Rendering a Reflection pass with alpha of the reflection.

[antokoman]

I am wondering if it´s possible to render a reflection pass which outputs the color of a reflection and the alpha of the reflected objects only. I mean, for example let;s say I have a red sphere I want to reflect on a plane. I want the red sphere reflection on the beauty but the shape of the phere reflection on the alpha. Is this doable in one pass via AOV;s. So far I;ve been rendering a reflection layer and then overriding the shader of the reflected objects for a white surface shader in order to get a black and white image of my reflection that I will use as an alpha, but it would be cool if I could do this only in one pass. 

Thanks

[sean.heasley]

Hi @antokoman

I believe the way you're doing it right now is the best workflow.

It's possible this could be set up to only need one pass but the amount of overrides and different shader set ups you would need would make it a bigger hassle than doing it with multiple passes.

Plus by doing multiple passes you have more control over the overrides and outputs 

Please let me know if this helps or if you need any more assistance!

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[antokoman]

Hi Sean, 

Thanks so much for your reply, I was doubting if this way was the most efficient. Still, I would be interested in knowing how to do it in one pass if that was remotely possible. 

Cheers,

Antoko

[sean.heasley]

Hi @antokoman

I just reread your post and I actually might have misunderstood initially. Do you want the reflection on the alpha channel or the render or do you just want the alpha of the object to be rendered?

[antokoman]

It´s difficult to explain. What I would like is, let´s say I want RED Sphere A to be reflected on Plane B. So what I want is the Reflection of a sphere on a plane, and the shape of the reflection of that sphere on the alpha. Is that doable? In this case I would see a Red Sphere reflecion on the RGB channel and a white Sphere on the Alpha Channel. 

Thanks Sean!!!

[sean.heasley]

Hi @antokoman

Ah ok in this case I don't believe its possible via one render pass. Your best bet would be to use two passes like we mentioned above.

Hope this helps!

If one or more of these posts helped answer your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

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[mpowers]

can someone post a screenshot or material that demonstrates this? Ideally the indirect_specular would have a way to add an alpha of the reflected objects. This an extremely common workflow, especially for compositing print stills.