[MTOA] Xgen flickering lights

[JWRodegher]

Hello everyone, I'm having an issue rendering xgen. I have some flickering hairs in a character's turn around.

https://vimeo.com/246673225

First thing I did was check on the alpha of the turn around and to my surprise, the hairs primitives aren't actually moving at all. It seems to be a sort of "light's threshold" (I checked and the setting is at its original value 0.001), as if the hair receiving a certain amount of light would make it fully lit, and under a certain amount is not affected by the light at all. So in every different angle I'm getting different values and makes the hair appear as flickering.

I'm using area lights (maya area lights), raytrace shadows, 3/6 samples each light, 512 in the resolution and a grey/dark skylight . In this version all hairs are set to min pixel width = 1 (I made another quick render of 5/6 frames with varying degrees of mix pixel width, none of them higher than .5, same issue). Version is Arnold 2.0.1 and Maya 2018, on Win7. All the xgen hairs are guides. I've attached a small quicktime to observer the issue.

I'm pretty new to xgen, so any tips regarding this kind of issue are absolutelly welcome. And also pay no mind to the actual grooming, it's a WIP.

Thanks in advance! Cheers.

[lee_griggs]

Wow, it really looks like the hairs are moving in that animation! It's hard to see whats going on with that Vimeo video. Are you able to do a slower camera turntable, maybe closer to the hair? What happens when you increase Specular Roughness? Does adjusting Melanin affect it much? Have you tried any of the Hair presets to see if they render the same way? Can you share a simplified scene?

[JWRodegher]

Hi Lee, thanks for answering.

Specular roughness doesn't change much. Render takes a while so making a slower or closer turntable is not that much of an option right now (might be able to do it in a couple days). I haven't tried the presets or changing the melanin a lot.

I will attach a simplified scene. It really looks like it's something to do with the lights rather than the shader, but I will investigate the shader further none the less.

https://wetransfer.com/downloads/d97976edb420e6d52ff9c954170b434e20171211105732/55c67fff42a6cecb0857...

Thanks!

[lee_griggs]

Hi Jonathan, those lights look very big and are very far away from the head. I would shrink them down a lot and move them closer. I am confident that will help.

There is a basic studio lighting scene here that might give you a better idea of size and distance. Nice head btw!

[JWRodegher]

Hello Lee,

Thanks again for taking the time to look into this.

On one hand, I'll download a test scene just to see if the issue dissapears in a different setting than my own.

On the other hand I've used this kind of set up extensively with hair before (even though it was yeti, and an older version of Arnold) and never find issues like this. I'm not sure how size/distance should be a problem as we should not be limited by hair to achieve certain effects from lighting. I'm also thinking ahead about actual lighting shots and not only lookdev rigs, etc.

Thanks a lot again!

[JWRodegher]

I've render a few frames with the following changes:

Two lights, a big one like before but with the resolution at 4096 (it used to be 512).

And a smaller and closer light, pretty much same angle, both look the same and the flickering is exactly the same.

However, the next test I did, was to render in batch render, and using the render sequence. Having made the export patch for batch render, there's a small difference in the grooming and the flickering is not there in the render sequence!

The flickering is much smaller now, I'm guessing because in the grooming there are changes that are beneficial to dodge the issue (like some of the chaotic hair now being a lot more thinner, etc).

Check on the vimeo video, it's better to download the quicktime cause it's only a handfull of frames.

The one above is from batch render, and the one below from render sequences:

https://vimeo.com/247008635

It's getting annoying to have so many issues with the batch render and xgen.

[JWRodegher]

Well, reporting back again. Seems like I found a solution.

I was using Maya 2018, tried 2017, problem solved.

Render below has been made with the original lightrig (same distance, intensity and area light resolution), only difference, Maya 2017. Take a look:

https://vimeo.com/247105558

Cheers.

[JWRodegher]

In case anyone is interested in some insight about this issue:

Somebody else has mentioned that one of the most important differences of Xgen in Maya 2017 vs 2018, is that 2018 uses hyperthreading for the generation of the primitives, and that might cause the flickering if the order is different in each frame.

Also, per color strands might not help in this hyperthreading context.

[JWRodegher]

Got a solution, other than jumping back to 2017.

In case anyone is interested, I managed to remove the flickering rendering with Maya 2018.I just used the curve_id, color jitter worflow and input that result into the melanin value. Getting my randomness from there and setting the melanin randomness = 0. I input this data to several properties in the shader, so the look is rather awfull in my sample, but I just wanted to test it was working. On the other hand, I think you might need a shader per description, given that each descriptio exports its own curve id? Does that make sense? Anyway, hope this is helpfull to someone. Cheers.

https://vimeo.com/247681040

[aronbaaijens]

Jonathan, I got a similar issue, but can't seem to get the 'color jitter workflow´ to work.

The issue seems to be coming from the melanin randomness of the aiHair shader.
When the melanin randomness of the aiHair shader is turned on the flickering of the hair ruins the render.

It looks like the randomness of the aiHair shader is calculated randomly every frame (not using the curve id).

I have the feeling I have to cache the xgen hairs so the curve_id gets stored onto every strand of hair but can't find where to do that.

Can you explain in more detail how you got the color jitter workflow to work?

Here's what I have so far:

I have attached a aiUserDataFloat with the curve_id Attribute to the aiColorJitter Data input, with Gain min on 0 and Gain max on 1, and attached this to the melanin value of the aiHair shader.

But this doesn't seem te be working.

Cheers,

Aron

[JWRodegher]

Hi Aron,

I actually kept rendering with 2017, cause it will be solved in 2018 eventually so I'm trying to keep shaders as clean/tidy as possible.

My setup sounds the same as yours, care to share some screenshots from your network and the result if possible? I'd have to check back on what I did but if anything changed and it doesn't work anymore, I will possibly be in the dark here (would need to render another sequence to find out).

Cheers.

[JWRodegher]

Here's a file with the setup, for those who don't know how it works.

I'm rendering the sequence now as a proof that it works. Might be useful for those with the same issue.

Rendering in Maya 2018, and Arnold 2.1.0.3 (Tried Arnold 3.0.0 in 2018, issue is still there).

Cheers.

hairjittertest01ma.zip

[JWRodegher]

Rendered with the scene posted above. On the left, with the curve_id/color jitter setup. On the right, melanin randomness from aiStandardHair shader.

They, of course, don't look the same. You gotta work it out, but I'm happy to get rid of the flickering.

Also yeah, didn't bother to make it look pretty, sorry about that.

hairjittertest.tar.gz

[denisVFX]

I had the same problem. Thank you for solution!

But what if working with xgen interactive groom splines? How to add custom float attribute curve_id there? Or maybe it has some attributes... How to watch them?

[JWRodegher]

Well, it looks like the shading data still comes from the xgen tab? It looks like IGS is just the interactive editor? I'm not sure about it though.

[JWRodegher]

There's also this:

https://forums.autodesk.com/t5/maya-forum/xgen-interactive-grooming-splines-id/td-p/8215391

You could use the IGS to guide classic Xgen as well.

[JWRodegher]

In case you guys still having the flickering issue, or want to get rid of the $id setup thing (specially if you just found that solution after lookdeving your character, and then find out the result is somewhat different). Well here you go:

Uncheck the "multithreading" option in the Arnold Advanced Settings, you'll find it in the Preview/Output section of each Xgen description. No need to touch your shaders at all, no need to do anything else. Goodbye melanin randomness flickering.

See image below: