1.4.3 Motion Illusions and Gestalt

In [Creative computing I, vol. 1, section 7.2], several principles of
Gestalt psychology were introduced. The Gestalt school of
psychology was largely initiated by experiments performed by
Wertheimer14, researching into the perception of motion. He and his 14Max Wertheimer (1880–1943),
Czech psychologist. His paper
Experimentelle Studien uber das
Sehen von Bewegung (1912),
Zeitschrift fur Psychologie 61,
161–265 is credited with launching
the Gestalt revolution.
colleagues used very simple stimuli, arrangements of dots switched
on and off in particular patterns, to elicit perceptual responses from
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CC227 Creative Computing II Perception and Information Retrieval
subjects. Two effects in particular were described by Wertheimer:
beta motion and the phi phenomenon.
Beta Motion
Beta motion is believed to be the perceptual illusion responsible for
converting a succession of still images (as projected in a cinema, for
example) into the perception of motion.
Learning activity
Type the following Processing code into a sketch, and run it. What do you observe?
boolean left;
void setup() {
left = false;
smooth(); frameRate(3); size(200,200);
}
void draw() {
background(0); left = !left;
if(left) {
ellipse(50,100,30,30);
} else {
ellipse(150,100,30,30);
}
}
Alter the sketch so that different stimuli are used instead of the circles: for example,
squares, text or an image. Does this affect your perception of the scene? What
happens if you use different colours?
If one of the stimuli is made smaller than the other, some people report that the motion
is backwards and forwards in addition to sidetoside.
Implement this, and see what
your perception is.
Comments on the activity
A typical response to this stimulus is along the lines of: “the circle moves from the left
side to the right and back again”
Phi Phenomenon
A separate peceptual illusion of motion was described by
Wertheimer, who called it the phi phenomenon or pure motion:
distinguishable from beta motion by the fact that it was objectless
motion: the motion perceived in the phi phenomenon is not that of
any of the stimuli, unlike beta motion. It is this observation which
was truly novel and serendipitous in 1912, and which kickstarted
the Gestalt movement in psychology.
Learning activity
Type the following Processing code into a sketch, and run it. What do you observe?
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Motion
int NSTIMULI = 6;
int pos;
void setup() {
pos = 0;
smooth(); frameRate(15); size(200,200);
ellipseMode(CENTER);
}
void draw() {
background(0);
int i;
for (i = 0; i < NSTIMULI; i++) {
float phase = 2*PI*i / NSTIMULI;
if (!(i == pos)) {
ellipse(100+50*cos(phase), 100+50*sin(phase), 30, 30);
}
}
pos = (pos + 1) % NSTIMULI;
}
Alter the sketch so that different stimuli are used instead of circles: text, images or
different shapes. Does this affect your perception of the scene?
Observe the sketch with different numbers of stimuli: is the effect always clear? How
does the number of stimuli affect the Processing frame rate necessary to see the
effect?
Comments on the activity
Typical response: “something with the same colour as the background is moving
round, obscuring the dots”.
The original phi phenomenon was described in terms of just two dots; however, many
people find the phenomenon much clearer with more stimuli.
Many textbooks are unclear over the distinction between beta motion and the phi
phenomenon; beta motion is the perceived motion of an object, elicited by successive
showing of two related stimuli in different spatial locations, while the phi phenomenon
is perceived motion of an unseen object with the visual properties of the scene’s
background, elicited by implied occlusion of stimuli at a relatively high frequency.
More information about the phi phenomenon can be found at the Magniphi
website15. 15http:
//www2.psych.purdue.edu/
Magniphi/index.html. See
also the paper referenced there:
Steinman, R. M., Pizlo, Z. and Pizlo,
F. J. (2000) Phi is not beta, and why
Wertheimer’s discovery launched
the Gestalt revolution: a minireview.
Vision Research, 40, 2257–2264.
Figure 1.10: Illustration of the Gestalt principles of proximity (left)
and closure (right): in each case, grouping into a structure or inference
of a shape occurs without there being a direct stimulus for that
inference.
There is no single authoritative list of Gestalt principles of
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CC227 Creative Computing II Perception and Information Retrieval
perception; the theory of Gestalt perception has been in existence
for about a century, and many distinct effects have acquired the
label of ‘Gestalt’. However, a conservative list of principles would
probably include the effects of proximity, closure, similarity,
continuity and common fate.
The common thread underlying these perceptual principles is the
grouping or inference of an entity without having a direct associated
stimulus: in figure 1.10, the grouping into units by proximity and
the inference of a triangle’s presence over three circles is performed
by the perceptual system without any direct evidence. It is exactly
this effect – the inference of an object or grouping without any
direct stimulus – that is present in the phi phenomenon’s objectless
motion.
In this course and others we have presented Gestalt perceptual
phenomena using visual examples. However, we will find in the next
chapter, on the subject of Sound, Hearing and Music, that many of
the same principles carry over to the auditory domain.
Exercises
1. Create an interactive, 3D Processing sketch to illustrate
schematically the structures in the eye: information that you
should impart to the viewer should include:
two distinct shapes to represent rod and cone cells;
three different pigments for the cone cells;
the spatial distribution of rods and cones over the retina;
the presence of the blind spot.
2. Convert the following XYZ colours into the CIE LAB colour
space, and compute the distances between each pair. Which
colours are closest together, and which are further apart?
Comment on your answer.
XYZ sRGB
{38.76,42.87,67.20} (140,180,210)
{40.05,43.39,73.97} (140,180,220)
{40.86,47.06,67.90} (140,190,210)
{40.53,43.78,67.29} (150,180,210)
3. (a) Write a Processing function implementing the
transformation given in equations (1.15) and (1.16), to
convert sRGB values between 0 and 255 into XYZ
tristimulus values.
(b) Write a Processing function transforming XYZ tristimulus
values to xyY chromaticity coordinates.
(c) Using these two functions, display the colours available to
you in Processing on a chromaticity diagram. You may wish
to include some animated or interactive component to your
sketch, to allow the viewer of your sketch better to
understand the chromaticity available to them on an sRGB
display.
4. Because of the nonlinear
transfer function in sRGB, a na¨ıve
approach to image scaling does not work (see for example
http://www.4p8.com/eric.brasseur/gamma.html for
discussion of this point). Implement a Processing sketch which
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Motion
correctly scales down images of even dimension by a factor of 2,
by converting each pixel’s colour into CIE XYZ space, averaging
blocks of four pixels, and converting back into sRGB. Compare
the results of your scaling on some test images with the results
from commercial or free image manipulation programmes.
5. Design Processing sketches illustrating the Gestalt principles of
proximity, closure, similarity, continuity and common fate. In
each case, illustrate in a manner of your choosing the inferred
structure that does not correspond to a direct stimulus.
6. Select a website with a large number of users, and examine the
use of colour from the perspective of accessibility: are the
colours used sufficiently distinct even for those with anomalous
vision? Are there any mitigating factors, such as alternate
stylesheets, which could improve the accessibility of such a
website?