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Kolin
09-03-2010, 01:04 AM
This must have been covered before but I cant find it in a forum search.

I have a sphere that is fully reflective (with and environment map) and is %100 transparent but I get nothing in the alpha channel. If I want to comp it properly over footage I have to use certain modes or luminance techniques to make it work. Is there a way to just get a useful alpha channel?

Mr Rid
09-03-2010, 02:54 AM
Not sure what you are going for, but in the Surface Editor- Advanced- Alpha Channel, set the Constant Value to 255 for the object to have a solid alpha.

Kolin
09-03-2010, 02:04 PM
I know about setting the Alpha to constant. That will just create a 'mirrored' ball and not a 'crystal' ball. You wont be able to see any of what is behind the ball through it.

Salv8or
09-03-2010, 02:20 PM
To make that work you have to use the image that you are going to composit it over at least as a background image in the compositing tab in the effects window. "CTRL+f7"
I would not use 100% transparancy and a 100% diffuse since a material is never (in realworld) more than 100% in total. (diffuse+reflection+transparancy=100%)
The easiest way in my opinion is to use the dielectric material in the node editor. (Add node->Material->Dielectric)
Plug the outgoing "material" to "material" of the surface in the nodetree.
Dubbelklick the "dielectric"-material node and enter some good values.
Something like 1.0 for absorbtion and the refraction value for Crystal 3.34. Check the "Partial internal reflection".
In the traditional surface editor uncheck "Dubble sided" since the dielectric material already calculates both front and back polygons.

I think that should solve it for you in a very easy way.
Good luck.

Mr Rid
09-03-2010, 02:42 PM
Yes, your surface is not realistic. But aside from that, if you render your sphere over a background image, then if you do another render over black and comp it over the same background image, you should arrive at identical results, depending on settings in your comp. The reflective surface of a 100% transparent sphere should appear luminously additive over the background either way.

toby
09-06-2010, 08:22 PM
The reflective surface of a 100% transparent sphere should appear luminously additive over the background either way.
In other words, you don't use an alpha channel at all, just an additive blend mode in your comp.

And don't forget fresnel if this is glass -