Volumetric not rendering with background images

[Mateusz]

Hi. I wonder if anybody came across that issue. I’m rendering scene that involves volumetric shape. Shape renders just fine when there is nothing behind it. As soon as I introduce some background or items that would appear behind given volumetric shape, volume “smoke etc.” no longer render in the shot parallel between camera view and the object behind volumetric. I tried tens of settings. Even alpha imaging whole sets of the scene and clumping it back together. Every time when there is something behind volume , be it original or from alpha image, volume disappears. Changing colours don’t help. density settings makes no difference.
Here is render in cycles with no objects behind and its what I’m striving for.

This is same thing but with other detail behind. Areas not crossing view path with back image are fine other than that they disappear. Any word of wisdom?

[NP5]

No idea sorry barely had any use for volumetrics. All I can say is it looks correct to me, flicking between the two it looks right. What am I not seeing?

[Mateusz]

His right arm disappears where wing is in the background. Also that face smoke. If you look close there is mist comming out of his face. And it is symetrical. Where on his left it is showing as there is nothing behind it. There should be right side tobthat but it disappears with helmet behind it.
Very annoying.

[FedPete]

My machine is too slow for the smoke sim. So I don’t have any experience with it.
But it looks like a Z-depth problem (maybe a bug). Smoke is done first then model mesh (wings).

My only idea it to switch over to Blender compositor.
Where you can combine several blender objects, scenes individually. Using Object ID and … (I forgot the term. basically filter out objects in post-production).

Basically renders smoke in a different post-composition layer and merged it.
We are getting used to the idea of rendering all at once, in a final shot. But there is the next step, in the Blender toolset. And that is post-processing. Find your solution there. And probably learn more about Blender too.

[Mateusz]

Youre right. I think piling too much stuff in to one render confuses blender. Compositing my be the answer. Using it to add glow. But there are tons of options there so will get digging.
By the way. This are not smoke simulations.
I came acros this gadget they introduced in blender2.9.
Its converting mesh in to volume.
Click on add menu and there is volume option. Choose add empty and you have empty volume “instance” . Than if you go to modifiers you have 2 to choose from. Convert mesh to volume and volume displacement.
It only works for density and not emission but its great way to create desired volumertic effect without tons of expensive smoke simulations.

[NP5]

To me it still looks exactly right as I would expect. White wings behind white smoke will make the smoke near invisible. Especially with bright lighting on the wings which also is strongly picked up in the very reflective metal armour.

I click on the image and then use the arrows at the side to flick to the next one, round and round.

But if it is not what you want fair enough.
Perhaps test it with red smoke?

[Mateusz]

I have been experimenting further with this and got to conclusion. That volume from modifier letting you create volume from mesh is behaving differently to normal smoke. I tested normal smoke in this situation and it renders nice and consistant. The one from modifier is very sensitive to light and background changes. Its hard to explain. In some places shows as you would expect and on some spots is as if it wasnt there at all. Propabily it is due to mesh its taking shape from. Although its hidden but information is there.
Got to plan scene more carefully or just use standard smoke.