Christian Froeschlin <chr### [at] chrfrde> wrote:
> Oh and I recommend a non-black sky because otherwise you never
> know if your black areas are trace level artefacts, shadows
> or some path in which the sky is visible:

> sphere
> {
>    0, 1000 hollow pigment {color SkyBlue}
> }

> And before the usual suspects complain, yes, I'm perfectly
> aware "hollow" has no effect here. But I do so dislike tracking
> down strange issues later when media enters the scene ;)

  What's wrong in using sky_sphere, which exists for this exact purpose?

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
>   What's wrong in using sky_sphere, which exists for this exact purpose?

Various things. For instance, if you're using an image for the sky, you cannot
brighten or dim it using finish { ambient mySkyBrightness } if you map it to a
sky sphere, because - well, there's no finish on a sky sphere.

Similarly, you cannot use different sky spheres for radiosity (you may want a
strongly blurred image for that) and reflections (you'll definitely want a
high-res image there). With standard spheres, you just define two spheres of
almost same radius, and give the inner one a "no_image no_reflection" to have
it only affect radiosity. sky_sphere doesn't allow any of these statements.

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clipka <nomail@nomail> wrote:
> Warp <war### [at] tagpovrayorg> wrote:
> >   What's wrong in using sky_sphere, which exists for this exact purpose?

> Various things. For instance, if you're using an image for the sky, you cannot
> brighten or dim it using finish { ambient mySkyBrightness } if you map it to a
> sky sphere, because - well, there's no finish on a sky sphere.

  I didn't see him using an image map for his "sky sphere". His stated
purpose was to use something else than black as the background color.

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
>   I didn't see him using an image map for his "sky sphere". His stated
> purpose was to use something else than black as the background color.

Right, but the issues I described may be a good reason of making it a habit to
use true spheres instead of sky spheres.

In *this* particular case, you're right: A sky_sphere will do fine. Just like a
true sphere will, too.

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clipka <nomail@nomail> wrote:
> Warp <war### [at] tagpovrayorg> wrote:
> >   I didn't see him using an image map for his "sky sphere". His stated
> > purpose was to use something else than black as the background color.

> Right, but the issues I described may be a good reason of making it a habit to
> use true spheres instead of sky spheres.

  Why should it be a habit? Writing a sky_sphere is much less typing than
writing a regular sphere. If you ever need to change the sky_sphere to a
regular sphere (which probably will happen rarely), it's rather easy to do.

  If you make a habit of creating a sphere, you are only going to encounter
problems which a sky_sphere doesn't have.

-- 
                                                          - Warp

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"clipka" <nomail@nomail> wrote:
> Warp <war### [at] tagpovrayorg> wrote:
> In *this* particular case, you're right: A sky_sphere will do fine. Just like
> a true sphere will, too.

... oh, and speaking of it: A simple background {color SkyBlue} will do the
trick, too...

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Warp <war### [at] tagpovrayorg> wrote:
>   If you make a habit of creating a sphere, you are only going to encounter
> problems which a sky_sphere doesn't have.

.... which would be...?

I mean, unless we're talking about infinite planes and the sphere being so small
that it visibly messes up the horizon...

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clipka <nomail@nomail> wrote:
> Warp <war### [at] tagpovrayorg> wrote:
> >   If you make a habit of creating a sphere, you are only going to encounter
> > problems which a sky_sphere doesn't have.

> .... which would be...?

1) A sphere makes the rendering slower. For each single traced ray (all of
   them, not just camera rays) povray has to test it against the sphere.
   A sphere may be fast to trace, but when you have to test against it for
   each single traced ray, the small times add up.
   A sky_sphere doesn't need to be traced, and if it suffices, it eliminates
   the useless overhead.

2) Light sources in your scene might interact in undesired ways with your
   sphere, even if you specify an "ambient 1" for it. If light sources have
   an effect on the sphere, it not only can change the illumination/coloration
   of the sphere, but objects might also cast shadows onto it. A sky_sphere
   will always look the same regardless of your light sources.

3) You don't need to worry about fog and media effects with sky_sphere, but
   you have to if you use a sphere.

4) Objects which are large enough might intersect with the sphere. Even if
   they don't, if they are large enough and eg. reflective, the reflection
   might become wrong.

5) If you move the camera too far from the center of the sphere, the sphere
   texturing may look distorted in an undesired way.

6) If you use things like ground fog, the sphere limits its visibility and
   might make this boundary visible. sky_sphere doesn't limit anything.

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
> clipka <nomail@nomail> wrote:
> > Warp <war### [at] tagpovrayorg> wrote:
> > >   If you make a habit of creating a sphere, you are only going to
> > > encounter problems which a sky_sphere doesn't have.
>
> > .... which would be...?
>
> 1) A sphere makes the rendering slower...
>    A sky_sphere doesn't need to be traced, and if it suffices, it eliminates
>    the useless overhead.

Interesting. I didn't know that.

KW

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Kenneth <kdw### [at] earthlinknet> wrote:
> > 1) A sphere makes the rendering slower...
> >    A sky_sphere doesn't need to be traced, and if it suffices, it eliminates
> >    the useless overhead.

> Interesting. I didn't know that.

  Even if sky_sphere had to be traced, it would only have to be traced for
rays which don't hit anything in the scene, so it would still be more
efficient.

  However, as I said, it doesn't even need to be traced. What POV-Ray does
is that when a ray doesn't hit anything, and there's a sky_sphere pigment
defined, it calculates the gradients of the ray and picks up a color from
the pigment as a function of those gradients.

  The effect is the same is if the sky_sphere was a unit sphere (with the
specified texture) centered at the starting point of the ray which doesn't
hit anything in the scene. But without the need to actually create and
trace such a sphere.

-- 
                                                          - Warp

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