VRay Maya -> Bring the sky into Nuke (deep compositing)

[patrick_ortman]

An admittedly novice question, thank you for your help:
I'm using the VRay physical Sky (the blue sky button in the shelf) to make my skies, along with the VRayGeoSun/Transform. It looks great for this project.
Problem happens when I bring the rendered EXR (deep) into Nuke for 3D/Deep compositing: I have not yet been able to figure out how to bring the sky into Nuke.
Is there something simple I am missing? I am hopeful the answer is yes! There's got to be a button somewhere...

I did find a thread from 2009 (almost 12 years ago!) which suggested just hiding your geo and outputting a JPG to place in your compositing scene. But that outputs an 8bit JPG according to Photoshop. Which... kind of?... works but not really. Because in Photoshop I can clearly see the 8 bit banding on the JPG. Help!

Thanks and happy 2022!

[patrick_ortman]

Blah, can also do the JPG trick with regular 32 bit EXRs, the old 2009 forum posts maybe VRay didn't support that back then. Although, if there is a way to just put the background (VRay Sky) into a deep EXR from the main VRay render and just extract it into Nuke in the right place for compositing- all the better! Is there?

[patrick_ortman]

So, group mind, is there not a better way in 2022?

[Morbid Angel]

Hey Patrick, there is one way I can think of of getting your sky to show up in deep. The issue is that deep needs to sample actual physically geometry points in 3d space. The sky is not a geometric object so it can't be sampled. The sky alpha is also black which means it only exists in the rgb part of the regular exr, but in deep it does not work the same. What you need to do is make a sphere, scale it up around your scene and place it into vrayobject properties node. There you need to check matte object, and uncheck all other attributes in visibility options except primary visibility. That way the sphere will act as a holdout matte, but it won't holdout anything and show the sky through yet write it into the deep data.
This works, but I would not really advice doing this. Its always best practice to render the sky as a second render layer, you can layer override vray exr per render layer, so deeps for your main bty, and multichannel or regular exr for sky.

[patrick_ortman]

Morbid Angel Thank you, Dmitry! It's nice to have options.