Inverted RGB of a MatteShadow or Alpha render element.

[LL_Will_J]

Both the Alpha render element and the Matte shadow render element seem to be carrying lots of information in their RGB values.

By inverting the RGB of these render elements in nuke I can see GI colour bounces and shadow colours from refractions.

I would love to be able to use these in Nuke - but I cant quiet figure out the correct maths to recreate what I am seeing when rendering with the background through Vray.

Does anyone know how to set this up? Seems like a killer workflow for MaterialWrappers to integrate stuff onto a plate.

(I'll stick together a couple of images to better explain what I mean soon!)

Cheers!

Will

[Svetlozar Draganov]

It is a good idea to show us some renders/scenes/compositions so we could understand better what you are after.
Not sure if such formula exists but I'll try to find out more information when some examples are available.

[LL_Will_J]

Thanks for getting back to me. I hope this is a clear explanation...











I am wondering what magic vray is doing to get these results - so I can match them in Nuke.

Many thanks

Will

[Rens]

What does the difference look like? Also use Over instead of Screen, or Plus if you just need to add a layer.

Edit: actually, just switching to plus should give you the correct result. Nuke's Over multiplies the background with the inverted alpha, then adds the foreground. Only thing is, is that a regular alpha then will be monochrome, so saving out the coloured 'alpha' separately as you did should hopefully give you the correct result?
Also worth looking into is a VRayShadow pass.

[LL_Will_J]

Totally nailed it! Thanks Rens.

Using plus fixes the issues I was getting on the edges of the objects against the background.

This is an awesome workflow. I will definitely be using this on every single job from now on. In a way this is a hidden feature - I cant find this documented anywhere. Glad to know how to use now.



for anyone that is interested you need to set Vray to save a separate alpha when batch rendering to avoid the monochrome situation Rens mentioned.

Also the MatteShadow does have this information in there as well but also shadows on the objects. Not sure how to use this pass as it stands. Maybe thats the next investigation!

[LL_Will_J]

I should also mention one thing i have noticed in the process when testing this stuff in Vray.

Explained here:



Hope this is handy for people.

Will

[Rens]

Great, glad it works. : )