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Particle children loosing their randomness
Open, Confirmed, MediumPublic


System Information
Windows 10, 980Ti, 6700k

Blender Version
Broken: 2.79, release version

Short description of error
Particle children are suddenly loosing their randomness. Pictures will say much more:
Left and right is the same project, but in different stadiums.
Left is old save, right is current.
Please note, all particle settings are exacly the same.


As you can see left one is cool, random. Right one has a pattern, no randomness. They are just slightly moved.


Changing segments on the left project from 5 to 4: still looks OK.


Changed segments back to 5 from 4 (original value). Suddenly: no randomness!
I can change those segments again and again and the randomness will not come back.
However (!) undo option does fix that!
But after two changes it will get broken again.

Exact steps for others to reproduce the error
I can't attach my .blend file due to copyrights, unfortunately.
I also don't know how many steps it takes to screw it up. But something is bad. I can't leave render overnight due to this problem. :(

Side note: changing the seed does not help, they are moved in different positions but the pattern is still clearly visible.

Also, this gets somehow fixed.
Look, <- bugged (I haven't noticed it then), the next day: - fixed! Without touching.



Event Timeline

Philipp Oeser (lichtwerk) triaged this task as Needs Information from User priority.

I have a hard time reproducing this unfortunately [though it is apparent theres something wrong]

couple of questions:

  • these are 'simple' (not 'interpolated') children, right?
  • what randomizations are we talking about? just the orientations? or rough?, kink?
  • no hair dynamics or particle edit mode, right?

It would be very helpful if you could try recreating the problem in a simple .blend that you can share, otherwise we might be stuck here.
Marking as inclomplete for now...

Yes, simple children.
No hair dynamics, about the second one I'm not sure what are you talking about.

I think it should be better if I'll send you this .blend file by private message, so it won't get public?

Yes, can you mail me [info at graphics-engineer dot com] on how to get the file?

I've sent you the .blend.

Sorry to get back so late, I've never recieved the .blend. Could you upload somewhere with a password or mail me how we can solve otherwise? [I'm afraid my mailbox cannot accept large files...]

It was ~11MB, so not this big.
I've sent second mail, with link , without the rest of the mail.

@Acrivec Cevirca (Acrivec) : sorry for the noise, found the blend in the mail, will have a look shortly...

Philipp Oeser (lichtwerk) raised the priority of this task from Needs Information from User to Confirmed, Medium.Jan 17 2018, 7:54 PM

OK, looking further there seems to be a problem with the dupli_group Use Count option.
So (if I observe correctly) what looks like children 'loosing their randomness' is actually the dupli group always picking the same element in the group (instead of a different element for each child)

So (as a workaround) I can successfully change segments in case I also enable Pick Random.
I can also (as a workaround) leave Pick Random disabled but change the count of an element.
Both these workarouds will restore the randomness.

That said, this really seems to be a bug, will look at actual code shortly.

on first sight, this is NOT the same element displaying again and again, needs further investigation...

I'm also experiencing problems with children's size (quite extreme, for example hair length is 1cm and children are 50cm), and random rotation - but I guess that would be a separate bug report.

Unfortunately I'll only be able to look at this in April again, so let others take over....