Some rendering/ compositing problems with Smoke

[ben_hamburg]

Hi,

I?m finally getting closer to finishing my underwater volcano shot and I?ve encountered some issues.

1. To add Fog (I?m using a Volumelight and not Vrayenvironment fog, which is just too slow to render and I can?t get nice godrays) I need a Zdepth pass, so I tried using the volumetric geometry mode, but even after increasing the max transparency level in Vray to 1000, I still get some artefacts. (see second attachment).
2. Another issue is that to composite the environment fog I used the zdepth element and it has no opacity for the transparent areas of the smoke volume 8at least if rendered with the rest of the scene). This is ok for DoF compositing, but for the environment fog I get a visible halo at the border, where the smoke becomes transparent.
3. I will probably just be able to fix that in comp, if I render phoenix in a separate pass, but I?m not sure how I?m supposed to do that then: just disable "visible to camera" in the object properties so it will still cast proper shadows and light the scene?
4. I haven?t tried it yet, but I remember that rendering the scene with the phoenix element invisible but still casting light, the areas where the fire was in direct contact became incredibly slow to render even though I was already using linear color clipping at 1.0 to avoid overbright areas. I?ll have to try again, but if that problem is still there, is there a way around this? Maybe just render fire with the rest of the scene and smoke+fire in a separate element?

[Svetlin.Nikolov]

Hey,

Looking very nice!

For 1 and 4 if you could send a scene and fix or figure out the reason for these if they are not bug, it would be great - I haven't seen neither so far. With a V-Ray atmosphere you need to be aware of this https://docs.chaosgroup.com/display/...VRayVolumeGrid, but in Volumetric Geom mode it should be okay. However, not sure how the volume light would work with this.

We will be implementing matte support for V-Ray soon, but it would take some time.

Cheers!

[ben_hamburg]

Hey, thanks for the quick reply, looks I am not the only one working on the weekend... Although...this is my personal project, so not quite working after all.
Anyways,
I just did another test on another workstation and as far as I can tell, I cannot reproduce the artefacts there. So either its something with the other workstation or possibly ram issues, since I have only rendered two elements in this test and not the whole scene, which also includes a lot of vraydisplacement.
For 2 I can work around by using a separate render pass for the smoke, it properly has the transparency in the alpha, so I can use this to comp a seperat zdepth for DoF and Fog compositing.

I have also found a solution for the geometry pass: "volumetric geometry" does not work with "visible to camera" unticked in the object properties. To get light and shadow from the phoenix element in the geometry pass I needed to use volumetric mode.
So this would be part one, and part two rendering only the phoenix element in volumetric geometry mode. This also confirmed I do not have to worry about overbright lights...I did not get any when rendering the geometry pass. In fact I got almost no light from the grid at all, even after increasing the light on scene multiplier from 0.01 to 100.. No idea what the issue here was with my very first tests and how to get more light on the scene here...I will probably just add an extra vraylight, possibly the smoke is just so dense that there just is not much light from the fire coming through...(emit even if not renderable is ticked).
I already rendered a volume light pass and it worked, I am not quite sure how I did it, maybe I had to use mesh mode for it to work.

[ben_hamburg]

Nevermind, I also found the solution for the missing light...I had turned down "physically based" to 0.0, once I increased that back to 0.01, I got my lighting back. I turned it down for artistic reasons and render times triple when having it on, so I am going to do some more testing to find a good balance. Lighting definitely looks better than with a vray light in place of the fire though...

I will probably end up using different values for fire multiplier/physically based to render the geometry pass and the phoenix pass.

[ben_hamburg]

Ideally it would be possible to just affect some of the geometry by the light cast on the scene, so I would not have to render separate passes for the volcano (which should have the lighting from the fire) and the rest of the scene (which would probably just be slowed down for almost zero light effect...)...

But you should consider either changin the way it works right now, or at least mention it in the manual, because it is a bit confusing, that the "affect scene" multiplier apparently has none or almost no effect on the outcome, compared to the effect the physically based and fire multiplier value have.

[ben_hamburg]

Hm, just did another test with an area further away from the volcano and the rendertime was almost not affected, so the further away he light, the less of a render time hit I should get. So maybe its ok to render all the geo in one pass with the light from the phoenix element on...

[Svetlin.Nikolov]

Alright, reproduced both of these! Will be back to you when I know more, or ideally when there is a fix. Thanks very much for reporting these issues!

The scene multiplier should act in combination with the black body multiplier - the black body should affect the emission in general, while the multipliers for the scene on for the simulator itself should regulate the effect on either of the two. However, there is a difference indeed between volumetric geom and just volumetric, as well as between disabling Renderable and Visible to camera.

[Svetlin.Nikolov]

Hey, the shadows and illumination with Visible to Camera disabled and Volumetric Geometry mode should work the same as in Volumetric mode in tomorrow's nightly (Jan 16)

[ben_hamburg]

Oh, ok, thanks... But I?ll probably still go back and use some vraylights instead of the volume light. The render hit on the full scene with forest objects and everything is just too big I?m afraid. It?s a personal project, so I?m trying to keep rendertimes down to below an hour per frame...
I also played around a but with vrays new adaptive lights, increasing it to 32, but that didn?t seem to have much of an effect on rendertimes. If you have any tips about that I might give it another try though.

[Svetlin.Nikolov]

Ah, yes - it's important to know that right now the adaptive lights won't collaborate with the Phoenix Fire lights, but if you use V-Ray lights, they will.

[ben_hamburg]

Ok thanks for the info, than I won?t bother with that right now...