Am 28.04.2013 07:42, schrieb Subclick:
> Their only difference is transparency.
>
> In earlier versions of POV-Ray, it was possible to have a background
> pigment showing up in reflections and radiosity but not directly
> visible, so the output image would have a transparent background, simply
> by using a “sky_sphere” and enabling alpha output. This isn’t possible
> now, because you don’t get a transparent background in the output file
> unless the “sky_sphere” is transparent, and this in turn removes its
> effects altogether, including reflections and radiosity.

Note that this is a "damned if you do, damned if you don't" situation: 
In earlier versions, it was impossible to have a partially transparent 
background that shows up both in reflections (with transparent areas 
filled in by the background) /and/ in the image itself (with transparent 
areas being... well, transparent); think of clouds for instance.

As you already noted, the obvious solution to this is not to go for one 
option or the other, but add the same flexibility that already exists 
for huge spheres.

There's even more benefit to such a solution than just having a 
patterned background to be reflected but not seen: Adding the full set 
of "no_image", "no_reflection" and "no_radiosity" keywords (with 
"no_shadow" being the obvious exception), it would also be possible to 
have a high-detail sky_sphere for reflections, while using a highly 
blurred one for radiosity to avoid blotches from a single small bright 
spot on the sky_sphere such as the sun.

This (or an alternative syntax providing similar flexibility) is 
actually on my to-do list already, but hasn't made it into 3.7 yet due 
to feature-freeze.

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