I had noticed something was off a while ago, but then had another artist I’m working with describe the same thing. So I dug around and tried to figure out if we were doing something wrong, but couldn’t find anything, and couldn’t find any gotchas noted in the docs. The issue manifests even with a completely default Particle POP.
In the Particle POP, the birth rate seems to behave just fine as long as your setting works out to at least 1 particle per frame. But for anything less than that, there are more particles being born than expected, and the discrepancy increases dramatically as you go to lower values.
In this test file, all the Particle POPs have a 1s Life Expect, and no Life Variance. They’re being emitting from 0, 0, 0, with an initial velocity of 0, 1, 0, and none of them are going past 0, 1, 0 before dying. So the life expectancy is working fine; it seems to be just the birth rate that is at issue.
This is an issue for particle systems where you want to emit particles very slowly. It’s really hard to find a number that behaves as desired.
Build: 2025.32280 MacOS ARM64
OS: MacOS 14.8.4 running on a MacBook Pro w/ M3 Max
Particle POP birth rate.1.toe (7.7 KB)
I think I ran into something similar a while ago when POPs were still getting polished, maybe even before Particle POP was a thing. In order to carefully control accurate count and/or slow emission I had to design a system that would “inject” particles via a more controlled process by merging new points into the feedback loop within a single frame and then making sure it didn’t happen on the next frame. Definitely seems like it could be a useful RFE to include a python function like the particle SOP has (createParticles) for controlled emission. Not sure what the dev lift is there since of course the POP works very differently under the hood, seems like you would need to feed it the attributes instead of getting the list back and manipulating like you do in the SOP.
The manual “injection” method may still work as a way to still lean on the feedback parts of the particle POP while controlling the emission differently, i think you would just need to make sure to match the attributes exactly?
Hello @gborreson
Thank you for reporting the issue and for performing these tests. We will look into this.
@archo-p When Emission from is set to Birth Attribute on the Particle POP, we can control how many particles are created for each source point each frame. Does this give you enough control over the emission?
Ah yes, i think maybe i ended up using that birth attribute option in a refactor after Particle POP was fleshed out. So yea, if you wanted to emit for instance 1 particle every 3 seconds, you would pop up the birth attribute for a single emission point up to 1 for a single frame, and the pull it back down at the end of the frame I think. Will have to dig it back up to test again.
The birth‑rate inaccuracy will be fixed in the next build we release.
