P3D The Stereo Window by DrT

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Eddie Bowers asked about the stereo window.  IMO, this is an very 
important aspect in stereo imaging that is frequently overlooked and 
not very well understood by beginners.  The nature of the stereo 
window is such that it is better understood seeing a few stereo images 
and it is learned to apply by practice.  So words cannot help much.  
But once the skill of setting a stereo window is mastered then stereo 
presentations can be greatly improved.  So by all means, EXPERIMENT!!!

As I mentioned a few postings back, the "Stereo Window" is the subject 
of my current "Tutorial in Stereo Photography" in the "Stereogram".  
To explain the concept I follow these steps:

0. Define the window as a universal (2D & 3D) concept, being simply 
   the boundaries of the image (like in a movie theater, the screen is 
   the window inside which the action is taking place).  In stereo of 
   course the window is more important because of the element of depth 
   and an object can be placed behind, in front or going through the 
   window.
 
1. Explain how a REAL window works:  When looking through a real 
   window, the left eye sees less in the left side and the right eye 
   sees less in the right side.

2. Apply a stereo window in a flat image:
   - Start with a far away scene with no depth in it.
   - The right and left parts of the stereo pair are *identical*.  
     Hence, no stereo. (Or, the stereo window is "at infinity")
   - To simulate the real window, crop a bit of the left side of the 
     left image and a bit of the right side of the right image. This 
     last operation results in stereo window in the far away scene 
     since the left eye sees less in the left side, etc.
   - Show how obscure the image looks if you crop the other way 
     around, essentially placing the flat scene in front of the 
     window.

3. Generalize from #2 on how the window is set by trimming or cropping 
   (pushing/pulling the film images under a fixed mask).

This much is covered in the current issue of the Stereogram.  The rest 
will be covered in the next issue:

4. Get into the "metrics" of the window.  Can a stereoscopically blind 
   person set a stereo window?  You bet!

5. Explain concepts like built-in windows in stereo cameras, the 
   window in projection, etc.

I hope to have a tutorial in next year's NSA convention where I will 
follow the same methodology but with very nice examples in projection. 
I am working on a method that will allow me to move the stereo window 
in "live" stereo projection.  I plan to do that by trimming the top of 
RBT mounts to access the pin bars that slide the chips horizontally.  
In my Brackett Dissolver I can access the top of the mounts for the 
slide that is being projected, plus I can quickly switch between two 
stereo images (through the dissolve function of the projector).

Look for this tutorial in next year's NSA convention!

George Themelis