Wasn't it Kaeru Gaman who wrote:
>I want to render a glass-sphere that refracts a detailed background.
>however, this glass-sphere should be the only object visible,
>the surrounding should be background-color that would be calculated
>into full alpha transparency...
>do you understand my problem?

Could you do it with reflections?

For example, place a circular mirror right through the centre of your
glass sphere so that it faces the camera. The camera will then see the
front half of the sphere and its reflection will look like the back half
of the sphere.

Then you can make all the other objects in the scene be no_image, so
they won't be seen directly but will be seen in the refracted
reflection. You can also scale them by -1 in the camera direction to
recover the original positioning and orientation.

A (probably unwanted) side effect of this technique is that the
reflected background colour seen through the glass sphere becomes
opaque, whereas refracted background colour would be transparent.

-- 
Mike Williams
Gentleman of Leisure

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