Voxels in animated clouds

[Max Dralle]

Greetings everybody.

I'm a long time lurker and bought TG4 Professional almost three years ago.

I use it mainly for sky replacements in feature films and TV series and it works out quite well, but recently I stumbled over an issue where there was some heavy shadow popping to be seen in the clouds, when they are backlit from the sun behind them.

Searching for such an issue in this forum, pointed me to using voxels for shadows might be causing this, so I made a couple of test renderings with various settings in my Easy Cloud.

Raising the amount of voxels (up to 1024 millions) only made the popping artefacts smaller and disabling "accelerate empty space" only reduced the amount of those artefacts, but they were still clearly visible and disturbing.

Changing the transition distance and/or using a 2D shadow map didn't help either.


So I had to disable "use voxels for shadows" and "accelerate empty space" alltogether to get rid of those popping shadows completely, but the visual result looks not as good as with voxels.

...

For sky replacements, I do not have a terrain in my scenes and my render/quality settings are as follows:

#####################################

GI Settings

Prepass
GI cache detail - 4
Gi sample quality - 4
GI blur radius - 8
GI prepass adding - 0.25

Image Pass
GI surface details - disabled
Exxagerate surface details - disabled 
(as there are no surfaces in my scene)

GI in clouds
Cloud GI quality - Sequence Quality x8
Cloud GI max ray depth - 2 (only one Easy Cloud Altocumulus Castellanus layer in the far distance, sunlight from behind)
Voxel scattering quality - 200

#####################################

Renderers

Quality
Micropoly detail - 0.75
Anti-aliasing - 8

Standard renderer
+Defer atmo/cloud
+Defer all shading

Filter
+Anti-aliasing bloom
Post effects
Bloom - 0.1

Tonemap
Soft clip effect - 1
+Compensate soft clip
Contrast - 0.25
Gamma correction - 2.4
+Pure gamma

Advanced
+Do ray traced shadows
Ray detail region  - Detail in camera
Ray detail region padding - 1
Max ray depth in PT - 4

Sequence/Output
RGB TIFF 16 bits
Layers (Alpha & Depth) in EXR 32 bits float
Ray detail region  - Detail in camera

#####################################

Writing all these settings down, I noticed that I haven't compared the effect of "Voxel scattering quality" just yet, but I'll start a few renders with different values and will report back, how that affects the resulting images.

...

Anyways, summing all my test so far up, I started wondering:

- am I missing something?

- is this a bug?
- are voxels maybe just useful for still images? (voxel shadows do look better and add more depth to the visual impression)

...

I'm looking forward to your feedback and any possible solutions.

Thanks for your time!
Max

[Kevin Kipper]

Hi Max,

We've found that increasing the Voxel scattering quality to 400 has been the sweet spot for the animations we've been working, for example the aircraft shot in the Terragen for VFX series, and some other shots we're working on at the moment.

Is the cloud layer drifting and evolving over time?

Another thing that might be helpful, is to identify if the popping is occurring from the direct or indirect lighting.  Using Render layers, you can enable the Cloud Direct and/or Cloud Indirect elements to be saved.  I've often found these helpful in identifying the source of the problem, i.e the cloud layer itself or the GI.  For example, if the indirect lighting element is popping but not the direct lighting element, then the issue is in the GI.

[Max Dralle]

Hi Kevin.

Thanks a lot for your feedback!

Over night, I rendered three versions of voxel scattering quality (100, 400 & 800), but the results are visually indistinguishable from each other and the shadows in the clouds are popping like crazy.

My clouds are drifting (speed 41.17m/s @ 25fps), but evolution is disabled.

I'll try your suggestion to render Cloud Direct and Cloud Indirect layers.

Cheers!
Max

[Max Dralle]

Small Update:

The popping clouds are visible in both Direct and Indirect Cloud layers...

Any other suggestions?

Thanks,
Max

[Kevin Kipper]

A similar issue was recently discussed on our Discord channel.

Here's a link, in case it helps shed some light on this issue:
https://discord.com/channels/979379973854994432/1040804434176901210/threads/1290287326773379174

[Matt]

Hi Max. Can you confirm that the popping goes away when you disabled all the voxel options? Since you said your prefer the visual look when using voxels, can you show us an example (maybe just a crop, if you prefer) of the difference between the voxel and non-voxel versions? Theoretically the non-voxel version should be more accurate, just take longer to render, so I'm curious if there's something we can do to improve the look of the non-voxel version.

[Max Dralle]

Hi guys.

Sorry for my late reply, I expected a mail notification, but didn't work out although I had enabled "receive emails and alerts".

Anyways... I guess, I'd need a discord account to have a look into that channel, but I don't have one and don't see a reason for yet another platform to follow. Maybe I'll bite sometime in the future.


About the visuals, it's obviuosly a very subjective impression and hard to describe, but to I have an impression that voxels add more depth, especially when the clouds a backlit from the sun.

As if the voxels add a little more contrast in their center and they appear to fade out to their borders so that this higher contrast is fading out as well.

I guess, I can achieve that look by increasing the density. It was just an instant impression after I had a first look at the result when I had voxels disabled.

And yes, the popping issue is gone when I disable "accelerate empty space" and "use voxels for shadows".

I'll try to post some examples later.

Thanks!
Max

[Max Dralle]

These images are the upper right quarter of UHD renderings and the filenames reflect the settings.
I rendered two images of each setting, so that you can see the issue.

Summary:
As long as voxels and/or accelerate empty space are enabled, there are popping artefacts in the clouds, so I have to disable both options for a clean rendering.

...

As for the visual impression, I really like the way the clouds are more faded to the outside when "accelerate empty space" is enabled and how the shadows inside the clouds are softly exaggerated when "use voxels for shadows" is enabled.

[Matt]

Thanks for these renders and choices of settings and frames. I can clearly see what the issues are. I am thinking about some solutions and will get back to you.

What value do you have set for "Ray-marching quality" on the Quality tab of the cloud layer? At the moment "Ray-marching quality" affects both the final image sampling and the voxel buffer generation, but there is inherent randomness which is one of the possible causes of this flicker. One of the solutions I'm considering is to add a separate quality control for the voxel buffer generation so you can reduce the voxel variance between frames independently of the image sampling quality.

Would you be willing to send us the .tgd? You can send it to us via email at support@planetside.co.uk and we'll keep it confidential.