Hey all,

I have some guide hairs in vellum and my orient (point-)attribute which is generated using the guide deform sop is "flipping" (sign changes on orient vector components). This causes the guides to freak out during simulation, showing randomly changing length, jittering and weird behaviour etc.

Knowing that vellum needs a stable orient attribute to produce stable sim results, I tried to delete that orient attribute coming from the guide deform sop and instead let it be calculated on the vellum constraint node using "compute missing orientation". The values are somewhat the same and somewhat also different, but I also see some "flipping" orientation values from frame to frame on some guides.

If I disable the orient attribute on the guide deform sop (create orient attribute off) and on the vellum constraints sop (compute missing orientation off), I assumed no orient is calculated for my guides, but to my surprise, I still have an orient attribute present after the vellum constraints sop. The even bigger surprise was, that this orient attribute produced a totally stable sim.

This left me rather confused, and I hope you guys can maybe shed some light on whats going on here.
Why is my orient attribute flipping, how can this be prevented, and what (where) is the best way to calculate a stable orient attribute?

I unfortunately can't provide a hip-file showing the issue cause my current project is under NDA, but I still hope you guys can make some sense of what I say. I am using Houdini 18.5.392.

Thanks in advance and all the best,
Scratch

Anyone? :/

Hi Scratch,
I was having the same issue as you, in a couple of shots, where I am simulating some guides.

For me deleting the orient attribute just before the Vellum configure node was fixing it in most shots.
However, there was a shot that deleting the orient attribute still didn't fix it.

So I tried your approach, by disabling the orient attribute on the guide deform sop (create orient attribute off) and on the vellum constraints sop (compute missing orientation off), and it worked perfectly!

Same as you, I have no clue why this happens...
Does anyone have an idea what is causing the issue?
Maybe a Houdini bug?

I am using Houdini 18.5.532
Thanks!