Render only smoke in pass Vray-Phoenix

[Mikeymore]

Hello, I have Vray next for max 2020 and Phoenix FD for max. I made some smoke following a path in my already made 3d model and want to render only the smoke to composite it in earlier made renders of the model animation. How can I set this up? I already tried VRAY render elements (with atmosphere) but it only outputs the whole render of the model with smoke and no seperate file in PNG of the smoke. I set the render elements to Vrayatmospheric, but it returns into a black pass render.

I want to render with Vray GPU. Actually I want to render only the smoke, and not the whole colorpass of the animation.
I now get a render pass of atmospheric black with only an alpha of the 3D model the smoke is in. Ideal is an aplha of the smoke too.

What do I do wrong and how to handle this?

Thanks in advance!

[georgi.zhekov]

Hey,

That's a bit tricky case for rendering on the GPU as the V-Ray atmosphere render element is still not supported there, but you have two options.

1. Assign a black V-Ray material as an override material for the scene - this way everything except for the Smoke will be rendered black - though if you have any colored objects that need to cast GI over the smoke - this won't do the job.
2. Add a Cryptomatte render element, switch to Bucket sampler in the V-Ray GPU options and render this way. This will give you a selection mask for the smoke, but the rest of the scene will be rendered as well.

If you need two you can use both methods to get the most flexibility later in post.

Hope this helps!

[Mikeymore]

Thank you Georgi for the quick reply!

Is that feature coming in the future of Vray GPU?

I managed to render it now the way you suggested.
Thanks again!

[georgi.zhekov]

It's definitely coming so stay tuned

[BeneZ]

Similar question: Is it possible to render colored smoke in a separate pass for compositing?
I imagine it is hard to separate, when it has whit and black smoke parts.
(CPU or GPU)
Thanks

[georgi.zhekov]

Can you elaborate more about the colored smoke?

You want to have the beauty render with a certain color for the smoke and then another one in the render elements?

[BeneZ]

georgi.zhekov My task: I would like to composit smoke in Nuke over a scene.
Normally with fog it is just white smoke on black background and you use a "Plus" node there to copy it over.
But this is not possible, when fog has very bright and very dark values (parts darker than the compositing background image and other parts lighter than background).

So what is needed is a rendering that gives the beauty as a transparent layer, which would be possible to copy over any background (like you do it also in photoshop).

How can I render the "beauty" with transparency out, without having it merged into a background color?
When the beauty is just a solid layer with an alpha, I don't know a way how to get it separated from it's background, because an alpha mask does not separate previously merged colors.
Or which would be the workflow?
Thanks

[georgi.zhekov]

What I mean by the beauty render - this is the whole final image.

In order to get the effect you're describing you will need two passes - the environment (this is all of your objects in the scene) and another pass for the smoke.

Depending on the shot you're after - in order to get proper lighting and occlusion for the smoke it needs the rest of the scene, so you have to render the smoke together with the environment.

You will need to separate the smoke on another pass so you can composite that later - you can use the V-Ray Atmosphere pass (this will work only on CPU for now) - this will give you only the smoke with the proper occlusion and lighting.

After that in Nuke you can Subtract the Atmosphere pass from the final render with the environment and and then add it back in on top - this way you have control over the two layers.
Using this approach you can render only once. Though this will work correctly only if you want the smoke to be placed on a specific place.

Another approach would be to render the smoke and the environment as two separate renders - one only for the environment and one for the smoke.
In this case it's best to render the smoke as a multichannel OpenEXR, this will give you the maximum bit depth and provide you with an alpha channel for the smoke. This way you will be able to overlay it on any background.
You might want to check this video out as well - https://www.youtube.com/watch?v=NDwqTuig7yQ

Hope this helps!

Cheers!