On 25-5-2014 7:58, skippy wrote:
> Thinking the normals of the planes would be opposing each other (hence y is 1
> for bottom plane and -1 for top plane). Reference says the distance value is how
> far along the normal from the origin. So if the normal is going y-negative, I'd
> need a negative number to bring the plane back 'up' above the camera and light
> source.
>
> But I can't seem to make this idea work. Suggestions?
>
>

Your normals are correct, however, you interpret wrongly the distance of 
the plane to the origin. -1 and -7 mean that the planes are positioned 
at -1 and -7 from the origin along the y-axis, irrespective of the 
normal /orientation/. If your camera and light are placed at y=0 for 
instance, you can place the planes at:

plane {<0, 1, 0>, -1}
plane {<0,-1, 0>, 7}

for the effect you are after.

Note that planes have no thickness, so normal orientation is irrelevant 
in your example, except if you want to use a media in between the planes.

Thomas

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