Optical Illusion in Which Line Segments Continue to Grow or Shrink

by Displaying Two Images Alternately

Kazuhisa Yanaka and Sota Mihara

Kanagawa Institute of Technology, 1030 Shimo-ogino, Atsugi-shi, Japan

Keywords: Optical Illusion, Motion Illusion, Animation, Visual Psychology.

Abstract: A new illusion has been discovered, wherein line segments, when alternately displayed with their tonal

inversion or monochromatic images for approximately 120 ms each on a monochromatic background, seem

to grow or shrink continuously. For instance, if the first image features black line segments on a white

background and the second image shows the inverse brightness, switching between these two images causes

the line segments to give the illusion of continuous expansion. Although a single line segment suffices,

aligning multiple line segments parallel to each other enhances the effect of this illusion. This illusion can be

achieved using achromatic colors, such as black and white, as well as chromatic colors, such as red, blue, and

green. Specifically, when using an image with a black line segment on a red background alongside its

brightness-inverted counterpart, the line segments appear to steadily decrease in length. Our hypothesis

suggests a comparison between the mechanisms of this illusion and the changes in water volume in a pond.

1 INTRODUCTION

Optical illusions are related to vision. Illusions

involving the movement of objects are collectively

called “motion illusions.” A new motion illusion, in

which the line segments continuously grow by

displaying two still images alternately for

approximately 120 ms each, was discovered by

Mihara and Yanaka. As depicted in Fig. 1, one image

features black line segments on a white background,

whereas the other presents a tone-reversed version of

these segments. When these images are presented

alternately, each for approximately 120 ms, the line

segments create the illusion of stretching

continuously in one direction. The moment when the

line segments appear to shrink is imperceptible,

despite their physical lengths remaining unchanged.

This phenomenon is likely a result of the

malfunctioning motion detectors within the human

visual system and the subsequent perception of line

segments becoming longer.

Conventionally, a phenomenon called “apparent

motion” is evident in railroad crossing alarms, where

a red light seems to shift from left to right despite

merely two red lamps alternating in blinking.

However, in this scenario, the movement of the light

spot will always be bidirectional instead of

unidirectional. Introducing a third state where both

lamps are extinguished can render the movement

unidirectional. Yet, this condition is akin to

expanding the number of images used to three.

Moreover, by further increasing the number of

images to four, a more seamless one-directional

motion, referred to as “four stroke motion,” becomes

possible [Mather, G.].

However, although only two images are used in

this illusion, the line segments seem to stretch

continuously. The moment the line segments shrink

is not perceived in this case. Notably, this illusion is

color dependent because its strength is influenced by

variations in the background and line segment colors.

Moreover, different combinations of colors can yield

a reverse illusion, wherein the line segments appear

to shorten progressively.

Figure 1: Example of a line-segment shortening illusion.

Display

alternately

120ms

each

The length of the line segments

is perceived as continuously increasing in length.

Yanaka, K. and Mihara, S.

Optical Illusion in Which Line Segments Continue to Grow or Shrink by Displaying Two Images Alternately.

DOI: 10.5220/0012462200003660

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 19th International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications (VISIGRAPP 2024) - Volume 3: VISAPP, pages

737-740

ISBN: 978-989-758-679-8; ISSN: 2184-4321

737

2 ILLUSION USING

MONOCHROME IMAGES

This optical illusion can be generated from

monochrome and color images. Here, we will discuss

the scenario where two monochrome images are used.

As illustrated in Figure 2 (a), when alternating

between an image featuring a black line segment on a

white background and its tone-reversed counterpart

every 120 ms, the line segment appears to elongate

continuously. This optical illusion remains consistent

regardless of whether the line segment is oriented

horizontally, vertically, or diagonally. Notably,

employing multiple parallel line segments, as

depicted in Figure 2 (b), amplifies the illusion

compared to using a single line segment.

Figure 2: Optical Illusion in which Line Segments Continue

to Grow.

