Proceedings ofThe Third International Workshop on Kansei

p. 1

A study on the angle visual illusion in the road image
Shuhei Fukuya", Misato Hayashi*, Atsushi Osa*, and Hidetoshi Miike** Graduate School ofScience and Engineering, Yamaguchi University 2-16-1 Tokiwa, Ube-shi, Yamaguchi, 755-8611

JapanE-rnail.' *{mO14vm,j036iflz,osaa,miike}@yamaguchi-u.ac.jp.

Abstract
In this study, we found a visual illusion; the angle of corner at the vanishing point drawn by parallel

white lines in road images is observed smaller than the correct one. We investigated relationshipbetween pictorial cues for depth perception in images and this visual illusion. Our results showed theevidence of this visual illusion, and the kind of pictorial cue was an important factor to control the visualillusion. In addition, we were interesting in directions of presented images, which also could control thevisual illusion. We expected that this il}usion has a relationship with Ponzo illusion. The effects of thepictorial depth cues and the direction of images were similar to the results of previous reports on thePonzo illusion. However, in the images observed the new illusion, Ponzo illusion was not clear, and viceversa. It is possible that this new angle illusion and Ponzo illusion aren't completely the samemechanism in our vision system.
Key VltOrdS Perspective, Vanishingpoint, Depth cue, Ponzo illusion

1. Introduction

and this error of angle estimation.

As for the visual illusion related to the sense ofperspective, Ponzo visual illusion is2. Experiment 1
We experimented on the perceived angle offamous[1][2]. This visual illusion was devised

corner at a vanishing point in road images. We

an Italian psychologist Ponzo in 1928. Whentwo horizontal parallel lines with the same

controlled pictorial depth cues in the images,
and direction of the images for presentation tolength put between crossing lines like a turnedobservers.

" V" shape, the upper line of parallel lines is

2.1 Method

perceived longer length than the lower line.

(Participants)

Not only simple crossing lines, but we canobserve this illusion in cave images, roadThe observers were 10 university students, 7males and 3 females, aged 21'28 years. They

images, and rail road images also. To observe
this illusion, it is important that a pair ofhave normal sense of color, and normal
eyesight (including the correction).

spread lines is drawn in the image like a(Stimuli and apparatus)

vanishing point and parallel lines in

We prepared 32 standard stimuli. At first, weperspective images. There are many studies
about this visual illusion, they have paid

set 4 levels for controlling pictorial depth cuesin each image. Figure 1 shows 4 example

attention to changing of objecVs size locatedimages, a road image, a paved road image, abetween the crossing lines[3].

road & cones image, and a no backgroundIn this study, we investigated whether theimage. Second, we set 4 variations of roadangle of corner at a vanishing point drawn bywidth, 2,750mm, 3,OOOmm, 3,250mm, and3,500mm. Third, when the stimuli wereparallel lines in perspective images wasperceived correctly, and the factors to observe

presented on a LCD display SONY,

this error of angle estimation. Then, wePCVD-17SM2/K), we selected the direction of

discussed a relationship between Ponzo illusion

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VVorkshop
on Kansei p.2

the standard stimulus, and compare thepresentation, the normal direction like Fig.1
and the upside'down direction. As a result, thestimuli were 4 times 4 times 2 variations (thestandard stimulus and the comparison
stimulus without watching two stimuli at the

pictorial cue, the road width, and the display

same time. The comparison stimulus wasdirection). The images were generated bypresented on a table from the smallest angle to3'dimensional computer graphics. The positionof camera was the center of the road and 1 m

the bigger angle or from the biggest angle tothe smaller angle in turn. The observer selectedheight. The focus distance of camera was 50mm. The size of displayed image was 25.9 by

the comparison stimulus when they thoughtthat the comparison stimulus was the same

34.5 cm.

angle as the standard stimulus. An observer's

On the other hand, the comparison stimuli
were a black isosceles triangle drawn on aperceived angle of each standard stimulus waswhite paper. The base line of the triangle was

2.2 results and discussion

located at lower position, but this line was notFigures 3 and 4 show the results of stimuli
displayed with the normal direction and thecalculated by the average of 2 selected angles.

drawn. That is, the comparison stimuli seem
like a turned "V" shape. 19 sheets between fromupside-down direction respectively. An analysis

30 degrees to 120 degrees by 5 degrees wereprepared. Figure 2 shows the experimentby using the 950/o confidence limits revealed
that there were significant differences from theenvironment to display these stimuli.

correct angle in all stimuli.
no background image •-• e--•road image
road &conesimage-' t'- paved roadimage- 15Ee- 20oo2750

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roadwidth[mm]3-way analysis of variance, pictorial cues forthe depth perception (depth cues), road width,

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Figure4. Inversion

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''- 15road & cones image no background image-xbackgroundimage--t'-pavedroadimage--

Figure2. Experimental environmentand display direction, revealed the main effect(Proeedure)of the depth cues [F(3)=8.708, p=O.OO03]. The

The standard stimulus was presented on theLCD display by random order. The observers

results were subjected to multiple post hoccomparisons by Ryan's method. There is awere directed to gaze at the vanishing point in

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Proceedings of The Third Internatt'onal VVorkshop on Kansei

significant difference at the pair of the roadshows the experiment envlronment to display

image and the road & corns image, the pair ofa

these stimuli.

no background image and a road & cones image(O.05 significant level). This results show thatin the images with rich depth cues thedifferenee from the correct angle was bigger
than that in the images with poor depth cues. It
seems that this regularized difference is a kindof visual illusion. We call it as the angle illusionof road images. We think that this visualFigure 5Standard stimulus

illusion relates to the sense of perspective like(Procedure)

Ponzo illusion.

