How is this polygon called? (the one over the circles)

square
rhomboid
concave square
concave rhomboid
In this Ehrenstein illusion the sides of a square placed inside a pattern of concentric circles take an apparent curved shape. But in reality, the sides are straight.
Debbie Fox
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Correct this question
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Very interesting!
reach: rather globalgeometryillusionsmathematicsoptical illusionstechnology

Answers by country

square
rhomboid
concave square
concave rhomboid
answers
flagPoland
69%
12%
9%
7%
30359
flagFrance
68%
20%
4%
8%
25
flagGreece
66%
14%
3%
14%
27
flagAustria
65%
9%
12%
9%
55
flagSwitzerland
63%
2%
25%
5%
36
flagthe United Kingdom
62%
8%
16%
10%
259
flagGermany
61%
11%
18%
7%
368
flagCanada
61%
12%
12%
11%
71
flagSingapore
61%
23%
15%
0%
26
flagUnknown
60%
9%
9%
19%
41
flagthe Netherlands
60%
7%
21%
3%
28
flagthe United States
59%
12%
17%
7%
1002
flagIndonesia
57%
5%
26%
10%
19
flagAustralia
55%
13%
13%
16%
79
flagRepublic of Ireland
48%
14%
14%
18%
27
flagMalaysia
47%
13%
21%
17%
23
flagPakistan
47%
9%
33%
4%
21
flagIndia
45%
16%
20%
15%
410
flagthe Philippines
41%
13%
16%
24%
65
flagthe United Arab Emirates
37%
25%
14%
17%
35