Brainteaser about MirrorTransp

Hi guys, this is something I’m really struggling to get it to work properly.
So, here my setup:

1 - I’ve a transparent object, a water sphere with IOR 1,33;
2 - I have an image I want to see behind my water sphere as a refraction color modifier for my sphere;
3 - and I have an image mapped to the sky used as a background image for my scene.

http://b.imagehost.org/0199/study_640.jpg

Now the problem I’d like to solve:
I’d want my sphere to refract only the “refraction” image and not to refract the world and the sky.
I do not want the image used for sky taking part in the refraction process of the sphere as you can see in the attached image.

I know I can add an image texture to the sphere’s material and map it as Refl as reflection, is it possible to have another image texture mapped as RayTransp for my sphere?
In other words is it possible to prevent the sky taking part in the RayTransp computation for my sphere and to choose what image is having to do it??

For you to know, I’ve just tried to add a plane parallel to my camera, have my Refraction texture applied to it, make it shadeless, moved it to another layer and excluded it from rendering but it’s not working 100% as I’d want because the plane is finite and behind its limits the sky is still visible.

I hope I could have properly explained the issue although my ugly english, thank you!

I would just render the first pass out without the sky. Render as a PNG sequence RGBA. Then just drop the sky into the background with your favorite compositor.

No proper solution? well, I think I’ll do that… or I’d try changing refractor plane’s mesh scale and location.

Thank you!

I don’t know why people see using compositing and render layers as not a “proper solution.”

Amen, harkyman.
Far from being an improper solution, comping’s how it’s done!