocity source.
I’m using the latest version of both plugins inside Max 2018.
Thanks in advanced.

Here’s a Youtube link to a video showing the problem.


#2

We handled this via a support ticket, but just to keep people updated for future reference:

The issue is unrelated to Retiming in Stoke, and in fact it is not directly Stoke’s fault to start with.
What happens is that the FumeFX simulation has the Adaptive grid option turned on, and on a certain frame (around 104 in the original unretimed simulation), the Stoke particles hit the bounds of the current adaptive grid one frame before it managed to grow in front of them. This causes some near-zero velocities to be assigned to the particles, and produces a more pronounced hickup in the retimed version, but it is visible even without retiming:

The obvious workaround would be to disable the Adaptive grid option in FumeFX before simulating to avoid these “edge” cases.

Remember that the Stoke particles do not have inertia, they do not retain the last velocity assigned to them, they follow the grid velocities verbatim. So if the sim says the Velocity is 0, Stoke follows…