polygon_integrals, a FORTRAN77 code which returns the exact value of the integral of any monomial over the interior of a polygon in 2D.
We suppose that POLY is a planar polygon with N vertices X, Y, listed in counterclockwise order.
For nonnegative integers P and Q, the (unnormalized) moment of order (P,Q) for POLY is defined by:
Nu(P,Q) = Integral ( x, y in POLY ) x^p y^q dx dy
In particular, Nu(0,0) is the area of POLY.
Simple formulas are available for low orders:
Nu(0,0) = 1/2 (1<=i<=N) X(i-1)Y(i)-X(i)Y(i-1)
Nu(1,0) = 1/6 (1<=i<=N) ( X(i-1)Y(i)-X(i)Y(i-1) ) * (X(i-1)+X(i))
Nu(0,1) = 1/6 (1<=i<=N) ( X(i-1)Y(i)-X(i)Y(i-1) ) * (Y(i-1)+Y(i))
Nu(2,0) = 1/12 (1<=i<=N) ( X(i-1)Y(i)-X(i)Y(i-1) ) * (X(i-1)^2+X(i-1)X(i)+X(i)^2)
Nu(1,1) = 1/24 (1<=i<=N) ( X(i-1)Y(i)-X(i)Y(i-1) ) * (2X(i-1)Y(i-1)+X(i-1)Y(i)+X(i)Y(i-1)+2X(i)Y(i))
Nu(0,2) = 1/12 (1<=i<=N) ( X(i-1)Y(i)-X(i)Y(i-1) ) * (Y(i-1)^2+Y(i-1)Y(i)+Y(i)^2)
The normalized moment of order (P,Q) for POLY is defined by:
Alpha(P,Q) = Integral ( x, y in POLY ) x^p y^q dx dy / Area ( Poly )
= Nu(P,Q) / Nu(0,0)
In particular, Alpha(0,0) is 1.
The central moment of order (P,Q) for POLY is defined by:
x* = Alpha(1,0)
y* = Alpha(0,1)
Mu(P,Q) = Integral ( x, y in POLY ) (x-x*)^p (y-y*)^q dx dy / Area ( Poly )
Simple formulas are available for low orders:
Mu(0,0) = 1
Mu(1,0) = 0
Mu(0,1) = 0
Mu(2,0) = Alpha(2,0) - Alpha(1,0)^2
Mu(1,1) = Alpha(1,1) - Alpha(1,0) * Alpha(0,1)
Mu(0,2) = Alpha(0,2) - Alpha(0,1)^2
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
polygon_integrals is available in a C version and a C++ version and a Fortran90 version and a MATLAB version and an Octave version and a Python version.
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