lw_segment_intersects()

int lw_segment_intersects	(	const POINT2D *	p1,
		const POINT2D *	p2,
		const POINT2D *	q1,
		const POINT2D *	q2
	)

returns the kind of CG_SEGMENT_INTERSECTION_TYPE behavior of lineseg 1 (constructed from p1 and p2) and lineseg 2 (constructed from q1 and q2)

Parameters

p1	start point of first straight linesegment
p2	end point of first straight linesegment
q1	start point of second line segment
q2	end point of second line segment

Returns

a CG_SEGMENT_INTERSECTION_TYPE Returns one of SEG_ERROR = -1, SEG_NO_INTERSECTION = 0, SEG_COLINEAR = 1, SEG_CROSS_LEFT = 2, SEG_CROSS_RIGHT = 3,

Definition at line 371 of file lwalgorithm.c.

References lw_seg_interact(), lw_segment_side(), LWDEBUGF, SEG_COLINEAR, SEG_CROSS_LEFT, SEG_CROSS_RIGHT, SEG_ERROR, and SEG_NO_INTERSECTION.

Referenced by lwline_crossing_direction(), rect_tree_intersects_tree(), and test_lw_segment_intersects().

{

        int pq1, pq2, qp1, qp2;

        /* No envelope interaction => we are done. */
        if (!lw_seg_interact(p1, p2, q1, p2))
        {
                return SEG_NO_INTERSECTION;
        }

        /* Are the start and end points of q on the same side of p? */
        pq1=lw_segment_side(p1,p2,q1);
        pq2=lw_segment_side(p1,p2,q2);
        if ((pq1>0 && pq2>0) || (pq1<0 && pq2<0))
        {
                return SEG_NO_INTERSECTION;
        }

        /* Are the start and end points of p on the same side of q? */
        qp1=lw_segment_side(q1,q2,p1);
        qp2=lw_segment_side(q1,q2,p2);
        if ( (qp1 > 0.0 && qp2 > 0.0) || (qp1 < 0.0 && qp2 < 0.0) )
        {
                return SEG_NO_INTERSECTION;
        }

        /* Nobody is on one side or another? Must be colinear. */
        if ( pq1 == 0.0 && pq2 == 0.0 && qp1 == 0.0 && qp2 == 0.0 )
        {
                return SEG_COLINEAR;
        }

        /*
        ** When one end-point touches, the sidedness is determined by the
        ** location of the other end-point. Only touches by the first point
        ** will be considered "real" to avoid double counting.
        */
        LWDEBUGF(4, "pq1=%.15g pq2=%.15g", pq1, pq2);
        LWDEBUGF(4, "qp1=%.15g qp2=%.15g", qp1, qp2);

        /* Second point of p or q touches, it's not a crossing. */
        if ( pq2 == 0 || qp2 == 0 )
        {
                return SEG_NO_INTERSECTION;
        }

        /* First point of p touches, it's a "crossing". */
        if ( pq1 == 0 )
        {
                if ( pq2 > 0 )
                        return SEG_CROSS_RIGHT;
                else
                        return SEG_CROSS_LEFT;
        }

        /* First point of q touches, it's a crossing. */
        if ( qp1 == 0 )
        {
                if ( pq1 < pq2 )
                        return SEG_CROSS_RIGHT;
                else
                        return SEG_CROSS_LEFT;
        }

        /* The segments cross, what direction is the crossing? */
        if ( pq1 < pq2 )
                return SEG_CROSS_RIGHT;
        else
                return SEG_CROSS_LEFT;

        /* This should never happen! */
        return SEG_ERROR;
}

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