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The normal human visual perception is binocular, that is with two eyes. Everyone of these two 
eyes is already a perfect visual mechanism,  for this everytime we watch an object that is in 
our field  of vision,  we  really look at it twice:  once with the left eye and once with the 
right eye.  As our eyes are on the face at a distance of about 2,5 inches between them, every 
eye  looks  at the same object  from a different perspective angle.  At this point  our brain 
overloads  two images,  the one from the left eye and the one from the right eye,  and merges 
them in a single image,  melting the identical parts and inserting in an intelligible way the 
differences that results  between them.  Scientifically  this process is called "Stereopsis", 
that is the merging  of two images.  Thanks to this our brain  can  build a three-dimensional 
sight of an object,  starting  from the two bidimensional  sights that our eyes produce while 
look at the object from different perspectives.  When we have a three-dimensional sight of an 
object, we consider not only its width and height,  but also its depth,  that is the distance 
where  the same object  is  in the space  compared with our position.  The term "Stereoscopy" 
means litterally "Spatial Sight", as etymologically it is formed from 2 greek words "Stereo", 
that means "Space", and "Skopein", that means "To See". The phenomenon of binocular sight was 
discovered for the first time in the 1838  from the english physicist Charles Wheatstone, who 
invented so the stereoscope:  a mechanism  that reassembled  with a mirrors system two images 
placed  at a few  of inches between them and showing the same object, but with a visual angle 
slightly different,  succeeding  in this way in obtaining  the sensation of spatial depth. In 
the year 1839 was discovered the photography and quickly it was evident that the human eye is 
like a camera.  So it was possible to obtain some images that were virtually an exact copy of 
those produced from anyone of our eyes. This was made with a particular stereographic camera, 
with  two  lenses  put  about 2,5 inches  of  distances  each  from  other.  Each lens made a 
photography of the same object but from a different angle, as for human eyes. Afterwards with 
particular photo viewers  or  also only with a special viewing technique,  it was possible to 
rebuild,  starting  from these two photos,  a three-dimensional sight of photographed object. 
The phenomenon  of photographic stereography became fashionable and for a long period was the 
centre of popular attention, after, slowly, it fell into oblivion.  Today it is possible with 
computer  to  make  some  images  of the same object  with  different  perspectives  as those
visualized from our eyes.  So we can realize  stereoscopic images  as those produced with the 
ancient cameras with two lens that so much success had in the past century. Who in interested 
to make  such images with the computer,  can read the appropriate Tutorial I have written for 
this.  A particular facet of Stereography is the one of the visualization of the steregraphic 
images,  so that it was possible to obtain a single three-dimensional image,  starting from a 
pair of bidimensional images.  In the past century  were special viewers and it was very easy 
make this sight. Today,  on the contrary,  it is very difficult to use them, because they are 
not more produced and marketed.  But it is equally possible  to see correctly a stereographic 
image, making use of some particular techniques,  without the help of extern tool, but making 
use exclusively of our eyes. The techniques are different, but two are the main techniques: 
The first, called "Parallel Eyes Vision",  consists of drawing your face 
near the monitor,  with the closed eyes,  till you skim  the screen with 
your nose.  After you must open the eyes,  looking  at the two seemingly 
identical images you have in front of you,  without focusing the look at 
the center  of the screen,  slowly going away from it,  with the relaxed 
eyes  that must be  in the starting position,  trying to look at a point 
that is  beyond the screen  you have  in front.  After a few the objects 
that are  on the screen  will seem as they had blurred borders and after 
some other moment  a third image  will appear at the center of the other 
two images  that  are  in front  of you.  This third image  will  become 
visible in the air, at a middle distance between you and the screen, and 
it will be three-dimensional, as an hologram, with a strong character of 
depth in some  of its parts.  Therefore this first metod  is realized in 
looking  at the images  as if we want focus a point that is a lot beyond 
them. In this way the lines of the sight of our two eyes go on the image 
as in parallel. Image on right will help you to understand this concept:
The second technique, also called "Cross-Eyed Vision", consists of trying 
to focus  our look  in a middle point  between our eyes and the couple of 
images that are on the screen. With this technique you will need to place 
the left-eye image on the right and vice-versa.  We can help ourselves to 
focus this middle point using our index finger,  fixing our look on it an 
so moving it toward and far away from the images on the screen, until the 
stereographic  three-dimensional image  will  not appear.  So this second 
method consists of looking the images,  crossing our look on a point more 
near us in regard  to the same images.  In this way the lines of sight of 
our two eyes  will go crossed on the images.  The image here on left side 
will help you to understand this concept.  Until now we have examined the 
Basic Stereogram,  alkso called Autostereogram,  composed  from a pair of 
images placed  side by side,  each of them 2,5 inches wide and apparently 
identical,   but  really  slightly  different   in  relation  with  their 
perspective point of view.  A such stereogram  is the faithful simulation 
of the three-dimensional human workings of viewing. 
We must also  shortly  remember that  afterwords  other kinds of stereograms were discovered:
- RDS, that is the abbreviation for "Random Dot Stereogram".  They  were published  the first 
  time in 1972 from Dr. Bela Julesz of MIT.  An RDS is a stereogram  that looks like a random 
  dot pattern without any dditional information, while if you observ them in a particular way 
  you will see a single three-dimensional, rational and definite image. 
- Shutter Stereograms, where it is applied  a technique  called  "Tachistoscopic".  With this 
  tecnique are generate  images  for the left  and  right eye  which are separated by shutter 
  glasses. The shutter closes one eyes's sight while the other one gets the proper image. The 
  speed  of the alternate shuttering  of the images  is  so fast  that  it is  not noticed at 
  conscius level. It is generally used by some virtual reality systems. 
- Anaglyphs, Polarization Filters: It is a stereoscopic method based  on different colored or 
  polarized images.  A pair of particular spectacles  gives each eye the right information by 
  special filter glasses, making a three-dimensional sight of the image. 
- SIS, that is the abbreviation for "Single Image Stereogram".  They were published the first 
  time in 1979 from Christopher W. Tyler. Today they are the most common form of stereograms. 
  They are formed  from a single image  and  not more from a pair of images as the steregrams 
  until here checked.  The Single Image Stereogram divided themselves  in several subspecies: 
  - SIRDS, shortly "Single Image Random Dot Stereogram". It is like the RDS, but start from a 
    single, colored or black and white, random dot pattern image.
  - SIRTS, shortly "Single Image Random Text Stereogram". It is like the SIRDS, but it is SIS 
    using ASCII text for two dimensional image information. 
  - SITS, shortly "Single Image Texture Stereogram".  It is like  the SIRDS,  but it is a SIS 
    using a texture instead of a random pattern.
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I have written this page in Italian language and after I have translated it in English language.
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