The rotating dancer: the rotation
direction is undefined!
The original site for the famous rotating dancer seems to be: http://www.procreo.jp/labo/labo13.html
from the "Flash Laboratory" http://www.procreo.jp/labo.html
People see the dancer rotate clockwise or counter clockwise and the direction can also change in an apparent random way.
Looking
to the images it can be noted that each of them can be interpreted in
two different and equivalent ways.
For example in the image number 3, the dancer can bee seen as standing
on her right leg, 3/4 frontside or standing on her left leg, 3/4
backside. http://www.afs.enea.it/bracco/varie/allframes/569317100_frame_003.bmp
The vision mechanism in the brain apparently is able to
lock into one of the two possible interpretations and remain locked
until some discrepancy is detected, so that the other interpretation is
selected.
If the interpretation is the one with the left leg standing the flux of
visual information is interpreted as a clockwise rotating dancer while
in the other case of the right leg, the returned rotation is in
the opposite direction.
Someone has noticed that the transition bewteen the two different
interpretations can be triggered by fixing the observation on the
feets. Indeed, by looking at the single images, it can be noted that there
are cases where the feet positions are not equivalent for the two
interpretations. The artist has been really smart but some of
the situations have really no compatible solutions for the feet.
Others have remarked that also an oscillating solution can be observed
and probably is related to the capacity of some observers to elaborate
in a much faster way the image flux.
To test if the described interpretation can be the correct one, I have
produced two slightly modified version of the animated GIF, by
adding only two white pixels in each of the images and the
pixel location has been chosen so that only one of the two
interpretation is the coherent one.
A test
with colleagues and friends shows that the stabilization work mostly
but not always, as someone seems to see the white spots sliding on the
back of the dancer.
