Particle motion blur in Houdini

I ran into a problem recently where I was trying to make some nice-looking embers in houdini, complete with nice motion-blurred trails. Typically with a particle system you use the velocity attribute to handle motion blur, but geometry velocity blur is always linear, so your motion trails will always be perfectly straight even if you have nice squiggly motions with your embers.

Deformation motion blur looks great, but in most simulations particles are being born and dying all the time, and deformation motion blur doesn’t work with a changing point count.

The solution is to force a constant point count. This can be problematic when your particles need to have a lifespan, so there are a few little tricks you’re going to have to pull in order to make this work…