holography...

[P3D Stephen J Hart <sjhart@xxxxxxxxx>]

>Date: Thu, 19 Sep 1996 15:18:47 +0100
>From: P3D Jacques Lajoie <lajoie.jacques@xxxxxxx>
>To: photo-3d@xxxxxxxxxxxxxxx
>Subject: holography...
>Message-ID: <v01530501ae670525b769@[132.208.112.53]>
>
>Rather thousands of them. And the holographic picture is a true stereo
>phenomenon because depth is created only when seen with both eyes. One eye
>will see only parallax movement. So it should be included in this
>respectable forum...
>
>Am I right?
>
>Jacques Lajoie

Not entirely! As a long time lurker on this list, and an active
holographer, here's my take on this. First off, there are many kinds of
hologram, even ignoring many "3D" things we've all seen over the years
which get called holograms but which don't deserve to bear the name
(because they are nothing to do with wavefront recording).

A classical full-aperture hologram offers the following *physical* depth cues:

        Accommodation -

                The eye has to change focus to look at things in the
                hologram which are at different depths.

        Convergence -

                The two eyes have to swivel inwards more to look at things which
                are closer.

        Parallax -

                Nearer things appear larger and move faster across the field of
                view, both left/right and up/down... this is true for both gross
                motion like when you move your head, and for subtle motion like
                saccades (rapid flickering motions of the eyes themselves).

        Stereo disparity -

                The two eye see differences because they see the world from
                different positions.

Accommodation and parallax only need one eye, convergence kind of needs two
(though even with only one eye you do know where it's pointed), and stereo
disparity needs two eyes.

A good way to experience all this is to hold up the index finger of one
hand close to your face, and the index finger or your other hand at a
somewhat greater distance. Now look at one finger, but "see" what the other
one looks like.

By the way, different folks use different terms for these effects (like
convergence = vergence, accommodation = focus, etc).

In addition to these physical cues, a classical hologram offers the regular
psychological cues such as shadows, glints, obscuration, and so forth. A
hologram of a deep scene containing fog (it'd have to be a pulse hologram)
would even have atmospheric cues.

However, most of the holograms we all see are in fact not of this nature.
Rather, they are holographic stereograms. These use holographic techniques
to encode multiple two-dimensional pictures of a scene. This gives a
regular stereo view without needing special optical devices to see it (or
free viewing). But this doesn't give different accommodation or convergence
values for different depths, and generally gives only left/right parallax.
Also, the parallax it does give is for a single fixed viewing distance,
whereas in the real-world (and in classical holograms) parallax depends on
distance: that's how perspective works.

Holographic stereograms are thus like a series of two-lens stereo
photographs: they give a stereo effect, but this has to disagree with the
depths judged by the other physical cues. Classical holograms are like
looking at the a part of the real world (though generally without color and
generally it's a static world): the depth cues can't disagree.

Stephen J Hart                      fax              +1 (714) 348-8665
Director of R&D                     e-mail            sjhart@xxxxxxxxx
                                    WWW URL      http://www.voxel.com/

VOXEL, 26081 Merit Circle, #117,    The opinions and "facts" expressed
Laguna Hills, CA 92653-7017, USA    herein are not necessarily correct



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