Postproduction in Vray Buffer and Alpha

[o-v]

Hello there

I wonder one thing (a couple actually): Why do the color/exposure corrections in vray buffer not affect the alpha? I save the final image from the buffer as a multichanel exr. The point is, I need the alpha for other purposes = using the images in Indesign and PS and illustrator, where alpha is crucial.

The lights (reflection, refraction, dome with hdri) do not affect alpha of a glas/refractive material. Ok. So no big issue, I just add them together in fusion (chanel boolean -> add) and add them on the top of the "empty" alpha. Thisway I have all these things together and can use it as an alpha mask e.g. in PS.

But if I do some color corrections on the glas (more contrast, less exposure, more tint) in the vray buffer, I loose all those corrections in fusion. the alpha seems to be a little to "dark", missing luminance. Hence I have to repeat those corrections in the final app (99,9% Photoshop). Unfortunately I have an production example only which is NDA. I will render something different so I can show you, what I mean.

[o-v]

Ok here we go

First image (vray-buffer) is the output from the vray buffer, saved from the buffer as multichanel exr.

Second image (photoshop) is the precomposed alpha from fusion, effects result channel in PS and the alpha applied on the effects result channel layer as mask on a white background underneath.

duh...

And thank you for asking: the glas material reflections are set to affect color+alpha and the refractions to "all channels".

[hermit.crab]

Not sure if I'm understanding right. The alpha channel is simply a channel containing the transparency information of pixels. If CCs are to affect it, it would yield wrong results. By adding lighting, reflection, and refraction information to it, you are then creating a custom mask, which excludes/enhances/suppresses the effects of certain lighting effects. The thing is that most post-processing software has presets to use the Alpha as a mask, but that doesn't mean that we should solely treat it as such - it's a data channel firstmost.

[o-v]

Right. To my knowledge the alpha chanel defines the "transparency" by value. 1 = 100% opaque, 0 = 100% transparent. So i'm thinkig of altering the "value" = "exposure" could alter the values in alpha as well. Less transparency (in laymans words = darker glas) the higher the value in alpha.

It is absolutely ok to exclude the lights from alpha. What I meant is, if I change the transparency value of a refrative object by an adjustment layer, this could be translated in some "effects resulst alpha" as well. This way I could use the postproduction done in the vray buffer without additional corrections in other software (Fusion, Nuke, PS, you name it).

1) effects result chanel
2) alpha chanel
3) turn exr into ps, combine reflections, spec and reflections and alpha, copy into Photoshop alpha chanel
4) remove other chanels and use it in some print production software

badabing badabum

[hermit.crab]

You can log the idea in the Portal here. Otherwise, one thing you can do is export the CCs as a LUT file (right-click the Display Correction layer in the VFB) and afterward apply it to the Alpha in post.

[o-v]

Thx a heap for the LUT hint!