P3D The Stereo Window (was: More Lenticular Queries)

[abram klooswyk <abram.klooswyk@xxxxxx>]

Interesting discussion initiated by (again) Bruce
Springsteen
and taken up by Greg Wageman, Tony Alderson, George Themelis

(P3d 3409-3419, 22-26 July 1999). I believe I share most of
Tony's views, although I'm not sure I can answer all the
questions of his thought experiments unequivocally, which
he seems to suggest should be done.

I have said (elsewhere) that a stereo window is common to
_all_ 3D images with visible borders.

All, including classical twin-view (Wheatstonian) stereo
pictures, whether printed or as slides, in stereoscopes or
projected; anaglyphs (all forms); lenticulars; and even
holograms.

Their visible physical or virtual borders necessarily are
_anywhere_ on a specific place in space, as is the 3D scene
they contain. Therefore the border has a defined perceptual
position with respect to the 3D scene.

Image points of flat 2D pictures have X and Y coordinates,
as have points of their border.
Image points of 3D scenes additionally have Z coordinates,
or, more simple said, lay at different distances from the
spectator. Visible borders also have a Z-coordinate, they
are at _some_ percepted distance from the viewing person.

[George Themelis (PHOTO-3D Digest 3414, 25 Jul 1999):
>when you overlap the edges of a slide mask on the screen
>during projection what do you get?
>A black frame.  It is not stereoscopic.

Not stereoscopic ?? The overlapping edges of the mask,
or stereo window for short, has the same right of being
called stereoscopic as any image point in the 3D scene.
Fight for the right of the window!  :-)]

In practise most borders of 3D images we see are
rectangular,
and I would call a rectangle at some distance in space:
a window.
Remember that we also speak (with twin-view stereo) of a
"stereo window at infinity", when there was no window set
up in the camera or in mounting.
So I'm not in favor of restricting the meaning of the
concept of "stereo window", as Greg seems to want.

I don't quite understand what Greg means with "monocular
frame" or "monocular border".
Monocular is: seen with one eye only. So it applies to
viewing techniques where the borders of one view are seen
by one eye only, that is: in stereoscopes and in stereo
projection, and in a part of anaglyphs.
The border of a lenticular is seen by both eyes.
-------

Now there are 3D images _without_ visible borders.
Examples are of course Wheatstones stereo drawings, where
the white background is the same paper as anywhere on the
page, no visible edge or border, unless you call the edge
of the whole page the edge of the drawing.
The reverse is a black background, for example a fireworks
slide, seen in a non-reflecting stereoscope.
I understand that Boris Staroski's "Surprise" falls in
the category of the white merging background, I have seen
pantogram-type anaglyphs of geometrical figures standing
out of the page, with the same effect.
And when the image is large, more than 180 degrees, you
cannot see all of its border at the same time, and sometimes

not at all, but it is there.

I my opinion these non-border examples are only special
cases, or tricks, which don't mean that having a window
isn't the general rule in 3D images.
(The trick status of firework shots is apparent from the
fact that films with those shots must be cut carefully at
the point of their non-visible edges!)

But is a 3D image possible which principally has no border,
so principally no window? Yes, but I don't know if it has
been made yet.
(Is Boris reading this? Maybe a new challenge :-))

The surface of a sphere, for example a globe, is curved
but two-dimensional. This 2D surface has no borders.
The surface of some other bodies, like kidney beans
and donuts, have similar properties.

Now make an anaglyph which covers a globe completely:
A 3D picture _principally_ without window.

It's a pleasure to discuss with you gentlemen. I believe
it is a French saying that for discussing it's necessary
to agree. I feel we agree on most things.

Abram Klooswyk


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