The duration of the stimulus presentation

influences the magnitude of the illusion. When the

two images alternate every 120 ms, the line segment

appears to grow infinitely. However, altering the

switching interval can influence this effect. For

instance, shortening the interval to 50 ms causes the

image to switch before the line segments are fully

stretched, thereby diminishing the illusion's intensity.

Conversely, extending the switching interval, such as

to 180 ms, results in a momentary pause after the line

segment stretches, thereby reducing the perceived

magnitude of the illusion.

Using an image featuring a black line segment on

a white background, along with its tonal inversion,

proved effective in creating the impression of

progressively elongating line segments. However, we

encountered challenges in generating the illusion of

decreasing line segment lengths using this method.

Therefore, we experimented with all-white and all-

black images, moving beyond sole reliance on tonal

inversion images. As a result, as shown in Figure 3,

we found that alternately displaying a black line

image on a white background and an all-white image

at 120 ms each produced the illusion that the line

segments were gradually shortening. Notably, this

effect is more pronounced when the line segment is

thicker, as depicted in Figure 3 (c).

Figure 3: Optical Illusion in which Line Segments Continue

to Shrink.

3 ILLUSION USING COLOR

IMAGES

Although black-and-white images have been used up

to this point, similar illusions can be created using

color images. An ordinary PC can handle

approximately 16 million colors, an extensive range

that is impractical to exhaustively examine. To

streamline our exploration, we have constrained the

color options: the line segments are limited to white

or black, while the background color is confined to

the three primary colors (e.g., red, yellow, and blue).

Consequently, the number of feasible combinations is

2 × 3 = 6.

120ms

each

(a) Stimulus consisting of a line segment.

(b) Stimulus consisting of multiple line segments.

Display

alternately

Display

alternately

120ms

each

(a) Stimulus consisting of thin line segments.

(b) Stimulus consisting of line segments of

medium thickness.

All-white image

Display

alternately

Display

alternately

Display

alternately

120ms

each

120ms

each

120ms

each

All-white image

VISAPP 2024 - 19th International Conference on Computer Vision Theory and Applications

738

3.1 Alternate Display with

Complementary Color Image

Inverting the gray scale of a monochrome image

corresponds to finding the complementary color in a

color image.

The outcomes of generating complementary color

images for each of the six images and displaying them

alternately for 120 ms each are depicted in Figure 6.

Each color component has been represented in 8-bit

integers as follows:

Black (0,0,0)

Red (255, 0, 0)

Green(0, 255, 0)

Blue(0, 0, 255)

Magenta (255, 0, 255),

Cyan (0, 255, 255),

Yellow (255, 255, 0)

White (255, 255, 255)

3.2 Alternate Display with Single-Color

Image

As described in the previous chapter, in scenarios

involving black-and-white images, the alternating

presentation of the image featuring drawn line

segments and the all-white image, each presented for

120 ms , induces the illusion that the line segments

seem to contract.

Likewise, with color images, the illusion of

contracting line segments is achieved by alternately

displaying the image with the all-white image, as

depicted in Figure 5.

4 RELATION TO KNOWN

OPTICAL ILLUSIONS

The phenomenon described here is similar to γ

motion, which is usually explained as follows: Upon

turning on a light in a darkroom, it is perceived to

radiate from the center to the periphery. Conversely,

when the light is turned off, it is perceived as

shrinking from the periphery to the center. This

phenomenon, in which an image appears to expand

and contract with the changes in luminance, is known

as γ motion, a type of apparent motion.

However, the phenomenon described in this work

has the following distinguishing properties.

1. Only elongation or contraction is perceived.

2. What is perceived depends not only on luminance

but also on chrominance.

Figure 4: Alternate display with complementary color image.

Background: red

Line segments: black

Background: cyan

Line segments: white

Line segments appear to shorten.

120ms

each

Display

alternately

Background: green

Line segments: black

Background: magenta

Line segments: white

The amount of illusion is minimal.

