Re: Converging lenses (was: Window reversal); D|s-TortiO|\|s

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

>I've had the same experience. I know what Keystoning does in 2D, but 
>I'm unclear what it causes in 3D, because the pictures I've seen that 
>everyone says have it, display no distortions to me at all!
>
>So what _is_ the effect in 3D?
>
>Gary N.

Okay, just imagine you introduced keystoning in a 2D image, and 
distorted the perspective. This "distortion" causes no problems
and is used accentuate or decrease the perspective effect, as shown
in the ascii figures below. A, B, or C (borrowing from Greg's ascii pics)

Note: The ascii pics shown below should be rotated 90 degrees
clockwise, to properly convey the concept. Thus in figure B,
the large side would be on the right, for example.

A)  no perspective distortion (picture taken, head on)
B) "Postive" perspective distortion (picture taken at an angle)
C) "Negative" perspective distortion (picture taken at another angle)

Now if you take the second snapshot, but move it sideways,
the degree of keystone distortion is the same as in the first
image. That is, if the first image is distorted as fig.A, the
second image will still be as fig.A. and so on. B,B or C,C. 
This causes no problems.

IF you move the camera sideways and toe-in, you introduce a
different amount of keystone distortion from the first image,
resulting in decreasing the 3D effect in general. The results
would be any combination such as, (A,B) (A,C) (B,A) etc. 


      A                  B                   C       
 -----------        ------------          ------     
|           |       \          /         /      \     
|           |        \        /         /        \    
|           |         \      /         /          \   
 -----------           ------          ------------   

P.S. Greg W., I loved your rundown on the origins of the word keystone.
     I knew about the keystone used in arches but I didn't put 2 and
     2 together with the classical explantion of keystone distortion!
     Thanks also to Paul T. from the explanation of where the Keystone
     Company got there name. I always wondered about that! :-)
     And thanks to Paul Pascu, who if it wasn't for him, I don't think
     we would have gone into this as far as we did! ;-)

Gabriel, who is always learning something from this great list!


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