Amiga Developer CD 2.1/Amiga Developer CD v2.1.iso/Reference/Amiga_Mail_Vol1/Misc/Stereoscopy

Text File | 1999-10-27 | 10.5 KB | 220 lines

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The information contained herein is subject to change without notice, and
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                       STEREOSCOPY AND THE AMIGA

                          by Tracy McSherry


Visual information comes to the brain from over seven million rods and
cones in the eyes that receive light through an adjustable lens and
aperture system.  All this information is processed by the brain in
real-time to produce an integrated image.  Visual depth perception
is part of that processing.  Depth perception is achieved through
visual cues intrinsic to a binocular visual system.


DEPTH PERCEPTION

These depth cues include Convergence, Binocular Disparity, Accomodation,
Motion Parallax and Pictorial Clues.

o  Convergence is a depth cue from the interocular angle formed by the
   eyes looking at an object.  Through trial and error over many years
   you have learned to estimate the distance of close objects by
   triangulation and feedback from the muscle tension and eye position
   of crossed eyes.

o  Binocular Disparity is the variance in images caused by the different
   path of light entering each eye.  Slightly different angles, sizes,
   positions, brightness and color values all result from the displaced
   geometry of each eye.  The two images are compared and fused and the
   difference is interpreted to give a depth perception.

o  Accommodation refers to the adjustment in focus by the lens
   muscles straining on the lens.  Here, feedback that results in a
   sharp image can also give depth information.  This doesn't get
   used much in computer displays since the image plane is constant -
   the monitor doesn't move.

o  Motion Parallax is a cue from the angular relative motion that is
   affected by distance.  The best example of this is driving down
   the street and seeing the stripes race by while the far away
   objects appear to move slower.  Motion Parallax provides a very good
   3D effect on a computer display and does not require 3D glasses.

o  Pictorial Clues include matting or overlapping, shading, horizons,
   perspective, scaling, texture, blurring, contours and shadows.
   Pictorial Clues also work well in computer applications and do not
   require special glasses.

You can demonstrate the effect of some of these depth cues on yourself.
For instance, hold your finger straight up in front of your face.  Now
close your left eye keeping your right eye open.  Switch eyes and
your finger should appear to jump.   The farther away the object, the
smaller the jump.  This is called parallax - the separation of the eyes
gives each a slightly different image and perspective.

By trigonometry we know that given one side and two angles, any triangle
can be defined.  The distance between a person's eyes remains constant,
so the brain learns to use the relative angles of the eyes as a clue to
how far away an object is - this is the convergence depth cue.

For instance, when you look at an object which is very far away, there
is no pronounced difference between the two images from each eye.  The
interocular angle is close to zero and the eyes look out almost parallel
to each other.  The brain uses all this information in gauging depth.

Here is another example you can try.  Hold one finger up at arms length
and the other halfway in between.  If you look at one finger, you see
two images of the other.  Since the eyes can only focus and cross at one
distance at a time, anything out of focus must be at another depth.  The
brain uses this as another depth cue.

Another example you can try is called the "floating finger".  Put your
hands one foot in front of your face with your index fingers pointing at
each other.  Now look out at something past the tip of your fingers, at
least three feet away.  If you focus on the distant object you should see
a third finger "floating" between the other two.  Of course you can
concentrate and ignore one image over the other or shift focus and resolve
one image.  But this shows how easily the brain integrates dual images,
sometimes producing a new object that isn't really there.  By taking
advantage of the how the brain processes visual data, we can produce
realistic 3D effects on the Amiga.


3D EFFECTS ON THE AMIGA

Three dimensional imaging is implemented on the Amiga by creating two
separate views for each eye.  The two images are displayed alternately
with the Amiga's interlace mode.  The display is viewed through a pair
of stereoscopic glasses such as the LASER GAMESMANSHIP STEREOSIGHT GLASSES,
the HAITEX X-SPECS 3D visor, or a suitably modified pair of SEGA 3D glasses.

Using a field sequential approach, the liquid crystal shutters in the glasses
separate the interlaced display into a left view and a right view.  The
liquid crystal shutters are synchronized with the interlace screen so that
each eye sees a separate field - a noninterlaced medium resolution picture.