120ms

each

Display

alternately

Background: blue

Line segments: black

Background: yellow

Line segments: white

120ms

each

Line segments appear elongated.

Display

alternately

Background: green

Line segments: white

Background: magenta

Line segments: black

120ms

each

Line se

ments a

ear to shorten.

Display

alternately

Background: blue

Line segments: white

Background: yellow

Line segments: black

120ms

each

Line segments appear elongate

Display

alternately

Background: red

Line segments: white

Background:cyan

Linesegments: black

120ms

each

Line segments appear elongated.

Display

alternately

Optical Illusion in Which Line Segments Continue to Grow or Shrink by Displaying Two Images Alternately

739

3. Stretching occurs only in the direction of the line

segment.

Therefore, investigating whether the discovered

phenomenon is a kind of γ motion or not is essential.

Figure 5: Alternate display with single-color image.

5 HYPOTHESIS OF THE

MECHANISM

We will elucidate the mechanism behind this optical

illusion by comparing it to water accumulation in an

elongated, oval-shaped pond, as depicted in Figure 6.

The bottom of the pond exhibits a concave structure,

with the central region deeper and gradually

shallowing toward the shore. When observing the

water surface directly from above, it appears as a line

segment, yet it is, in fact, a lengthy, slender ellipse.

Given that evaporation steadily occurs from the water

surface, the water volume diminishes, causing the

apparent length of the line segment to reduce

progressively. During continuous rainfall, the pond

water level rises, causing the line segments to

lengthen. In instances of intermittent rainfall, the line

segments extend during rainy periods and contract

during rainless intervals. This elucidation clarifies

why the line segments exhibit a cyclic expansion and

contraction pattern.

However, the actual process involves distinct

actions: when a line segment stretches, it solely

elongates, and when it contracts, it only diminishes in

length. This phenomenon manifests when the speeds

of expansion and contraction remarkably differ. For

instance, if a line segment extends gradually but

contracts swiftly, then the duration for contraction is

too brief to be perceptible, resulting in the continuous

perception of growth. Conversely, when a line

segment elongates rapidly but shrinks at a slower

pace, it appears consistently to contract. This concept

constitutes our hypothesis regarding the mechanism

underpinning this optical illusion. However, at this

stage, it remains a hypothesis, warranting further

verification in future studies.

Figure 6: Hypothesis comparing the mechanism of this

illusion to changes in the volume of water in a pond.

6 CONCLUSION

By alternately displaying an image with a black line

on a white background and an image with the

reversed gradation at approximately 120 ms intervals,

an optical illusion in which line segments appear to

continuously grow occurs. Furthermore, by coloring

the background with a specific color, a reverse

illusion, in which the line segments continue to

shrink, can be created. The actual length of the line

segment does not change. It is proven by viewing the

screen with a ruler. Nevertheless, the line segment

appears to expand and linger or increasingly become

shorter . In this respect, this illusion is similar to

Kitaoka’s optimized Fraser– Wilcox illusion

[Kitaoka]. We hope that this discovery will provide

clues for understanding our visual system more

deeply and lead to the development of a new

animation system based on a new principle.

REFERENCES

Kitaoka, A. (August 2013), The color-dependent Fraser-

Wilcox illusion: motion direction is reversed depending

on luminance, http://www.psy.ritsumei.ac.jp/~

akitaoka/Barn2013.html

Mather, G. (2015). Four-stroke Apparent Motion,

http://www.georgemather.com/MotionDemos/Fourstro

keMP4.html

Mihara, S., and Yanaka, K. (February 2023), Elongation

Illusion of Line Segment by Alternate Display of Two

Images. In Proceedings of the 303rd meeting of the

Institute of Image Electronics Engineers of Japan (in

Japanese).

Background: re

Line segments: black

Line segments appear to shorten.

All-white image

Display

alternately

120ms

each

Side view

of pond

Top view

Rain

Evaporation

Length of a line segmen

Wate

VISAPP 2024 - 19th International Conference on Computer Vision Theory and Applications

740