Optical Illusion

Posted by Junjie Hua on January 27, 2020


Start from a typical versual illusion(2020/01/27 update)

Since I am a student who is doing research about thermal illusion based on the existed optical illusion, I am going to describe some optical illusions from today.

I will start my work from a book which is called 新 錯視図鑑: 脳がだまされる奇妙な世界を楽しむ・解き明かす・つくりだす.

Due to limit of my spare time, I may possibily have not enough time to describe all about the illusions. I will introduce those illusions that are related to shape and brightness because these kinds of illusions are almost the most famous ones and can be transfered to some other percepts such as thermal percept. I will introduce them from from 4 basic parts

  1. What is illusion?

  2. Mechanism of illusion

  3. Illusion of shape or size

  4. Illusion of brightness

1. What is illusion?

(From wikipedia) An optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. We human being observe the world, recognize the world through our eyes in our daily life. 70% of the information processed in our brain come from optical information so it is really very important for us tounderstand what we see through the eyes. On the other hand, optical illusions are not rare things for us so we also need to understand the illusions well.

A typical optical illusion may be barber’s pole.A spinning barber pole creates a visual illusion, in which the stripes appear to be traveling up or down the length of the pole, rather than around it. This illusion occurs because a bar or contour within a frame of reference provides ambiguous information about its “real” direction of movement. The actual motion of the line has many possibilities. The shape of the aperture thus tends to determine the perceived direction of motion for an otherwise identically moving contour. A vertically elongated aperture makes vertical motion dominant whereas a horizontally elongated aperture makes horizontal motion dominant. In the case of a circular or square aperture, the perceived direction of movement is usually orthogonal to the orientation of the stripes (diagonal, in this case). The perceived direction of movement relates to the termination of the line’s end points within the inside border of the occluder. The vertical aperture, for instance, has longer edges at the vertical orientation, creating a larger number of terminators unambiguously moving vertically. This stronger motion signal forces us to perceive vertical motion. Functionally, this mechanism has evolved to ensure that we perceive a moving pattern as a rigid surface moving in one direction.

A barber's pole

However, illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect (where, despite movement, position remains unchanged). An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.

Pathological visual illusions arise from pathological changes in the physiological visual percept mechanisms causing the aforementioned types of illusions; they are discussed e.g. under visual hallucinations.

(atmosphere –> mechanism)

2.Mechanism of illusion

3.Illusion of shape or size

3.1cafe wall illusin

source: http://nekomosyakushimo.hatenablog.com/entry/2017/09/06/012510

python program
from __future__ import division
from __future__ import unicode_literals
from numpy import array
from PIL import Image, ImageDraw

black1 = array(range(5)) * 120 + 30

image = Image.new("RGB", (620,123), color=(255,255,255))
draw = ImageDraw.Draw(image)

for i in range(5): #上段の四角形を描く
    draw.rectangle(((black1[i],0),(black1[i]+60,60)), outline = None, fill = (0,0,0))
for j in range(5): #下段の四角形を描く
    draw.rectangle(((black1[j]+30,63),(black1[j]+90,123)), outline = None, fill = (0,0,0))
draw.line([(0,61),(620,61)], width = 2, fill = (128,128,128)) #真ん中の灰色の線


4.Illusion of brightness