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
flagFrance
72%
18%
4%
4%
22
flagPoland
69%
13%
9%
7%
29721
flagGreece
66%
14%
3%
14%
27
flagCanada
65%
12%
12%
7%
63
flagAustria
64%
14%
14%
7%
28
flagSwitzerland
64%
0%
24%
8%
25
flagGermany
61%
10%
21%
6%
230
flagSingapore
61%
23%
15%
0%
26
flagthe United Kingdom
60%
8%
17%
10%
237
flagUnknown
60%
9%
9%
19%
41
flagthe Netherlands
60%
8%
20%
4%
25
flagthe United States
59%
12%
17%
7%
962
flagAustralia
53%
14%
14%
17%
76
flagIndonesia
52%
5%
29%
11%
17
flagPakistan
50%
10%
30%
5%
20
flagRepublic of Ireland
48%
12%
16%
20%
25
flagIndia
47%
15%
19%
14%
385
flagMalaysia
47%
14%
23%
14%
21
flagthe Philippines
40%
13%
18%
24%
61
flagthe United Arab Emirates
39%
27%
15%
15%
33