Food for thought:

 Ok we know that a sky sphere is a virtual effect and that for all intensive
purposes it is not a real object. On the same token it is not really a pigment
either because it cannot be applied to an object. This leave us with the
following observations that seem somewhat undefined as far as it's usage
(at least from the simple description in the docs)...


Attempting to render the following produces an error from Pov stating that
there are no objects in the scene...

--
camera{location z*-5 look_at 0}
sky_sphere{pigment{gradient y pigment_map{[0 blue 1][1 red 1]}}}
--

So it is not an object is it ?



Rendering the following with the addition of a box allows the render to
go without error because of the addition of an object to the scene...

--
camera{location z*-5 look_at 0}
box{-.1,.1 pigment{rgb 1}}
sky_sphere{pigment{gradient y pigment_map{[0 blue 1][1 red 1]}}}
--

So it is an object only when there is a real object in the scene ?
What if I only want to raytrace a sky_sphere for a background image ?




The next little ambiguity that we can try is the following...

--
camera{location z*-5 look_at 0}
box{-.1,.1 pigment{rgb 1}}

#declare Sky =
sky_sphere{pigment{gradient y pigment_map{[0 blue 1][1 red 1]}}}

object{Sky}
--

Whoops ! Now we get an error that an object is expected but a sky_sphere is
found instead. Now we know for sure that Pov does not recongnize it as an
object.



So we change things around again to accomodate the parser and try the
following code...

--
camera{location z*-5 look_at 0}
box{-.1,.1 pigment{rgb 1}}

#declare Sky =
sky_sphere{pigment{gradient y pigment_map{[0 blue 1][1 red 1]}}}

sky_sphere{Sky}
--

Well that does work but the question is WHY ? Because a sky_sphere is a sky_sphere
is a sky_sphere ! 


Which leaves us with the still unanswered question: What is a sky_sphere ?


Enjoy your meal,

-- 
Ken Tyler

See my 850+ Povray and 3D Rendering and Raytracing Links at:
http://home.pacbell.net/tylereng/index.html

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