ptarray_signed_area()

double ptarray_signed_area

(

const POINTARRAY *

pa

)

Returns the area in cartesian units.

Area is negative if ring is oriented CCW, positive if it is oriented CW and zero if the ring is degenerate or flat. http://en.wikipedia.org/wiki/Shoelace_formula

Definition at line 998 of file ptarray.c.

References getPoint2d_cp(), POINTARRAY::npoints, pixval::x, POINT2D::x, and POINT2D::y.

Referenced by lwpoly_area(), ptarray_isccw(), and test_ptarray_signed_area().

{
        const POINT2D *P1;
        const POINT2D *P2;
        const POINT2D *P3;
        double sum = 0.0;
        double x0, x, y1, y2;
        int i;

        if (! pa || pa->npoints < 3 )
                return 0.0;

        P1 = getPoint2d_cp(pa, 0);
        P2 = getPoint2d_cp(pa, 1);
        x0 = P1->x;
        for ( i = 2; i < pa->npoints; i++ )
        {
                P3 = getPoint2d_cp(pa, i);
                x = P2->x - x0;
                y1 = P3->y;
                y2 = P1->y;
                sum += x * (y2-y1);

                /* Move forwards! */
                P1 = P2;
                P2 = P3;
        }
        return sum / 2.0;
}

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