The perception threshold of the panda illusion, a particular form of 2D pulse-width-modulated halftone, correlates with visual acuity

Hana Langrová,Laura Kuehlewein,Anne Kurtenbach,Torsten Straßer,Eberhart Zrenner

doi:10.1038/s41598-020-69952-6

Abstract

To call attention to the danger of extinction of the panda bear, the Lithuanian artist Ilja Klemencov created the artwork “They can disappear”. The illustration is composed of black-and-white zigzagged lines, which form the famous panda logo of the World Wild Fund For Nature (WWF) when seen from a distance. If one is too close to the artwork, it is difficult to spot the bear, however, if one steps back or takes off one’s glasses the panda suddenly appears. This led us to ask if the ability to see the panda is related to the visual acuity of the observer and if therefore, the panda illusion can be used to assess the spatial resolution of the eye. Here we present the results of the comparison between visual acuity determined using the Landolt C and that predicted from the panda illusion in 23 healthy volunteers with artificially reduced visual acuity. Furthermore, we demonstrate that the panda illusion is based on a 2D pulse-width modulation, explain its technical history, and provide the equations required to create the illusion. Finally, we explain why the illusion indeed can be used to predict visual acuity and discuss the neural causes of its perception with best-corrected visual acuity.

Highlights

To call attention to the danger of extinction of the panda bear, the Lithuanian artist Ilja Klemencov created the artwork “They can disappear”
From the remaining parameters f defines the frequency of the triangular function, whereas α, and rh/w define the angle of the zigzag pattern between 0 and π, and the ratio between height and width of one tile of the zigzag pattern, respectively, and the phase shift of the triangular function (Eq 4)
The results of our experiment show that blurring, induced by natural or artificial degradation of the visual acuity, up to a certain degree facilitates the decoding of the pulse-width modulation (PWM)-modulated visual illusion into darker and brighter areas, providing a cue for figure-ground segregation and the perception of the encoded image

Summary

Introduction

To call attention to the danger of extinction of the panda bear, the Lithuanian artist Ilja Klemencov created the artwork “They can disappear”. The basic working principle of the panda illusion, increasing the distance to the image or introducing blur by taking off the glasses or squinting the eyes, is similar to other artwork, like “All is Vanity Ambiguous Figure” by Charles Allan Gilbert (Fig. 2, left), where the figure is perceived either as a skull or as a woman looking at a mirror. In this painting, two figures with different spatial frequencies are overlaid and a spatial low-pass filter removes the finer details of the one figure (woman), which reveals the other figure (skull)[2]. A comprehensive demonstration of similar illusions can be found in Nicolas Wade’s excellent book “Art and Illusionists”[11]

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Conclusion