The homogenous representation of a circle is given by x 2 </msup> + y

Craig Mendoza

Craig Mendoza

Answered question

2022-06-24

The homogenous representation of a circle is given by x 2 + y 2 + 2 g x z + 2 f y z + c z 2 = 0 (or, equivalently, if we set z = 1, x 2 + y 2 + 2 g x + 2 f y + c = 0). Now, given 3 points (in a homogenous form), we can solve a system of linear equations and retrieve the unknowns f, g and c.
This is all very nice (because of linear algebra), but what do these unknowns actually represent with respect to the circle? Which of these numbers represent the x and y coordinates of a circle and which one represents the radius?
Apparently, f and g would be the x and y coordinate of the center of the circle? Why is that the case? I would like to see a proof/derivation of it. Also, what is the radius then?

Answer & Explanation

Braylon Perez

Braylon Perez

Beginner2022-06-25Added 34 answers

0 = x 2 + y 2 + 2 g x + 2 f y + c = x 2 + 2 g x + g 2 + y 2 + 2 f y + f 2 + ( c g 2 f 2 ) = ( x + g ) 2 + ( y + f ) 2 ( f 2 + g 2 c )
This equation says that the squared distance of the point ( x , y ) from the point ( g , f ) is f 2 + g 2 c, which describes a circle centered at ( g , f ) with radius f 2 + g 2 c .

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