skydome visible to camera with transparent alpha?

[aaronfross]

Hi,

Is there a way for a skydome to be visible to the camera, but invisible to alpha? I want the skydome to render in the RGB channels, but alpha should be transparent. That's the way it works if I use the Environment dialog, which in turn creates an invisible Arnold skydome.

So it is possible to do this in Arnold, it just looks like the function is not exposed in the Arnold Light. Is that correct? If so, can we add an Alpha component in the Contribution rollout of the Arnold Light?

Thanks,

Aaron

[Stephen.Blair]

It's not possible in Arnold. Are you using Environment > Background > Environment Map?

That comes out as a background shader.

[aaronfross]

I beg your pardon, although it may not be possible in the Arnold core, it is absolutely happening in MAXtoA. See screen capture. The environment map is rendering in the RGB channels, but the alpha is transparent.

This is necessary in workflows where the background needs to be rendered, but the foreground and background need to be adjusted separately in post.

[aaronfross]

And related to my other post about GPU rendering... this is a scenario that currently only works when rendering on the CPU. On the GPU, we can light the scene with an environment and hide the environment from RGBA, but we can't hide it from alpha only.

[aaronfross]

By the way, 3ds Max environment map is hard wired to be transparent alpha, and there is actually no way to make it opaque... except by rendering on the GPU, which is a bug.

[Stephen.Blair]

I don't think this is a skydome light though. If you export an ass file, options.background should be a shuffle shader, which links to a rayswitch.

[aaronfross]

Someone told me it was a skydome... it acts like a skydome. So maybe there's a rayswitch AND a skydome

[nicola.stefano.jannuzzo]

The environment case works because what we do is exporting the selected map both as
- the skydome light color, with camera visibility off
- the background, with alpha overridden to 0 by a shuffle node.

For now I don't see a way to override the alpha with just a plain single skydome light.

[aaronfross]

Thank you Nicola, it would be great if the Arnold core could have an Alpha Contribution component for Arnold lights.

[aaronfross]

And since you're overriding the background alpha anyway, it would be great if you could expose that as a parameter in the Environment dialog. In other words, give users the option of either opaque or transparent alpha. This would be pretty useful when users render to certain formats. E.G. if I render to PNG, the background is always removed. And since PNG transparency is embedded, not a separate channel, there's actually no way to un-remove the background. This means PNG format is no good for any scenes with a visible background. Making the background alpha opaque would fix the problem. PNG is the best 8-bit format for many use cases because it's lossless and supported by every app and OS in the universe.

[nicola.stefano.jannuzzo]

Ok Aaron. Our understanding was that historically Max users always expect to have transparent background alpha. I'll open a ticket.

[aaronfross]

That's true, Nicola, Max users have never had the option of opaque alpha in the environment background. But since you've already built the option, but just hidden it from the UI, I think it would be a cool "new feature", and easy enough to implement.

Thanks