3D Imaging


STEREOGRAPHIC  IMAGING
IN
THEORY AND PRACTICE

by
Terry Blackburn

Note:  This document is not complete.  Many reference figures are missing and some sections are incomplete.  I will finish them as I have time and material.  I promise!   tb!
    TABLE  OF CONTENTS:
          i.  Introduction
    1. What is Stereographic Imaging?
    2. Stereoscopic Theory
    3. Stereographic Presentation Methods
    4. Creating Stereographic Images for Distribution via the Internet
    Introduction:
      Stereographic imaging is at once art, science and practiced technique.  We take three-dimensional imaging so much for granted that we sometimes forget the intricacies and difficulties associated with presenting 3d images.  This document is designed to present an overview of the theory and techniques of stereographic imaging.  To the veteran, the examples presented are standard fare, easily observed images.  To the first timer, it is impossible to view the examples without instruction in the equipment and techniques necessary to realize the images.  It's a two-edged sword.  The examples in the section on stereographic theory depend on information only provided much later in the document; specifically, the viewing of anaglyphs and stereo pairs.  If you are new to stereographic imaging, you may wish to initially skip ahead to the section on Stereographic Presentation Methods.  This section will help you acquire the necessary knowledge to view the examples presented in the section on Stereoscopic Theory.  If you find this document interesting and helpful, you're welcome!
    What is Stereographic Imaging?
Rules of Composition  (the practical stuff...)
In addition to the standard rules of photographic composition, stereographic photography introduces another set of basic rules to be considered in addition.  A basic rule of stereographic imaging is to include foreground objects in the picture to optimize the characteristics of the technique.  Otherwise, there's no point in taking a 3d picture, is there?  Foreground objects are important because the effect of depth falls off dramatically as distance to the subject increases.  In general, 3d scenery pictures shooting off to the horizon are ineffective.  Sometimes the desire to capture a scene is so overwhelming that we ignore good judgment and snap the picture anyway, and are disappointed with the results later.

A simple technique can be employed to enhance depth and turn a less than ordinary shot into a spectacular demonstration of depth.  Estimate the distance to the closest object in the scene.  Then, determine the ideal placement of the object in the scene.  Suppose the closest object is at a distance of 100'.  Ideally, an object within 5' to 10' feet is desired.  Let's place the object at 7 feet.  By extending the baseline we increase the convergence angle and reduce the apparent distance to the object.  Using simple ratios we can compute a new baseline using the following equation -  100 / .21  =  7 / x.  The value .21 is 2.5 inches converted to feet.  100  /  (.21 * 7) = 1.47 feet or slightly less than 18 inches.  The psychological implications of this technique can be extrapolated further by comparing the new baseline to your own height.  If you are 5'10" looking at an object 7 feet away, then an object 100 feet away with the same convergence angle would proportionately make you 83 feet tall!!  That's why the objects in the image appear smaller with an increase in separation.

This technique will drive purists nuts, and my apologies.  Art only imitates nature.  It doesn't necessarily reproduce it verbatim.  There are obvious drawbacks to this technique as well.  Placing a very large object virtually too close within the observation space will cause convergence difficulties.  The brain is well experienced at quickly guessing and presetting the convergence angle based on the image contents.  Too much stretching of virtual depth can result in 'fishing' as the eyes search horizontally attempting to converge at expected depth rather than the actual virtual depth.  This is not a good thing.  Techniques for avoiding this will be discussed later.
Oh, by the way...
Your eyes can be trained to do all sorts of amazing tricks.  There are, however, a few things your eyes could do for which there is no practical application.  And, in fact, if they do some of these things, a trip to your local ophthamologist might be in order.  Your eyes are designed to work together on the same horizontal plane.  If one eye were to look up and the other look down, for instance, really bad and probably painful things are happening.  Your eyes only do things that make images converge, and this situation just doesn't occur in nature.  

Divergence, or 'wall eyed' behavior is anomalous as well.  It is actually possible to force a small degree of divergence under controlled conditions, but more than a few moments in this situation will result in a massive headache.  It is not recommended even for brief experimentation.

Why do I mention these issues?  Because in designing and presenting 3d images it is important NEVER to expect these things to happen.  An image that is not properly aligned both horizontally and vertically will cause fatigue, pain, and general ocular discomfort.  It is important to have a solid understanding of the mechanics involved so your images can be fine tuned for optimal viewing effect AND comfort.

1I asked my friend Ralph.  He is an ophthamalogist.