Re: [photo-3d] Stereo x-rays, ortho/pseudo, front/back , up/down

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

----- Original Message -----
From: <abram.klooswyk@xxxxxx>
To: <photo-3d@xxxxxxxxxxxxxxx>
Sent: Tuesday, February 27, 2001 5:34 AM
Subject: [photo-3d] Stereo x-rays, ortho/pseudo, front/back , up/down


> John Rupkalvis:
> >(...)   In the example of the shell, a clockwise spiral from
> >one side would appear as a counter-clockwise spiral from the
> >other side, and if you did not know if the first or the second
> >image were pseudo, how would you tell which one was correct,
> >even if you knew the correct direction that the spiral was on
> >the real subject?  You would have to know not only the correct
> >direction, but also from which side it was imaged.
>
> Pseudo, short for pseudoscopy, and its opposite orthoscopy, do
> not refer to single images, but first of all to the cyclopean
> 3D percept. In looser usage, you can also speak of
> pseudoscopic mounting or presentation when, with side-by-side
> twin view images, the viewing mode (e.g. parallel)  is
> prescribed.

Pseudo, by itself is a rather loose term, as it simply means "false".  Scope
means "the range of one's perceptions".  Therefore, pseudoscopy means "a
false range of perceptions".  Stereoscopists have specialized this to apply
to images that are transposed (NOT flipped) such as to reverse parallax
directions (NOT reversing the image), thereby reversing only the perception
of the direction of depth, resulting in a perception that is inverted (close
objects appear far, distant objects appear close).

Orthoscopy is not the opposite of pseudoscopy.  Ortho, by itself means
"straight or perpendicular".  A common stereoscopic application is in the
term "orthostereo" or, more completely, "orthostereoscopic" (not
orthoscopic).   Here, the term in its simplist form means that all lines in
an image that are straight and/or perpendicular in the original subject
remain so in the perception of the stereoscopic image.  Orthoscopic is a
much more general term, in that it can apply to a flat "monoscopic" image as
well as a stereoscopic one.

> When a stereo x-ray of a right hand is mounted as a pair, say
> transparencies in a 5P mount, and viewed in a normal viewer,
> then you can see for sure if the 3D percept is ortho or
> pseudo, because pseudo mounting will result in the perception
> of a  _left_ hand. Viewing the slide from the back, or even
> upside down, will not change this property of the percept.

It will if it is also transposed (see pseudoscopic, above).  See also, my
original statement.

> It is also independent of the original positioning of the
> right hand for the x-rays.

Not at all.  It is very dependent on positioning.  If the right hand is
turned over and X-rayed, it will appear as the right hand as viewed from the
opposite side.   If the right hand is turned over and the images are flipped
(left side for right side), It will appear to be the left hand as viewed
from the same side.  If the right hand is turned over and X-rayed, and then
the images are flipped and transposed (left image for right image) it will
appear to be an image of the left hand as viewed from the opposite side.

> A similar reasoning applies to Peter Abraham's shells.

Similar to the above description.

> Peter said that some stereo x-ray pictures looked better in
> one way than in the other. I have also noticed this effect
> with many stereo x-rays, and I believe that it is a
> psychological effect.
>
> I must stress again that the subtle density, definition and
> magnification differences in x-ray images have hardly any
> practical influence on viewing stereo x-rays.

How do you know?  The brain is capable of extracting a surprisingly large
amount of information from very subtle differences.  Parallax differences of
a fraction of a millimeter can be interepreted by the brain as actual depth
differences.  Why not density, and/or definition, and/or magnification?

> John Rupkalvis:
> >Changing the aperture size mainly affects the apparent
> >sharpness, as a larger source area tends to make edges less
> >distinct.
>
> The source area of the x-rays is called the "focus". The size
> of this focus is not set by an aperture. All x-ray machines
> have a window in their shielding and also diaphragm, situated
> at some distance from the tube, to collimate the outgoing
> rays. A larger focus results in less definition, a wider
> diaphragm aperture deteriorates the images due to scatter.

A window is an aperture.  A diaphragm is an aperture.  To collimate is to
make parallel (as with optical lenses and visible light). Strictly speaking,
the diaphragm aperture does not collimate the X-rays, they remain as
radiating in all directions from the source, so that off-axis they are still
not parallel.  However, the diaphragm aperture does limit "cut-off" of much
of the off-axis radiation.  Changing the spacing of the diaphragm aperture
relative to the window aperture and the source, will change the amount of
the cone angle that is cut off, and therefore the apparent sharpness, since
the narrower the cone angle, the smaller the effective aperture (for a
specific distance) and the greater the apparent sharpness.

JR



 

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