Cloud rendering switches from perfect to noisy in the same render

[midnight.mangler]

Hello All

I have a curious issue. When I render clouds, the top of the render is perfect, but the bottom is very noisy. This can start from the upper middle part of the image or with the same clouds, but different perspective or including obj) it can be at the very bottom of the image. I can't figure out why it changes (or why its there at all). I wish I could attach an image as an example but for an unknown reason the upload reaches half way and stalls. I tried several times. So I'm hoping someone has hit this issue before and understands what I might be talking about.

Running build 4.8.23 on an Apple Silicon Mac. Did not yet have a chance to see if the same issue occurs on Windows but will try tonight.

Btw its odd you can't upload a tif without converting to another format as that is the default save to disk image format for Terragen.

Love my Terragen but baffled by this issue. Appreciate any help.

[Kevin Kipper]

Can you send us the project file so we can take a look at it?  

[midnight.mangler]

testfile.tgd

Thanks! :-)

[midnight.mangler]

Here is an actual render. Top of image is flawless, then starts to get very noisy.

[Kevin Kipper]

Thanks for sharing the project file.  The source of the noise is in the indirect lighting.  By adding a Render Layer I was able to save out the direct and indirect lighting elements and easily see the noise pattern.

You'll need to increase the Anti-aliasing value to clean up the noise.  This will increase the render time, but you can also reduce Easy Cloud 01's Ray-marching quality from 0.832301 to the default value of 0.5 to help compensate for that.

Since you're using the Path Tracer to render, you might try to increase the Max paths per sample value.  This can help reduce render times because the "smoother" sampled result means the anti-aliasing doesn't have to work as hard to clean up noise. (but test it first).

Lastly, if you don't need the Path Tracer for other reasons in your project, you could try rendering via the Standard renderer.  The standard render time was 03:52 verses the path tracer 23:50 with an anti-aliasing value of 8 for both renders.


Path tracer render


Standard render

[midnight.mangler]

Thank you so much for these tips! Greatly appreciated

[Matt]

There does seem to be a bug here. As Kevin said, switching to the standard renderer fixes the issue, but I am surprised by the excessive noise with these settings. I'll see if I can figure out the cause.

[Matt]

@midnight.mangler Thank you for reporting this and not just forcing it through with higher render settings. There was a bug which only occurs when using the Path Tracer, and was reducing the scattering quality in clouds on pixels (or subpixels) where there was a surface behind the clouds.

Technical details: Clouds are not directly rendered by the path tracer, so we would not expect the clouds to look any different when using the path tracer. But path-traced surfaces are illuminated by the clouds (i.e. the surfaces scatter paths that may go into the clouds), and the path tracer adaptively adjusts a variable to speed up the cloud samples during this process. The bug was accidentally leaving this variable in the wrong state and it affecting the appearance of clouds that are visible directly to the camera, where it should not.

I think there will be an update to 4.8 very soon to fix this.