The standard stimulus was presented by

3-way analysis of variance also revealed therandom ordez The observers were directed tomain effect of the display {ilirection [F(1)=9.701,

adjust the angle of the comparison stimulus top=O.O124]. The visual illusion of theupside-down direction images was smaller thanthat of the standard stimulus by manipulatingof a mouse device without watching two stimuli

that of the normal direction.

at the same time. Then the observers were

3. Experiment 2
We experimented on relationship the angle

asked about the length of the line S (see Fig.5).

illusion found by the experiment 1 and Ponzo

number of the line R.

They answered the length as the multiple

illusion. We controlled the angles of turned "V"stimolut;vntl ltpa rg.;-Il$

shape and "V" shape, investigated the both

Sfnncinrd stimtta"st"" :"illusions.

3.1 Method60(Participants)

50um

The observers were 7 university students, 4males and 3 females, aged 21-28 yeaTs. TheyeObserver

have normal sense of color, and normal

Figure6. Experimental environment

eyesight (including the correction).

3.2 results and discussion(Stimuli and apparatus)

Figures 7 and 8 show the results of stimuli
displayed with the normal direetion and theWe prepared 30 standard stimuli (Fig.5). First,
we set 5 variations of the angle e (30, 60, 60, 90,

upside'down direction respectively. An analysis

120, 150 deg.). Second, we set 3 variations ofby using the 900/o confidence limits revealed
that there were significant differences from thethe distanee X, 5.6cm(line A), 9.2cm(line B),

13.lcm(line C). Third, when the stimuli werecorrect angle in table 1.

standed on a desk, we selected the direction of/presentation, the normal direction like Fig.5 or+line A '`."'- line B

lineC-8

the upside-down direction. As a result, then286
p ti

stimuli were 5 times 3 times 2 vaTiations (theE842Eeopts
8 -2ttic' e-4e -6

angle, the line position, and the display

ttt

t-g• :th ,direction). The size of a standaTd stimulus was

22 by 30 cm.

On the other hand, the comparison stimulus

were presented on a LCD display

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angle of standard stimulus[degree]

Figure7. Normal d irec tion

257

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--t:-.

(SONY,PCVD-17SM2/K). The comparison
stimulus was a turned "V" shape, and the size
was the same as the standard stimuli. Figure 6.

.

mttProceedings of The Third International V;,rorkshop
p.4on Kansei

the depth clues.

+lineA'"'."'lineB A lineCoT
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3. Quantity of visual illusion decreases rapidly

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by the depth clues perceived from the given'1.'e.

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4. 0rigins of Ponzo visual illusion and that ofangle of standard stimulus[degree]

Figure8. Upside'down direction
displaydirection

NormalNormalUpside'down
Upside'down
Upside'down
Upside'down
Tablel

angre
90
150
60
120
120
150

the proposed angle illusion in the road
image seem to be different.

lineR

ReterencesA[1]Ponzo,M(1912)Rapports entre quelquesBB

illusion visuelles de contrite angulaire etA

1'appreciation de grandeur des asters aB
B

1'horizon , Arshives Itakiennes de Biologie[2]Coren, S. & Girgus, J. S. (1978) Seeing isdeceiving : The psychology of visual illusion.

Hillsdate : Lawrence Erlbaum Associate, Inc.

In the Ponzo image, it is known that visual[3]Fisher, G. H. (1973) Toward a newquantity of optical illusion tends to decrease

explanation for the geometrical illusions, ll .

with increase of the angle, which made by two

Apparent depth or contour proximity? British

lines contacted at a point. In our experiment,

Journal of Psychology[4] Pressey, A. W., Burchard, N & Scrivner, L.

however, such the tendeney was not confirmed.

(1971) Assimilation theory and the Ponzo

This discrepancy can be explained as follows.

illusion : Quantitative prediction. Canadian

The angle illusion seems to be induced by our

Journal of Psychology

perception characteristics. We tend to pay ourattention to the triangle formed by reversely

V'shaped lines and line S in the Pnzo image.

Consequently, we can not perceive aperspeetive feeling in the image. Thus, we canconclude that origins of Ponzo visual illusionand that of the proposed angle illusion in theroad image are different. We have to clarify
the difference in the future experiment.

4. Summary
In this study, the followings became clear.

1. The angle of corner at the vanishing point
drawn by parallel white lines of road images

is observed smaller than the correct one(visual illusion of angle in a road image).

2. The quantity of visual illusion depends on

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