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

[abram.klooswyk@xxxxxx]

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.

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

This all follows from the funny laws of asymmetry, direction 
of deviation and transposition of stereo images (some time ago 
I have written on deviation direction to the list). 

A similar reasoning applies to Peter Abraham's shells. 
Since we know they all have a clockwise spiral, we can tell if 
a certain side-by-side presentation (for parallel viewing) is 
pseudo or ortho, and this does not depend on the original 
direction of the x-rays, nor on turning of the pair (as a 
_pair_, for horizontal left-right turning of both _single_ 
images is the same as transposing them, of course).

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. 

Obviously the information content of stereo x-rays does not 
change whether viewing is ortho or pseudo. 
But I have several stereo x-rays in my collection (some day 
I might scan a few) for which most observers prefer one of 
the two viewing types. A typical example is a stereo x-ray 
of a pelvis with a calcified myoma of the uterus. Viewing 
this from the front is preferred. Viewing from the back, 
through the bony pelvic structures, is quite possible, 
but unpleasant. 

It seems that the preferred viewing direction (ortho or pseudo) 
depends on the structures with are most interesting or conspicuous, 
with a strong bias for having them closest in the 3D percept.

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. 
(I drop this x-ray technique subject, it is too much off topic 
for this list.)

Abram Klooswyk


 

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