I'm seeing very strange behavior from the orthographic camera, where
sometimes the whole image seems to disappear from the camera. I worked up a
simple script to demonstrate the problem that creates four spheres at a
Z-coordinate of 1. (This script works as I expect, the problem comes
later):

camera {
    orthographic
    #declare WINDOW=2;
    right WINDOW*4/3*x
    up WINDOW*y
    translate <0.5, 0.5, 0>
}

#macro RefBall(loc)
sphere {
    loc 0.2
    pigment {rgb loc}
    finish {ambient 1}
}
#end

RefBall(<0,0,1>)
RefBall(<0,1,1>)
RefBall(<1,0,1>)
RefBall(<1,1,1>)

As I expect, I see four circles. Now if I change the camera to say:
    translate <0.5, 0.5, 0.5>
I would expect to see these same four spheres. But instead I see nothing.

What's really strange is that if I translate the camera to anything up from
<0.5, 0.5, -10000> to <0.5, 0.5, 0.2> then I *do* see the four spheres with
a Z-coordinate of 1. But as soon as I hit <0.5, 0.5, 0.2001> everything
disappears. Strangely, the point where everything disappears matches the
diameter of the sphere - if I make the diameter of the sphere 0.3, then I
can move the camera to <0.5, 0.5, 0.3> before everything disappears.

Is this a bug, or do I have some fundamental misunderstanding of how the
orthographic camera works?

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