Weird grain in clouds?

[blattacker]

I'm working on a new scene, trying to get a better handle on populations and the like, but I noticed during a test render that in an area where there's clouds in front of a mountain formation, the clouds that cover land are significantly grainier than the clouds that are in front of open sky, even within the same cloud layer. I've tried messing with cloud density, enabled and disabling "receive shadows from surfaces", playing with the sun strength (which is why the scene is a little underexposed right now), and playing with the cloud layer coverage. Has anybody else run into this? I'm trying increasing the samples right now, but that significantly increases render times, so I figured I'd check here while I was waiting, see if anyone had any other ideas!

[pixelpusher636]

As well as the grass. Might start with the atmosphere samples. Crank them up a bit at a time and see if that grain disappears as you do.

Edit: Increasing atmosphere samples will increase render time.

[blattacker]

I don't have a picture for you all this morning cause, to be quite honest, it looked identical. I tried just increasing the cloud layer Ray-marching quality to max, as well as significantly increasing the anti-aliasing on the render (from 3 to 5), as well as increasing the max paths per sample for the path tracer (from 25 to 64), and then hitting render right before I went to bed. It took my computer around 6 hours to render, and again, it was almost identical. If anything, the grain was a bit easier to see. Right before heading to work, I started a new render where all the settings changed before were left at their higher settings, and then increased the Millions of voxels slider significantly as well as increased the Transition dist. setting. When I get home from work, I'll post whatever it spits out, even if it's mostly identical (I upped the resolution closer to the final image size on the off chance that somehow solved it), see if there's any more ideas.

Editing to note: I don't think it's "true" grain on the grass, I think those are just hotspots as I'm dialing in the reflections on the grass object.

[Kevin Kipper]

Hi blattacker,

Increasing the anti-aliasing value will probably make the biggest impact on the noise you've indicated in the clouds.  Increasing the anti-aliasing value from 3 to 5 is not that big a change, and for a final render you may have to go much higher, i.e. 8, 10, 12.  To determine how much anti-aliasing the image requires, render a cropped area of the project where the noise is most noticeable, using the different anti-aliasing values.

Be sure to review the wiki documentation concerning "ray-marching" and the other cloud parameters, as setting them to high may result in longer render times as well as unexpected results.  For example, enabling a cloud layer's "Receive shadows from surfaces" setting is great for when your landscape will cast shadows onto a cloud layer, but it will definitely increase render times.

https://planetside.co.uk/wiki/index.php?title=Cloud_Layer_v3
https://planetside.co.uk/wiki/index.php?title=Easy_Cloud

[blattacker]

Be sure to review the wiki documentation concerning "ray-marching" and the other cloud parameters, as setting them to high may result in longer render times as well as unexpected results.  For example, enabling a cloud layer's "Receive shadows from surfaces" setting is great for when your landscape will cast shadows onto a cloud layer, but it will definitely increase render times.

For sure, I have a basic awareness about what most of these settings do, just for this issue I'm mostly throwing everything at the wall and seeing what sticks ahaha!

Increasing the anti-aliasing value will probably make the biggest impact on the noise you've indicated in the clouds.  Increasing the anti-aliasing value from 3 to 5 is not that big a change, and for a final render you may have to go much higher, i.e. 8, 10, 12.  To determine how much anti-aliasing the image requires, render a cropped area of the project where the noise is most noticeable, using the different anti-aliasing values.

So I'm running a smaller test on my work computer in the background right now (significantly increased render times because weaker computer) where I just tried increasing the global illumination settings, is that likely going to be a useless test that I should abort and switch to testing AA options, do you think?

[WAS]

If you have Terragen Free version, you can kinda trick the AA by lowing the pixel noise threshold. Go into Sampling, and lowering your pixel noise threshold. Also make sure the cloud layer quality is high as well. I'm only at my computer for a minute making sure it works (power issue in our RV) but took a minute to demonstrate:

Default Terragen Settings:
Cloud Layer v3
AA 3 - MPD 0.5



Custom Terragen Settings:
Cloud Layer v3 -> Cloud Layer Quality: 1 (Extreme)
Edit Sampling -> Pixel Noise Threshold 0.01 (from 0.1) (Extreme in conjunction with Cloud Layer Quality vs Resolution)
AA 3 - MPD 0.5



Naturally, this will increase render time. But these features should really be talked about more, because you can balance Cloud Layer Quality, and AA without the heavy hit of just using higher AA settings. This is an extreme example, but can be used to balance a scene for speed in rendertime.

PS I don't know if these features have been locked in Terragen Free, I sure hope not considering AA limitations in general. But it used to work for me.

[blattacker]

I have the free version on my work computer right now, and lowering the pixel noise threshold definitely improved it! I'm excited to see what happens when I get home and don't have quite as many restrictions to operate within!

[Matt]

It is strange that the noise occurs exactly where the cloud overlaps the mountain, but that could happen if the sky is rendered with a different method from the terrain. What are your settings for "Defer atmo/cloud" and "Defer all shading"? I would recommend using "Defer all shading".

[blattacker]

The image was rendered using the Path Tracer, so the defer settings were greyed out, unless I'm looking at the wrong node?