Cardboarding and JPG

[P3D Larry Berlin <lberlin@xxxxxxxxx>]

> John Urbanic writes:

>I can do some simple, back-of-the-envolope, math that demonstrates that
>foveal resolution far exceeds (almost 2 orders of magnitude) JPEG resolutions
>for anything 50% compressing a 1 pixel vertical edge of a 640x480 pixel
>image displayed on a 17" monitor at 30" viewing distance.  It's basic
>trig plus the knowledge that a 50% compression on a vertical 1:5 pixel lined
>image results in a 3 pixel (2.6 mean extension rounded to 2) line.  A good
>test case if one wants to actually quantify things.  Given this awful loss
>off parallax information, you get considerable cardboarding.  Stereo-base
>width can also certainly effect things, but that is basically competant
>photograpahy.  I take that for granted, although I do appreciate the
>tremendous skill that it takes.  If anyone doubts my logic, a simple
>experiment would be to display an image that you believe has no cardboarding.
>Increase the lossy compression and see if such a thing appears.  Let us know
>your results.
>


****  I don't give two cents for what you work out on the back of an
envelope. Sure, the math provides some insight into the whole situation, but
it doesn't prove cardboarding comes from JPG compression AT ALL. No matter
how often you do your calculations the image still has to be viewed with two
eyes. That's where my information comes from, direct observation. It's fine
to know the math is there, but it's meaningless until applied to the images
themselves. 

Your argument goes more in the direction of proving that a computer monitor
screen lacks resolution, which is no surprise to anyone. In the process of
getting a stereo image to that screen compression proves useful at several
stages. Compression itself can degrade a stereo image, but in the case of
JPG, retinal rivalry is far and away the major culprit. It takes a lot of
compression before cardboarding might be created by compression alone. The
averaging processes themselves don't degrade the edge factors as much as
ruining surface textures with those ugly squares. Yes, they can degrade
parallax, of which cardboarding is an example, but the picture is ruined
long before cardboarding alone becomes noticeable, compared to the same
image in a standard TIF format viewed on the same monitor.

I see cardboarding in photos and slides too. They are known to be superior
in terms of resolution to anything you do on a monitor. If those specific
images are so marginal in film, guess what happens on a computer screen! It
still has nothing whatsoever to do with compression as even a high res TIF
image would show cardboarding. Compression might accelerate things, but
there are other techniques that can deal with the cardboarding issue aside
from compression.

You suggest comparing file examples and letting you know the results. Where
do you think I was speaking from? I gave you my response to that in the
original post. I've created hundreds of images and done all kinds of things
while viewing them. Compressed to varying degrees and different techniques
and using different programs, placed in various file formats, viewed as
anaglyphs, interlaced and page flipped or freeviewed. I tell you, based on
experience, not envelope calculations, compression is not the cause of
cardboarding, though it might contribute to it to a degree, but not a
sufficient degree that it gets used as an excuse to not use JPG for stereo
files. It makes a good argument for not using large amounts of compression,
but to avoid compression completerly, that's absurd.

Compression tends to maintain parallax factors, though partially obscuring
them with the smear effect. The eyes don't require a completely solid line
to determine from neighboring pixel groups that a line exists. Sure, I'd
like better resolution, but I don't get cardboarding from a slightly less
defined line. Only straight perpendicular lines might disappear during
compression. The diagonal ones tend to suggest themselves from somewhere
along their extent. Artists have used and proved over and over again the
power of suggestion relating to detail. Stereo is a more exacting
enviroment, but if you *suggest* to both eyes, you've done the job.

Larry Berlin

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


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