hi francois,

thank you VERY much for the excellent explanation.  i had not
grasped the idea that the second number was in the direction
of the normal, so my plane was off behind the camera, and a
default background grey that i have in my code was kicking in.

thanks again, miker



"Francois Labreque" <fla### [at] videotronca> wrote in message
news:3BF### [at] videotronca...
>
>
> MR wrote:
>
> > hello,
> >
> > i've got my camera sitting a little ways off in the -z area, and
> > looking towards z positive infinity.  i've put a plane out there
> > for a sky, plain black right now.  i would expect the normal to
> > be coming out of the surface i'm viewing, which, in my mind,
> > would make the normal -z.  but i notice that i only get the sky
> > color as black with a normal of z.  its grey with a -z normal.
> > here's the sky code:
> >
> > plane
> >   { z, 1000
> >     pigment
> >       { color Black
> >       }
> >   }
> >
> > the camera:
> >
> > camera
> >   {  location   <1, 1.5, -6>
> >      look_at    <0, 1.3, 0>
> >      angle      45
> >   }
> >
> > could you straighten out my thinking here, please?  why isn't
> > the normal  -z?
>
>
> Your thinking is straight.  Almost
>
> For the purposes of CSG (and other stuff), planes have an "inside" and
> an "outside".  a plane would be represented like this:
>
>                  |
>                  |
>    (inside)      |-----> normal  (outside)
>                  |
>                  |
>
>
> The location of the plane is measured by sliding the plane along the
> normal by so many units IN THE DIRECTION of the normal.  therefore:
>
> plane{ z, 1000 }
>
>                                |
>                                |
>                  .             |-----> z
>                (z=0)           |(z=1000)
>                                |
>
> plane{ -z, 1000 }
>
>           |
>           |
> -z <-----|          .
> (z=-1000)|        (z=0)
>           |
>
> Therefore, by just changing the orientation of your plane, without
> changing the distance, you are sending the plane 994 units behind the
> camera, so the grey you are seeing in your picture is probably other
> objects.
>
> What you need is:
>
> plane{ -z, -1000 }
>                               |
>                               |
>             .        -z <-----|
>           (z=0)       (z=1000)|
>                               |
>
> If you don't do CSG or media, plane{ z, 1000} or plane{ -z, -1000} will
> be equivalent (apart from the "Camera is inside non-hollow object"
> warning), but doing it right is sound practice if you ever want to
> improve on your scene a few months down the road.
>
>
> --
> /*Francois Labreque*/#local a=x+y;#local b=x+a;#local c=a+b;#macro P(F//
> /*    flabreque    */L)polygon{5,F,F+z,L+z,L,F pigment{rgb 9}}#end union
> /*        @        */{P(0,a)P(a,b)P(b,c)P(2*a,2*b)P(2*b,b+c)P(b+c,<2,3>)
> /*   videotron.ca  */}camera{location<6,1.25,-6>look_at a orthographic}
>

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