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Why is Hair so flaky?

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Steve Bentley
Why is Hair so flaky?
on Aug 20, 2018 at 9:47:00 pm

(there's a dandruff joke in there somewhere).
To be fair we're trying to use noise and gradient maps to limit placement and size etc. It seems that every time we open the project the settings have changed. Even working on the project, sometimes those maps will work and sometimes not and sometimes kinda (density, length scale slots).
The manual implies that we can assign a texture in those slots, which might work better, but I can't see a way to reference a texture like we can with the shader effector, or to even just choose one.

I've also tried the density slot in the hair object - same results.

It feels like a cache needs to be cleared, and cleared often, but we haven't cached anything yet so I don't think that's available.

Is there a trick.



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Brian Jones
Re: Why is Hair so flaky?
on Aug 21, 2018 at 2:59:47 pm

anything to do with mapping? (Hair/Hair Material/Mapping Hair materials)


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Steve Bentley
Re: Why is Hair so flaky?
on Aug 23, 2018 at 7:26:51 pm

Thanks Brian,
It turned out to be two things. First the scale we are working in huge. I still think the density maps weren't working properly, but they are now. We're using a few layers of noise to filter which hairs show up, and I think the scale of the noise wasn't high enough - there's no indication what space that noise is being generated in - I think its the object's space, which seems correct now, but I don't understand why it wasn't working before.

The other issue was a carbon based one - the dynamics were still turned on for a few of the generators (we thought we had turned them all off) so depending on what keyframe you were on, you may or may not get the results of a dynamics sim all the way from frame 1. I think if you interrupt the dynamics run-in, C4D gets lost and just does the best it can, which can produce different results from test render to test render.

In the end we turned off the dynamics and went with an evolving noise pattern projected on the wheat "hairs" to simulate that shimmer that wind gives to a field.



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Brian Jones
Re: Why is Hair so flaky?
on Aug 23, 2018 at 7:57:10 pm

yeah, that's definite for dynamics - each frame depends on the last so it has to calculate all up to where you are, unless you cache it. Baking to alembic in 20 looks like it will be a good thing.


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Steve Bentley
Re: Why is Hair so flaky?
on Aug 24, 2018 at 6:48:12 pm

Well so much for that. Everything looked good, test renders at low AA rates were fine. But once the big render started at 2 and 8 sampling and Gauss. It all fell apart. Initially all the "hair" looked good but then about 100 frames in, the hair started changing (no dynamics and even the world gravity is turned off) - hair clumps would pop off here and there, change places, seeming geometry changes but it might just be generating the hair cluster again. Even editable plane geometry that is visible and also being used as a generating surface for a few clumps of hair would turn off during the render - the plane itself would become invisible.
Before, I was putting hair on cloned objects but it seems the clones are generated every frame so to the hair, so every frame was different. So I ended up "flattening" every object that is a hair generator and figured it was solved.
The Issue seems to build up as the render goes along. Am I asking to much of memory? (128GB)
If I render later frames as singles it all looks good. If I render the problem frames as singles it all looks good. Does an error build up or is there a memory leak with hair?
I'm going to try rendering each set of hair separately and see if I can get away with that and comp it all together.



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Brian Jones
Re: Why is Hair so flaky?
on Aug 25, 2018 at 1:11:48 am

AA for the rest of the scene? With the new Bi-render AA for hair is automatic as I understand it. Haven't heard of anything like the hair/hair substrates suddenly moving... I've seen hair disappear off clones that are moving by dynamics but that was fixed by caching the dynamics


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Steve Bentley
Re: Why is Hair so flaky?
on Aug 25, 2018 at 1:42:56 am

The high AA is for the rest of the objects - there's some deep noise textures that get a little to chattery on lower settings.
Today I've been rendering the hair alone in groups with shadow catchers and alphas - so far no jumping. The jumping was only on the objects that were clones but have since been flattened to geometry, and they rendered for many frames before suddenly looking like something from Jacobs Ladder.



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Steve Bentley
Re: Why is Hair so flaky?
on Aug 25, 2018 at 2:11:36 am

And the shadows of course - the new engine might look after the hair's AA but the shadows the strands casts still need a high AA rate to keep them from being too noisy.



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