Path: utzoo!utgpu!water!watmath!clyde!rutgers!umd5!uvaarpa!mcnc!gatech!bloom-beacon!mit-eddie!mit-amt!dlleigh
From: dlleigh@mit-amt.MEDIA.MIT.EDU (Darren L. Leigh)
Newsgroups: comp.graphics
Subject: Re: ray tracing 3d holograms
Summary: vertical parallax is undesireable
Message-ID: <1913@mit-amt.MEDIA.MIT.EDU>
Date: 21 Jan 88 19:27:16 GMT
References: <1987Dec15.152944.3586@mntgfx.mentor.com> <390006@hpfcdq.HP.COM> <1225@mips.UUCP>
Reply-To: dlleigh@media-lab.MEDIA.MIT.EDU (Darren L. Leigh)
Organization: MIT Media Lab, Cambridge MA
Lines: 43

In article <1225@mips.UUCP> hansen@mips.UUCP (Craig Hansen) writes:
>In article <390006@hpfcdq.HP.COM>, wanger@hpfcdq.HP.COM (Leonard Wanger)
 writes:
>> 	Has anyone heard about a computer generated hologram created
>> in squares (opposed to strips)? This would square the number of
>> images needed, but would give the true 3-dimensional effect (in
>> the vertical and horizontal).
>
>There's no particular obstacles, except that
>
>1) it takes a lot longer to make 10000 images than 100 images
>    (100 times longer).
>
>2) binocular vision and head motion parallax is usually left-to-right,
>   so if you're going to go in only one direction, vertical strips
>   are more important than horizontal strips.

I tuned into this discussion a little late so forgive me if this has
already been brought up.

Computer generated holograms created in strips (stereograms actually)
do not have vertical parallax because they are usually meant to be
viewed in white light.  Because holograms work by the principle of
diffraction, the different wavelengths in the illumination beam are
refracted at different angles and can cause an unacceptable amount of
color blur.  The color blur is caused by blue light from a higher
vertical view and red light from a lower vertical view mixing with
light of in-between wavelengths from the central view because of their
differing diffraction angles.

We can get around this problem by using a rainbow hologram, which is
made by throwing away all vertical parallax and using only one
vertical view.  When the hologram is viewed in white light, moving
your head up and down will change the color of the image through the
spectrum instead of changing the vertical parallax.

The computer generated stereograms are made basically the same way
which is why you can get such nice color by making multiple exposures
at different angles.

A computer generated hologram with both vertical and horizontal
parallex wouldn't look very nice unless you viewed it under
monochromatic light.