Render elements change the appearance of VRayEnvironmentFog

[OPKJoe]

Hey everybody,

I am currently creating an underwater scene which works great.

I have two VRayPlanes inside the scene, one for the ocean with a simple water material and a black one for the ground of the ocean.
I have a box gizmo for my VRayEnvironmentFog and use a simple sun/sky system for lighting. I have one additional VRayLight to fake some caustics on the sphere's surface.

Everything looks fine so far.

Only when I add any render element, the visual appearance of my scene drastically changes.
For example the image below where you see the black surrounding is the exact same scene as the other image only with a simple Multimatte render element activated.
Not only does the look change but also the render time kind of explodes.

Is there something I am doing wrong or what causes this strange behaviour?


Thanks in advance,
Joe

[hermit.crab]

Mind if you attach the scene so we can troubleshoot?

[OPKJoe]

Here you are
Just add a multimatte or zdepth for example and you should see that the final rendering differs a lot from before adding those elements.

https://drive.google.com/file/d/1Xlo...ew?usp=sharing

Thanks in advance!

edit:
I am using 3ds Max 2023
yesterday I updated my V-Ray version from 61009 to 62006 without solving the issue

[Vizioen]

If you turn off Deep Output in the environment fog, your issue is fixed. Turning off deep output in the render elements the issue is still there. So might be related to that.

Edit: from the documentation:

Volumetric Support

---

In addition to deep data related to shading surfaces, V-Ray can also generate deep data from atmospheric plugins. The VRayEnvironmentFog atmospheric effect has an option to generate deep fragments. Phoenix FD can currently generate deep data if used in geometry mode. 3rd party plugins would need to use the V-Ray SDK to write deep fragments through the writeDeepFragment() function. Note that deep volumetric data can generate quite big deep files. You can use the VRayOptionRE render element to control the precision of the deep data in the resulting file.

Note: Make sure that the Optimized atmospherics evaluation in the V-Ray System rollout is enabled; otherwise you will not get correct results for surfaces behind atmospherics.

The thing is that option has been removed as I can't find it anymore. Not even sure if it would fix things either.

[hermit.crab]

FYI, we are investigating.

The thing is that option has been removed as I can't find it anymore. Not even sure if it would fix things either.

The option is hidden from the UI and is active by default.

[hermit.crab]

Here's a summary of what we discussed with the devs:
To generate the fog's deep output, the "ray marching" functionality is activated. Even if the "deep output" in the fog is active, the image won't be deep unless the output is also set to export deep images or as it is in your case render elements. The settings controlling the ray marching are "Max Steps" and "Step size", which limit the depth seen through the camera. In your case, 1000 Max Steps and 4mm Step size is not enough to encompass the whole fog (it's quite thin with a distance 13000 mm). 1000 max. steps with a step of 4 means the depth would be limited to only 4000mm.

As a rule of thumb, to get similar results with fog global (infinite) fog: the fog depth (Max Steps * Step Size) must be around 7 * Fog distance (to reach the cutoff threshold of 0.999);
For fog contained in a gizmo - the Fog depth should encompass the size of the gizmo (though the Fog distance should also be considered to avoid overkill). Overall, it's a question of adjustment (sometimes more max steps may be needed).

Based on our tests, in your scene, a step size of 100 should yield similar results and be quite fast.

[OPKJoe]

Thank you for your investigation and detailed explanation.
Seems clear so far even though I still don't get it why the image looks different without deep output activated. I guess the max steps and step size are valid for non-deep renders, too.

But either way round I now know how to solve this issue
Thanks again!