distance2d_sqr_pt_seg()

double distance2d_sqr_pt_seg	(	const POINT2D *	C,
		const POINT2D *	A,
		const POINT2D *	B
	)

Definition at line 2316 of file measures.c.

{
        /*if start==end, then use pt distance */
        if ((A->x == B->x) && (A->y == B->y))
                return distance2d_sqr_pt_pt(C, A);
 
        /*
         * otherwise, we use comp.graphics.algorithms
         * Frequently Asked Questions method
         *
         *  (1)         AC dot AB
         *         r = ---------
         *              ||AB||^2
         *      r has the following meaning:
         *      r=0 P = A
         *      r=1 P = B
         *      r<0 P is on the backward extension of AB
         *      r>1 P is on the forward extension of AB
         *      0<r<1 P is interior to AB
         */
 
        double ba_x = (B->x - A->x);
        double ba_y = (B->y - A->y);
        double ab_length_sqr = (ba_x * ba_x + ba_y * ba_y);
        double ca_x = (C->x - A->x);
        double ca_y = (C->y - A->y);
        double dot_ac_ab = (ca_x * ba_x + ca_y * ba_y);
 
        if (dot_ac_ab <= 0)
                return distance2d_sqr_pt_pt(C, A);
        if (dot_ac_ab >= ab_length_sqr)
                return distance2d_sqr_pt_pt(C, B);
 
        /*
         * (2)
         *           (Ay-Cy)(Bx-Ax)-(Ax-Cx)(By-Ay)
         *      s = -----------------------------
         *                      L^2
         *
         *      Then the distance from C to P = |s|*L.
         *
         */
 
        double s_numerator = ca_x * ba_y - ca_y * ba_x;
 
        /* Distance = (s_num / ab) * (s_num / ab) * ab == s_num * s_num / ab) */
        return s_numerator * s_numerator / ab_length_sqr;
}

References distance2d_sqr_pt_pt(), POINT2D::x, and POINT2D::y.

Referenced by lwline_split_by_point_to(), ptarray_closest_segment_2d(), and ptarray_locate_point().

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