The brain integrates the two views, merging together the objects identified
as the same object by crossing the eyes until the overlap makes a single
object.  Since the eyes are crossed either more or less than they would be
normally when looking at the screen, the stereo image appears either closer
or farther away.

The stereo objects that merge also seem to be more in focus.  Other objects
remain doubled unless they have the same separation or effective parallax.
The brain assumes these other objects are out of focus and ignores them the
way it is used to doing.  Some people don't expect this to happen on the
computer screen when there are two objects at different depths.

Note however that the eyes focus on the screen surface as a single image
plane.  Since the computer screen does not move, the eyes will not change
focus as you look from one object to another.  Hence, Accomodation cues
cannot be simulated.  With stereo imaging we can only play with the
convergence.


SIMULATING CONVERGENCE

Independent control of what each eye sees makes it possible to simulate
convergence.  To see how this works, consider a few examples.  Think in
terms of a triangle with the base of the triangle the line between your
eyes. (Hopefully we can take that distance as a constant!)  If you don't
move your head, the distance to the monitor is constant.  Assume that the
bridge of your nose is directly in front of the center of the monitor
as in Figure 1.  In this case the image appears at the same depth as the
screen,  because both eyes see an identical image and the visual cues
correlate and reinforce the depth perception.


----------------------INSERT FUGURE 1 HERE-------------------------


By offsetting the two images so the eyes cross as in Figure 2, the
convergence indicates that the object is at the point where the eyes cross.
In this case the virtual image appears closer than the screen.  This can be
reinforced by giving a different view or perspective of each stereo image.
This is a good effect but crossing the eyes too severely, too often will
cause the eye muscles to strain and should be avoided.


----------------------INSERT FUGURE 2 HERE-------------------------


More common and easier on the eye is the opposite separation as in Figure 3
where the eyes cross past the plane of the screen.  In this case the virtual
image appears farther away than the screen.


----------------------INSERT FUGURE 3 HERE-------------------------


At the extreme, if the screen images are almost six centimeters apart,
the eyes are looking straight ahead as in Figure 4.  In this case the
virtual image appears at infinity.


----------------------INSERT FUGURE 4 HERE-------------------------




ON MAKING 3D IMAGERY

The nice thing about doing stereoscopic images on the AMIGA is the number
of 3D rendering packages.  All of them can be used to generate stereoscopic
images, by moving the camera or the observers position.  Unfortunately none
of them currently fully support stereo imaging and so they allow you to make
embarrassing mistakes.

To get the best results, the camera movement should be small and understated
to avoid inducing eye strain or confusing the viewer.  Excessive parallax
will prevent comfortable viewing and is usually unnecessary.

Use stereo effects with discretion.  If you did a stereoscopic drawing
right now, would you put everything in the drawing in 3D?   You only want
to make the focal point of the picture finely detailed - this could include
stereo effects but don't apply 3D to everything just because you've paid
fifty dollars for a pair of glasses.

Proper use of stereo images can make a complex screen display appear
simple by allowing the eyes to look at it a layer at a time. But
remember, there is no magical way to increase the amount of information
the interlaced display puts out, so keep in mind that an increase in depth
perception costs in detail or resolution.  Conversely we can provide more
information at different depth levels if detail is not important.

For CAD and circuit layout, depth information is critical and can make life
much easier.  For a nice picture of an orange, color and resolution are more
important.  To create realistic action and interactive entertainment, a
compromise is in order.

When doing three dimensional or stereo displays you have to take care to
avoid inducing eye strain or confusing the viewer.  Try crossing your eyes
and then looking at something.  You'll notice a fairly long period of
unfocused viewing.  You don't want to quickly change the separation of
your stereo images or you might induce the same effect.

Avoid conflicting information.  Big cars and small buildings will do more
to give wrong information than you can compensate for by stereo images.
It will always seem that the cars are closer than you want them to be.
Putting an object in front of another object means it is closer, no
matter what other clues you add.

A temptation exists to try to use stereo pairs as the only clue to depth.
You need to remember that although you can get depth information from
many different types of cues, the best perception of depth is achieved
when several are used in conjunction.  Size, shading, perspective and
peripheral clues are just as important.  Careful use of each will
maximize the effect and really enhance your applications.

With careful manipulation of objects and experimentation, you can create
stunning stereoscopic images and given, the power of the AMIGA, even
stereoscopic animations and games.