Hi there;

I have a pyro sim which I like, but which I'd like to slow down to like 1/10 of it's native speed. I thought that I could be slick by just lowering the timescale on the Pyro solver (which is apparently exactly what this parameter is meant to do), but doing so drastically changes the behavior of my sim.

Further, I need some “runup” to my sim (it's a campfire, not an explosion, so I want it to be in a stable state when my shot opens). Rather than needing 10 times the runup, I thought I might be able to keyframe my Pyro timescale so that it's 1 before frame 1, and then drops to 0.1 at frame 1. But, again, doing this doesn't simply just slow down the sim as I'd expect; I see a jarring change in the character of the sim.

I'm curious if I'm just not using this the way it's meant to be used, or if I'm missing something obvious that I should also be doing to get the behavior I'm expected. I tried, for example, multiplying my incoming source density by timescale, thinking that maybe there was some sort of scale that wasn't being accounted for there, but that only seems to cause a “different kind” of look difference.

Any clues or thoughts?

Pyro ain't my area of expertise, but if you already have a sim you like and just want to retime it, I'd cache it out and then use the Retime SOP to change its speed. It works really well for slowing stuff down.

Mucking with the timescale sounds like trouble

I'm exploring the Retime node, and while it seems promising, I'm experiencing quite a few artifacts with it. Searching around turns up little information, which leads me to either suspect this approach typically “just works” for people and I'm doing something wrong, or these waters aren't well-charted.

The thing I'm trying to do is slow my sim down to 0.1x it's normal speed. I can use the Retime node set to “by speed” mode, with Speed set to 0.1, and using Advected Blend Mode for volumes, but I see a flicker in the result presumably each time I hit an integer frame of the original sim. Changing my interpolation settings, adjusting my input ranges to non-integer values, and modifying other parameters don't seem to remedy this. Yet I see lots of videos on Vimeo demonstrating people slowing fluid sims down to super-duper slo-motion. What am I doing wrong?

The one parameter that remains obtuse to me is “Advection StepSize”, which I can find no further info on beyond what the docs state (and the docs aren't terribly descriptive explaining what this parameter does).

I haven't used the new retime mode for volumes yet, but in the past when building my own advection based volume retime tool I had to add a blur that modulates in time with the sampling of integer frames to eliminate flicker. Finding out the amount of blur needed took some trial and error.