Maya spotlight renders against object but not alpha pixel

[Anonymous]

This is tricky to describe.... I'm using Maya spotlight and Arnold render.  There are a couple of semi-opaque objects in the scene and some background pixels where the objects are not located.  I'm using Arnold aiAtmosphereVolume to see the spotlight propagate through the air.  I need the alpha channel so that I have flexibility during post processing and have turned the 'camera visible' setting on a aiSkyDomeLight to off/no so that I can control the background color post processing.  Here's the issue... when I render as a .png, the spotlight shows up great across the air volume, but it has no alpha channel so the background is black.  When I render as a .tiff, I get the alpha channel and can control the background in photoshop, but the spotlight only renders on the pixels that contain the objects.  It doesn't render on the pixels that have no objects.  I think its because these pixels are assigned 0 in the alpha channel but I need the spotlight to show up.  Any suggestions?

Thanks

[hamsterHamster · Accepted solution]

It is hard to advice anything specific, since I see no image.

The general flexibility mantra goes like: if you want it, then render in layers and passes.

PNG is mostly delivery, not production format. Better choice would be to use EXR, which can hold multiple channels and floating-point values, which gives flexibility in extracting faint values, like alphas of atmospheric phenomena. My personal preference is to manipulate even still images in videocompositing packages, like AfterEffects or Resolve, which gives access to specific channels.

[Anonymous]

I think your advice is good.  I saved as an .exr and got better results although not perfect.  Adding render passes will help too.

Thank you.

[hamsterHamster]

If you happen to have switched to AFX, then change Project bit depth to at least 16, better - 32 bits, as working space matters. Use Color Correction filters to manipulate Alpha levels for the spotlight, mask out irrelevant things, composite over. If you manage to include in EXR custom channels and passes, then use 3D Channel>EXtrctoR filter to access those values.

[Anonymous · Accepted solution]

So I have been working with the various render passes and created a grey scale alpha channel by rendering everything in white/black.  Composited in photoshop seems to be working.  I'm still learning a lot about AOV and render set up workflow so your suggestions were very helpful.  Thank you.