Re: Ghosting

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

"Boo" (AKA Casper) writes:

>There was some recent discussion regarding inefficencies of polarizers
>introducing ghosting in projection 3d. All I can say that so far 
>everything mentioned is all wet. Increasing the ambient light as was
>mentioned helps to decrease ghosting but higher output lamps used in 
>projection doesn't contribute to ghosting but helps scare them away.

I couldn't decide if you were joking or not, playing on bright light
scaring away ghosts.  When I read you sig, I was sure you were.  But
you also say this:

>A good analogy would be anaglyphs displayed on monitors. There is 
>similiar problems as in polarized 3d projection.

OK, let's review how anaglyph works.  The information for the left
eye is displayed or printed in blue on a white background.  The
information for the right eye is displayed or printed in red.
You wear a red filter over the left eye, and a blue filter over
the right.  The red filter only passes red light, thus turning the
white background red.  The red (right) image disappears into the red
background and is not seen by the left eye, but through the blue
filter appears black, since the red ink reflects no blue light.

Similarly, the blue image disappears into the blue background, and
appears black through the red filter.  Where the blue and red images
overlap, the image appears black to both eyes.  So now you see a black
stereo image on a ground that appears red to the left eye and blue to
the right, causing major retinal rivalry. :-)

The problem with ghosting in anaglyphs is due to a mismatch between
the color as reproduced by the monitor, and the color of the filters,
right?  If the match isn't perfect, some of the off-red light (for
example) is visible through the red filter and is seen by the wrong
eye.  This would be uniform across the image, though, unless your
monitor is displaying the single color differently across the image,
or the color in the image varies, or your filters vary across their
surface.  I'm going to assume that the problem is just a color purity
problem, because the others are broken hardware or software.

So, in my opinion this is more like de-polarization, in that it affects
the entire image equally (like having the polarizers not quite at
ninety degrees to each other).  Ghosting, as far as I can tell, occurs
because of light leakage also, not because of depolarization, but
incomplete extinction.  It's a minor point, but there is a difference.
You notice it where a bright object as seen by one lens overlaps an
adjacent dark area as seen by the other lens.  It may resemble the
color purity problem in an anaglyph because the common (black) image
areas mask the problem.

Since I think you weren't joking, I'd appreciate some elaboration
about why you believe brighter lamps decrease ghosting.  If I understand
the problem correctly, they should either make it worse, or at best,
not make any difference at all.  I'd really like to understand exactly
what's going on, and what the best "fix" is.  If you have information
that contradicts anything I've said, please provide it!  Thanks.

        -Greg W.


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