My Turn Now...

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

Bob's question has some of us practicing our ASCII skills!  Here is my
version (in the most elaborate ASCII drawing I have ever made!!!) 

There are four different placement combinations for a stereo pair:

A: ortho & straight shows "W" at window level and "9" behind it.
B: is the inverted version of A but still ortho with "6" behind "M".
C: is the pseudo version obtained by cutting and transposing A (now "9" is
   in front of "W" which is still at window level.
D: is both pseudo and inverted, "6" is in front of "M".


     A         A                      B         B         
 |-------| |-------|   <------>   |-------| |-------|
 |   9   | |    9  |    invert    |   M   | |   M   |
 |   W   | |   W   |    *pair*    |  6    | |   6   |
 |-------| |-------|   <------>   |-------| |-------|
A: Ortho & Straight \            B: Ortho & Inverted
                     \          /
     |         |      \        /      |         |
     |   cut   |        invert        |   cut   |
     |    &    |         each         |    &    |
     |transpose|         chip         |transpose|
     |         |      /        \      |         |
                     /          \
     C         C    /            \    D         D
 |-------| |-------|   <------>   |-------| |-------|
 |    9  | |   9   |    invert    |   M   | |   M   |
 |   W   | |   W   |    *pair*    |   6   | |  6    |
 |-------| |-------|   <------>   |-------| |-------|
C: Pseudo & Straight             D: Pseudo & Inverted


It is interesting that each placement can be obtained from each of the
other three by a combination of inversion/transposition... For example, D
can be obtained by inverting C as a pair, or by cutting and transposing B,
or by inverting each chip separately from A.

It is also interesting that inverting the picture *as a pair* does not
create a pseudo but inverting *each chip individually* does give pseudo. 
As we know, each lens in a stereo camera inverts the image.  The result is
pseudo on the film.  If the real scene looks like A, the film records D. 
To go back to normal stereo we first turn the film upside down to remove
the inversion (going from D to C) and we then cut and transpose the chips
to get ortho (going from C to A).

As Greg said: "It's not simply a matter of which image is "left" or
"right"; it's the position of homologous points that matter."

BTW, Bob, "pseudo" is a Greek word... it means false.  It does not
stand on its own.  The proper word is "pseudoscopic" but Americans
like to abbreviate long words and have reduced it to "pseudo" the
same way that "stereoscopic" has been reduced to "stereo".  The 
opposite of "pseudoscopic" is "orthoscopic", "ortho" means correct.

George Themelis, Greek pseudolinguist and orthostereographer :-)


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