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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] videotron ca> wrote in message
news:3BF### [at] videotron ca...
>
>
> 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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