find the circularity of diamond, inverted triangle and octagon shape

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hi,currently I working with road sign sign detection, based on the observations from regionprops of matlab value, to detect circular shape, the circularities should be < 1.2, for square shape, circularities <1.6 and triangle shape should be <1.8
Can I know what is the circularities values for diamond , inverted triangle and octagon shapes respectively?

Answers (1)

Image Analyst
Image Analyst on 15 Oct 2012
Circularity is obtained by solving for the radius in terms of both the perimeter and area for a circle:
P = 2*pi*r
r = P/(2*pi)
A = pi*r^2
r = sqrt(A/pi) = P/(2*pi)
A/pi = P^2 / (4*pi^2)
1 = p^2 / (4*pi*A) = circularity
circularity = Perimeter^2 / (4 * pi * Area)
To find out what it is for those shapes, just create those shapes, and measure the perimeter and area.
  2 Comments
Tulips
Tulips on 15 Oct 2012
for octagon and diamond shape , to calculate the circularity , should I know their area?how to find the area of diamond and octagon shapes?

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