[photo-3d] Chromostereopsis

[Gabriel Jacob <3-d@xxxxxxxxxxxx>]

Dale replied he doesn't wear glasses. This was in response to my
inquiry after he reported he perceived the blue squares being closer
than the red squares. See,
http://groups.yahoo.com/group/photo-3d/files/3d/chroma.gif
This was viewing the image directly and not freeviewing.


I checked the Internet and found most sites mentioning that people
will see the red closer. A few however did also mention that some
people will see the opposite (blue closer). They didn't shed any
more information on why and what percentage. There was also
some confusion or ambiguities on some of them. What I could
figure out was the red/blue apparent  distance manifests itself in
different ways.

1) Viewing the red and blue squares with one eye, the red boxes are
supposed to look closer to everyone concerned. The reason given is
due to chromatic aberrations of the eye. The blue box being a shorter
wavelength, will get refracted more than the longer wavelength red
color. This will cause the red square to look sharper and closer. The
blue will look slightly out of focus and farther away. These are
evidently monoscopic depth cues.

2) Viewing the same red/blue squares with two eyes, still not fusing
the stereopair, another additional depth cue is introduced. Namely
a stereoscopic depth cue. This stereoscopic depth cue is again due
to chromatic aberrations but the depth mechanism at play here is the
parallax introduced due to the color fringing.


This still doesn't explain why Dale and some others see the blue
boxes as appearing closer. After some experimentation, I think the
reason might be how one is fixating on the image. Looking at the
colored boxed with two eyes but using the outer parts of both pupils
will have a different effect compared to using the inner part of both
pupils.

I couldn't readily test this theory out since it seems to be an
involuntary reflex, so I tried to simulate this by using a punched
out hole of paper. I used a piece a few mm in diameter from pre-
punched loose leaf paper. The actual size is not really important,
rather it's the edges. In this simulation,  we're taking advantage of
the diffraction effect of the edges.

Take two pieces of punched out paper and place them in front of each
eye and look at one of the 3x3 matrix red and blue boxes (see link
above). Now slowly move the left paper to the left and the right paper
to the right, till you almost reach the inside edge of the respective
punched out holes. The red boxes should look closer.

Now move the papers the opposite direction till you reach the other
edge. Now the blue boxes should look closer. This is irrespective of
ones natural tendency to perceive the depths of the colored boxes a
certain way without the paper (one should certainly experience this,
since it was very interesting).

Without the paper, if our eyes fixate on the colored objects in a similar
manner, this might explain why some (most?) people see the red
closer and others the blue.

In the above example with the two papers, you can even try it with
one piece of paper, and/or use no hole but rather the edge of the
paper. Other ways include using your fingers, or even the side of
your nose (turn your head slightly sideways while maintaining your
gaze at the colored boxes)! The effect will not be as pronounced but
still observable. One shouldn't confuse the above with Ken Dunkley's
3-DVG effect which works on another principle.

The effect regardless of which color appears closest is explained
by 2) above. One can readily see when viewing the red and blue
boxes with an edge near the eye and varying it relative to the eye, how
the blue box is displaced left to right with respect to the red box.
This of course causes a change in parallax.

This also explains why Ron perceived the blue box as being smaller.
Depending on the eye level relative to the monitor, the blue box can
look smaller or even bigger. Moving the edge of the paper, up and
down will illustrate this. Alternately, you might one to try fixating on
the colored boxes and move or tilt your head up and down relative to
the monitor.

Chroma, a rather colorful subject! ;-)

Gabriel