Resembling 3D

[P3D Larry Berlin <lberlin@xxxxxxxxx>]

>Date: Mon, 01 Sep 1997
>From: P3D Guy Fiscus artist & internist writes:
>..............
>If my question is simplistic you can thrash me but: Why do we see
>something resembling a 3D image when two identical images are juxtaposed
>and viewed in the crossed modality?


There is a definite sense of difference in the experience of fusing two
identical images as opposed to using both eyes to look at one of those
images. It does not make the image 3D however. You are using the eyes in the
same way, that is decoupling convergence and accommodation, but the
information your eyes are receiving contains absolutely no parallax other
than perhaps minor distortions in your view caused by the crossed eyepath. 

Set up two sets of images on your computer screen and view both at the same
time. In the first one pick out a nice stereo pair with good stereo
properties. In the second set, use just the left image and repeat it twice.
Now crossview and look from one to the other. Since you are viewing both in
the special mode of crossed viewing, you can equally compare 2D with 3D and
disregard environmental effects. You will readily observe the unique
difference between 2D and 3D.

Note that not all computer screens place the pixels in exactly the right
proportionate spacing across the screen. You may observe this inaccuracy as
depth factors in the 2D version, but which aren't in the actual images. It's
easy to test your entire monitor screen for accuracy in this respect. Put up
a wallpaper pattern that you then freeview. As the pattern repeats across
the screen it is still the same pattern with identical pixel counts and
other proportions. In theory, this should be seen in stereo as a perfectly
flat surface some distance in front of or behind the monitor. (depending on
crossed or parallel viewing)

Most monitors tend to bulge forwards in parallel mode, right in the center.
That means the pixels are crowded closer together near the center. Others
correct the display by various means and obtain a more accurate screen
display which is flatter. The pincushion control can fix a certain small
amount of error, but tends to leave the center alone and twist the edges
forwards or back, sometimes creating extreme stereo distortions.  

In 2D these effects are not noticeable. Learn what your screen does, so you
can properly interpret stereo images and recognize when the screen
distortion is present and pretending to be a part of the image you are viewing. 

It pays to perform this test on monitors if you are shopping for a monitor.
Many common brands are terrible in stereo. In one store I visited, every
single monitor had appreciable distortion, even major brands. After shopping
around I did find a couple brands that provided much better results. Price
alone won't tell you about this level of accuracy. They probably don't test
their products in this fashion, or if they do, they hope you won't know how
to. ANY wallpaper repeated pattern works.

Larry Berlin

Email: lberlin@xxxxxxxxx
http://www.sonic.net/~lberlin/
http://3dzine.simplenet.com/


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