Clouds fly-through : strange overlay in the first seconds and "blocky" clouds in the closer areas

[Ambrozie]

Hi all,

I rendered this flythrough clouds with different render settings and cloud densities to find the look I wanted. I got rid of the flickering with brute force in reasonable render times, but I get this strange overlay in the first 3 seconds. Any idea what is causing that?

Thanks, Ambrozie

[bdancer]

Ambrozie, I guess there should be a link to video or smth...

[Ambrozie]

https://rendering.no/houdini-tests/v...ds_test002.mp4

something happened when I pasted the link in the link tab, so here it is as text .

[bdancer]

Ambrozie, any chance to get the files? Could you email me the scene? or may be Gosho.Genchev have seem smth like that...

[Gosho.Genchev]

Hey there,

I could be wrong but those look like volume grid light cache artifacts to me. Can you try disabling Smoke->Light Cache Speedup to see if that helps? This will slow things down a bit but resolve the problem. You can then re-enable it and reduce it until they artifacts disappear.

We recently prepared this page to help people get familiar with all the volumetric rendering issues you may stumble upon. Here's the link: https://docs.chaosgroup.com/display/...ering+In-Depth

Hope that helps!

[Ambrozie]

Thanks for your reply,

I did those steps before rendering this test. At the moment the light cache speedup is at 0,1. Unfortunately, if I disable the light cache speedup, the render time is way off the reasonable limits .
This was made before going on holiday, but if I remember correctly the same vdb rendered well with mantra (but higher render times of course ).

I'll check the in-depth link you sent and keep get back with updates.

Best regards, Ambrozie

[Gosho.Genchev]

Feel free to send the scene to support or the mail in my signature so we can check what's causing this if you hit a roadblock

Cheers!

[Ambrozie]

Hi Gosho.Genchev bdancer

sorry for my late reply, I was traveling. I'm back on this issue, and after testing with a simpler/smaller cloud I think you are right. After increasing the volume density divs the cloud looks better. However that creates a new problem since I have a large cloud, the closer areas that were low rez before, look great but the further parts now look too dense, not cloudlike at all . But I'll post this on the SideFX forum, maybe you have some suggestions? Once I find a solution to my cloud I'll render again the animation see if the artefacts are still there.

here's a snip (in blue, cloud sampling divs= 750, one of the old issues that might have caused the artefacts, green: loud sampling divs 3000 divs new cloud looks good on closer areas, in red the further away parts look way too dense)

Also, do you recommend using the "cloud light" node? Is it compatible with Vray light?

Thanks, Ambrozie

[Gosho.Genchev]

Hey Ambrozie ,

first of all, a colleague mentioned having seen a similar problem to the one in your original post with a smoke simulation + adaptive dome light. If you have a dome light with Adaptive enabled, please try disabling it to see if that helps. We already have it logged as a bug so if that's the problem, I'll ping you once fixed.

As to the density changing as the distance from the camera increases - I can't think of a single reason why V-Ray would do that. Can you screenshot the Opacity graph on the volume grid shader? Personally, those areas in red look like very high opacity cut off by the curve.

Finally, the cloud light - I'm on this at the moment. I think it won't be as straightforward as plug and play, though. The Cloud Light node is generating what is essentially 3 emission color fields. There is no way to use those directly at the moment, without any pre-processing. If we manage to figure something out for the short term, I'll ping you. Alternatively, what you could do is use the points you're feeding to the cloud light SOP to instance omni lights.It won't give you the exact same result but it's an option nonetheless.

Cheers!

[Ambrozie]

Thanks for the answer Gosho.Genchev ,

I was using the adaptive dome, I'll turn it off now.

I tried with both the default curve on the opacity and a custom curve with some bezier points (below the snip. crazy curve...not sure how I got to that shape, it was working for closeups I guess )

Thanks for checking out the cloud light node, no stress on that.

Cheers, Ambrozie

[Gosho.Genchev]

Awesome, please tell us how it goes. I could not reproduce either of those here. A hip file would be awesome, too, if you can provide it.

As to the Cloud light - turns out it's possible to hack things so somewhat it's usable. I'm uploading a hip file here. Things of note:
1. The VDB Resample is there to fix the pivot point / transform of the individual fields (density, Ce.*). V-Ray always expects those to be in the same place so things break if you don't add it.
2. The VDB Convert is there because the Ce fields are native volumes, not VDBs.
3. The Wrangle is used to amplify the Ce field. When values go above 1, it appears as self-illumination (altho it's not).
4. The Ce fields are provided as Smoke Color in the shader

Link: https://drive.google.com/open?id=1qr...MKgHXuO_IOy8kR

[Ambrozie]

Thanks,

I'll send you the file in a few minutes. Here's what I get when I open your file.

[Gosho.Genchev]

It's basically saying there are parameters on the /out/vray node which it doesn't know what to do with. I'm not sure when we added those from the screenshot specifically but it was probably after the build you're using was released. You can safely ignore the message.

Cheers!

[Ambrozie]

Thanks again for the solution. Super cool that it works. Adds a lot to the overall look of the cloud.