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XGen grass rendered with Arnold looks grainy/noisy

Message 1 of 6
2212 Views, 5 Replies

XGen grass rendered with Arnold looks grainy/noisy

Hi everyone! I'm having some trouble rendering a shot where I'm using XGen grass. I followed this tutorial to create the XGen grass: , and when I rendered the sequence I noticed the grass looks grainy/noisy. I'm attaching two video files here, they're both the first 10 frames of the shot but one has the XGen grass and the other doesn't (I hid the character for demonstration purposes). For lighting I used an arnold physical sky because it's an exterior shot and I didn't really need any other lighting. As you can see in these videos when I render without the grass everything looks fine and not noisy at all, but when I render with the grass it's noisy. For lighting I'm using Is there a way to fix this? Or is there a better way to do grass? Thanks in advance! 

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Message 2 of 6
in reply to: fATIMAANIMATION

This is a pretty common issue in general and on this forum. This thread has a lot of useful info:

Solved: Re: Noisy hair in animation with Arnold - Autodesk Community - Maya

To solve your issue you will have to drive up render times. I would also recommend also turning on "Lock Sampling pattern". It's under the Arnold Global - Arnold Renderer - Advanced. It'll help the look, but AA is probably your simplest bet.


Good luck with this!

Message 3 of 6
in reply to: daunish

Hi @daunish , thanks so much for your help! I didn't realize this was a common problem, I had tried searching for this problem but I used the wrong words when searching and couldn't find anything. 

After going through the thread you linked and another thread that was linked in the thread you linked I realized that when I created the XGen grass it assigned a HairPhysicalShader to the grass and someone said that that's a mental ray shader, so I switched it to an ai Standard Hair but it was still grainy. 

While reading the threads I found this article and I wanted to try playing with the Min Pixel Width and the Transparency Depth but I couldn't find those attributes, so instead I played around with the specular and camera AA samples as you suggested. I tried increasing the specular samples to 10 but that didn't work, then I tried increasing the camera AA simples and that didn't work either (although I think it did help a little). Then I remembered I hadn't turned "Lock Sampling pattern" on so I turned it on and now it looks much better. I think it's still noisy though, and the "Lock Sampling pattern" is just keeping the grains in the same place on each frame right? I don't really mind it because it's grass and the shot is wide so you can't really see that it's grainy because it looks like it's the grass and the grains aren't moving each frame so it looks fine (hopefully this makes sense). Just out of curiosity, do you know how I can change the Min Pixel Width and the Transparency Depth in the hair shader? Thanks for your help!

Message 4 of 6
in reply to: fATIMAANIMATION

Yes, that does make sense.

And you are right about that lock sample pattern does. It's not a fix, but a visual camouflage for the issue, as it reduces how noticeable the flickering is in an animation.

However, adjusting the global AA values should do more than help a little! This value is a multiplier for all other samples, such as diffuse, spec, and light samples. That said this will dramatically increase render times. Instead of using this try upping Diffuse samples significantly and see if that helps. Specular samples will contribute very little to this issue.

Also, just as a suggestion. aiStandardHair is great if you're doing hair but I found it's a pain when it comes to getting grass looking good. I usually use aiStandardSurface.


As for the location of Min Pixel Width. you'll find that on the xgen description:

Xgen description - Preview/Output - Arnold Curve settings

If it doesn't exist you need to set: Output settings - Renderer to Arnold. The option will then exist.

In general, this setting will be found on curve shape nodes and descriptions as you will want to adjust them separately from what the shader is doing.

What it essentially does is clamp the width of a rendering curve to a specific pixel value. it will for sure help by reducing the visual complexity of the grass.


Grass and curve rendering is pretty straightforward and kinda heavy to render. There arent a ton of options besides upping your sample count. But if after all this you still aren't getting what you want. Upload some more pics of how your grass is looking and the sample count of some of the lights and globals.


I hope you have more luck getting your grass looking good!

Message 5 of 6
in reply to: daunish

Ooohhh ok maybe I'll just use an ai Standard surface, that will simplify everything. And I'll also increase the diffuse samples to see if that helps too. And thanks for telling me about theMin Pixel Width! I found it this morning but haven't gotten a chance to play around with it, I might use it if everything else fails but I have a feeling that switching the shader will help a lot. Thanks again for your help! I'll keep you updated.

Message 6 of 6
in reply to: daunish

Hi! Just wanted to give an update about this issue. I switched the material to an aiStandard material and increased the diffuse samples as you suggested and that definitely helped but didn't get rid of the noise completely. The noise in the grass that was closest to camera was reduced significantly by increasing the diffuse samples but the grass in the foreground was still grainy because it was super tiny so I increased the Min Pixel Width and I also turned lock sample pattern on (I had turned it off to assess how grainy the sequence was). So the section of the grass that's farthest from the camera is still a little noisy but it's so tiny you can't even see the grains and with lock sample pattern the sequence looks great. Thanks again!! 

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