On Wed, 19 Nov 2003 15:53:51 -0600, Lee Brown 
<mr_### [at] acruelandbeautifulworldcouk> wrote:>
> difference {
>   sphere {
>     <0,1,0>
>     1
>
>   }
>   sphere {
>     <0,1,0>
>     .9
>
>   }
>   inverse
> ...

What is the inverse doing there? Now you are creating a glass filled scene 
with a small spherical shell without glass. As you perhaps notice the 
rendering messages warn you of a camera inside a non-hollow object.

> Now if the light is repositioned to <0,1,0> it doesn't seem to produce 
> the
> expected result.

I don't know what effect you expect, but that seems to be the right 
behavior. (In the center of your "sphere construction" the light won't 
bend and focus to a bright area on the ground)

-- 
light_source{20*y,1}#macro _(M,X,Y,P)#macro L(N,D)#if(N)#declare 
P=P+D;box{-
0.5,0.5translate z*mod(9*P.gray,4)pigment{rgb P}rotate 45*x+clock*y 
translate
P}L(N-1,D)#end#end#if(M)L(mod(M,8)<mod(X,3)mod(Y,3)1>-1)_(div(M,8)div(X,3)div
(Y,3)P)#end#end _(2301603551,12850,60365,20*z-5*x)plane{y,-9pigment{rgb 1}}

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