I see you are getting plenty of help here, so I won't add anything except
about what you asked me in e-mail about doubling the arrow. If I understand
right, this is very simple. One way to do it:

// double-ended arrow
union {
cone { +y, 1, +2*y, 0 } // points toward + y (pluses not needed)
cylinder { -y, +y, 0.5 } // centered on 0 y
cone { -y, 1, -2*y, 0 } // points toward - y
pigment {rgb 1}
scale 4
scale <1,2,1>
rotate <0,0,-180>
translate <200, -20, 0>
}

Replying late because of making a new car buy after my 10 year old truck's
waterpump decided to fail. Lot's of researching done to finally narrow that
down to a 2004 Subaru Impreza Outback Sport. I was going to get a small SUV
at first then happened across that car (or wagon).

Speaking of "focal point", as in the original message here, I was getting
new eyeglasses. In fact, replacing lenses of the new eyeglasses I just got
because I couldn't read very well using "progressive" lenses (for both
reading and regular seeing).
Changed to regular-only again and don't even want to try bifocals. I'll get
reading glasses someday. For now I can hold things out to read them, even
though words can be small that way. :) I don't know how people read with
those progressive things! I had about one word in focus at a time, the rest
were a blur.

Bob H.

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