P3D Re: Chromostereopsis, Leonardo effect (was: the Dunkley Effect)

[abram klooswyk <abram.klooswyk@xxxxxx>]

Ken Dunkley wrote (PHOTO-3D Digest 3035, 21 Oct 1998):
>I discovered, in 1985, that with 15 minutes of practice it is possible
>to train your vision to make any 2-D picture appear to be fully 
>three-dimensional. In other words, the picture will appear to be 
>truly stereoscopic. Apparent depth will extend to infinity and each 
>element within the picture will be perceived in its proper depth 
>relationship relative to other picture elements. Professor Bela Julesz
>experienced the effect in 1989 and named it the Dunkley Effect. 

Boris Starosta wrote (PHOTO-3D Digest 3042, 25 Oct 1998):
>I have also observed the Dunkley Effect (...) There is in fact a 
>perception of depth, though I disagree that the depth is always
>perceived correctly.  In other words, that objects always appear at
>their "correct" depths.  (...)
>...it is the lens of the eye itself that is utilized to shift the 
>colors (...) the eye's lens will be acting much like a prism (...)

Boris description of his 'more careful observation' is an excellent
example of how observations should be done, especially in not accepting
previous descriptions. 

The proclaimed "Dunkley effect" is an effect which is known for some 
time in several variations. Jim Crowell has discussed the "Dunkley 
effect" in some detail in PHOTO-3D Digest 2011, 21 Apr 1997, it is in
the Archives (Bob Wier should get an award, medal, five stars).
It seems a mix of chromostereopsis and the "Leonardo effect".

Chromostereopsis is known since the 19th century. The Dutch
physiologist Willem Einthoven (1860-1927) wrote his thesis on this
subject (1885). That treatise was only 32 small pages long.
  [In 1901 Einthoven invented the string galvanometer which made
   electrocardiography clinically feasible. He received the Nobel 
   Prize for this in 1924.] 
Recent literature on chromostereopsis is found through the (American) 
National Library of Medicine (NLM), which offers (free of charge) 
access to the medical literature database MEDLINE, accessible *anywhere*
on the globe by *anyone* with Internet acces. The database also covers 
physiology, including research on binocular vision and depth 
perception. (http://www.ncbi.nlm.nih.gov/PubMed/)
When you enter chromostereo* (star for wildcard, as usually), you get 
the references, clicking on an authors name often mostly gives the 
abstract of the article (if recent).  

Most of the recent articles on chromostereopsis appeared in "Vision 
Research", for example the paper by Winn B et al.: "Reversals of the 
colour-depth illusion explained by ocular chromatic aberration", 
Vision Res. 1995 Oct;35(19):2675-84. Their abstract begins with:
"Although many colour-depth phenomena are predictable from the 
interocular difference in monocular chromatic diplopia caused by the 
eye's transverse chromatic aberration (TCA), several reports in the 
literature suggest that other factors may also be involved." (...)
"... red, blue, and green test stimuli which were viewed monocularly 
or binocularly (haploscopically) through 1.78 mm artificial pupils. 
These apertures were displaced nasally and temporally from the visual
axis under controlled conditions to induce a variable degree of TCA."
(...) "...we found that a given coloured target could be located in 
different depth planes within the same display when located on 
different background colours. These seemingly paradoxical results 
could nevertheless be explained by a simple model of optical TCA
without the need to postulate additional factors or mechanisms. "

The literature shows that after a hundred years the books have not quite
been closed on the chromostereopsis subject, and there seems to be 
some disagreement on various aspects of the depth illusion among the 
researchers. 
I can add that the studies in physiological literature of course only
use special targets, without other depth cues, so no photographs having
various kinds of monocular depth cues.

Leonardo da Vinci already noticed that in looking with one eye through
a tube at a painting a depth effect is seen. I have used his advice 
often in art museums, making a 'tube' of my hand (like children do in
imitating binoculars), and looking with one eye through it (you have 
to ignore other visitors who think you've gone mad, I didn't use a 
toilet-paper roll tube to prevent being expelled). 
Jim Crowell mentioned several effects due to this procedure, a number
of 'flatness cues' are eliminated, and the edges of the tube hide the
edges of the picture. The effect is marked on landscape paintings and 
portraits, but even cubistic paintings gain depth from it.

I have also been on the Atlanta I.S.U.-N.S.A. congress. Ken Dunkley
was there, and he is a magician. After listening to him I believe
everything :-). I even believe that "Bela Julesz (...) named it the 
Dunkley Effect". But I'm not sure which effect Julesz meant, it could
be the show effect of Ken's performance. 
It would be very convincing when some written proof of Julesz' 
judgement existed. So far I cannot find "Dunkley effect" in MEDLINE.
As it probably is a mix of a number of effects it will be hard to
provide any scientific evidence on it.

The "ChromaDepth process" and the socalled "Dunkley Effect" are, as 
Boris wrote,: " a neat illusion, worthy of study. But it is not magic.
It does not create two views of a scene, where there is only one."
Obviously chromostereo effects and the "Leonardo effect" can sometimes 
add up to produce a depth illusion, which certainly would be great if 
stereoscopy didn't exist.

Abram Klooswyk


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