Re: Random Dot Stereograms

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    |                    /  \_  /     @)  ''//_ //
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: 	I'm sorry, but just what exactly am I supposed to be looking
: at? Is it encoded?
Maybe this helps
    Humans have two eyes, roughly              Critical Design
 level and separated by only a few                 Feature
 centimetres, but it is this 'design
 feature' that lets us see in three               ->|   |<- 6-8cm
 dimensions.
                                                   |||||||
    The key to stereoscopic or three              /       \
 dimensional vision is the ability               | (o) (o) |
 to 'line up' the separate views of              |    .    |
 an object seen from each eye to form             \  \_/  /
 a single image.                                   \_____/

    Normally, when looking at something, the brain is presented with two
 images, one from each eye. In order to make sense of the information it
 is receiving, the brain alters the angle of the two eyes until the images
 overlap. The brain then uses the muscles that surround the eyeballs to
 alter their shape to create a sharp image. Using the eyeballs to provide
 a sharp image is an independent action from lining up the eyes and does
 not contribute to the 3D effect, just the clarity. This is known as
 focusing. The brain then measures the angle between the eyes and using
 simple trigonometry, calculates the distance to the object in focus. The
 point at which the two images overlap is called the focal point and the
 distance is referred to as the focal distance.

    The two pictures below show how the eyes normally see an image on a
 monitor. The right-hand '3-D' seen by the right eye overlaps with the
 right-hand '3-D' seen by the left eye (same for the left-hand one) and
 the brain locks the eyes as the image is focused. The diagram on the left
 shows the angle between the eyes whilst focusing on a screen image. The
 diagram on the right shows what the brain sees and what distance it
 perceives it to be.


 screen       ________3-D___3-D________        ________3-D___3-D________
                          .
                        .   .
                      .       .
                    .           .
 eye level        <O>           <O>                <O>           <O>


    The problem for the brain is that it can only focus by comparing the
 two images that it sees. It needs two images of an object to focus upon it
 properly (making it difficult to focus on a point in mid air). Normally,
 the brain is only presented with two images of each object, so is able to
 overlap them easily. It is when the brain is unable to decide at what
 point the images should overlap that it can be fooled.

    As stated earlier, aligning the two eyes onto an object and obtaining a
 sharp image are separate functions for the brain. This means that the eyes
 can view an object clearly without locking onto it.

    If we use the same two images as above, the brain will normally focus
 correctly on the screen. However with a little training, it is possible to
 let the eyes 'drift' outwards, so that they are effectively focusing on a
 point *behind* the screen.

    The best way to learn how to 'unfocus' is to stand at arms' length in
 front of a wall, keep your elbows against your chest and hold both hands
 in front of your face, with both index fingers extended vertically.

    If you focus on your fingers,             _           _
 you should simply see them both             |_|         |_|
 as shown on the right. The wall             |L|         |R|
 behind will be blurred and out              | |         | |
 of focus.                                  Solid       Solid

    Now focus on the wall. You will         _   _       _   _
 probably see four translucent             |_| |_|     |_| |_|
 (semi-transparent) fingers (don't         |L| |L|     |R| |R|
 worry, they're alright!). This is         | | | |     | | | |
 because the brain is now aligning
 the images of the wall instead of      Translucent   Translucent
 your fingers.

    Next, keep focusing on the wall,         _      _      _
 but step backwards or forwards             |_|    |_|    |_|
 until the two centre fingers meet          |L|    |L|    |R|
 and overlap, forming a solid finger        | |    |R|    | |
 in the middle. Even though you are
 still looking at the wall, it should        ^      ^      ^
 be possible to see the 'combined'           |    Solid    |
 finger clearly.                             '-Translucent-'

    It is this skill, being able to look behind an object but still keeping
 it in focus, that allows the brain to be fooled into seeing 3D pictures.

    Returning to our screen with our new-found (and practiced!) skill, we
 allow the eyes to focus behind the screen as shown below. Initially, the
 brain sees two different images. However, as the eyes focus further back,
 the right-eye's view of the left-hand '3-D' and the left-eye's view of the
 right-hand '3-D' overlap (Yes, go back and re-read that again!). At this
 point the brain becomes confused. It sees a solid combined image at the
 centre of its vision (the only bit it is interested in) despite the rest
 of the image being jumbled. If the viewer holds his or her eyes in that
 position, the brain simply resolves the paradox by deciding that it has
 locked on correctly to the object in front.


 virtual screen                                ________3-D___3-D________
                          .
                         . .             3D image->       3-D
 real screen  ________3-D___3-D________
                       .     .
                      .       .
                     .         .
                    .           .
 eyes             <O>           <O>                <O>           <O>


    Now convinced that it has the correct lock-on angle, the brain will
 focus the eyeballs for a clear image and then recalculate the distance to
 the object, but with an incorrect angle. Because the eyes are at a wider
 angle, *everything* appears further back (although it might not seem that
 way) but more importantly, the combined image appears in front of all the
 other screen images - the brain appears to view everything unfocused as
 being even further back.

   It is worth noting again that crossing your eyes achieves a 3D effect
 in reverse. This is not advised because the eyes have difficulty in
 focusing on the two images and if the wind changes, you'll stay that way!

And now...
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