lwline_crossing_direction()

int lwline_crossing_direction	(	const LWLINE *	l1,
		const LWLINE *	l2
	)

lwline_crossing_direction: returns the kind of CG_LINE_CROSS_TYPE behavior of 2 linestrings

Given two lines, characterize how (and if) they cross each other.

Parameters

l1	first line string
l2	second line string

Returns

a CG_LINE_CROSS_TYPE LINE_NO_CROSS = 0 LINE_CROSS_LEFT = -1 LINE_CROSS_RIGHT = 1 LINE_MULTICROSS_END_LEFT = -2 LINE_MULTICROSS_END_RIGHT = 2 LINE_MULTICROSS_END_SAME_FIRST_LEFT = -3 LINE_MULTICROSS_END_SAME_FIRST_RIGHT = 3

Definition at line 477 of file lwalgorithm.c.

{
        uint32_t i = 0, j = 0;
        const POINT2D *p1, *p2, *q1, *q2;
        POINTARRAY *pa1 = NULL, *pa2 = NULL;
        int cross_left = 0;
        int cross_right = 0;
        int first_cross = 0;
        int this_cross = 0;
#if POSTGIS_DEBUG_LEVEL >= 4
        char *geom_ewkt;
#endif
 
        pa1 = (POINTARRAY*)l1->points;
        pa2 = (POINTARRAY*)l2->points;
 
        /* One-point lines can't intersect (and shouldn't exist). */
        if ( pa1->npoints < 2 || pa2->npoints < 2 )
                return LINE_NO_CROSS;
 
        /* Zero length lines don't have a side. */
        if ( ptarray_length_2d(pa1) == 0 || ptarray_length_2d(pa2) == 0 )
                return LINE_NO_CROSS;
 
 
#if POSTGIS_DEBUG_LEVEL >= 4
        geom_ewkt = lwgeom_to_ewkt((LWGEOM*)l1);
        LWDEBUGF(4, "l1 = %s", geom_ewkt);
        lwfree(geom_ewkt);
        geom_ewkt = lwgeom_to_ewkt((LWGEOM*)l2);
        LWDEBUGF(4, "l2 = %s", geom_ewkt);
        lwfree(geom_ewkt);
#endif
 
        /* Initialize first point of q */
        q1 = getPoint2d_cp(pa2, 0);
 
        for ( i = 1; i < pa2->npoints; i++ )
        {
 
                /* Update second point of q to next value */
                q2 = getPoint2d_cp(pa2, i);
 
                /* Initialize first point of p */
                p1 = getPoint2d_cp(pa1, 0);
 
                for ( j = 1; j < pa1->npoints; j++ )
                {
 
                        /* Update second point of p to next value */
                        p2 = getPoint2d_cp(pa1, j);
 
                        this_cross = lw_segment_intersects(p1, p2, q1, q2);
 
                        LWDEBUGF(4, "i=%d, j=%d (%.8g %.8g, %.8g %.8g)", i, j, p1->x, p1->y, p2->x, p2->y);
 
                        if ( this_cross == SEG_CROSS_LEFT )
                        {
                                LWDEBUG(4,"this_cross == SEG_CROSS_LEFT");
                                cross_left++;
                                if ( ! first_cross )
                                        first_cross = SEG_CROSS_LEFT;
                        }
 
                        if ( this_cross == SEG_CROSS_RIGHT )
                        {
                                LWDEBUG(4,"this_cross == SEG_CROSS_RIGHT");
                                cross_right++;
                                if ( ! first_cross )
                                        first_cross = SEG_CROSS_RIGHT;
                        }
 
                        /*
                        ** Crossing at a co-linearity can be turned handled by extending
                        ** segment to next vertex and seeing if the end points straddle
                        ** the co-linear segment.
                        */
                        if ( this_cross == SEG_COLINEAR )
                        {
                                LWDEBUG(4,"this_cross == SEG_COLINEAR");
                                /* TODO: Add logic here and in segment_intersects()
                                continue;
                                */
                        }
 
                        LWDEBUG(4,"this_cross == SEG_NO_INTERSECTION");
 
                        /* Turn second point of p into first point */
                        p1 = p2;
 
                }
 
                /* Turn second point of q into first point */
                q1 = q2;
 
        }
 
        LWDEBUGF(4, "first_cross=%d, cross_left=%d, cross_right=%d", first_cross, cross_left, cross_right);
 
        if ( !cross_left && !cross_right )
                return LINE_NO_CROSS;
 
        if ( !cross_left && cross_right == 1 )
                return LINE_CROSS_RIGHT;
 
        if ( !cross_right && cross_left == 1 )
                return LINE_CROSS_LEFT;
 
        if ( cross_left - cross_right == 1 )
                return LINE_MULTICROSS_END_LEFT;
 
        if ( cross_left - cross_right == -1 )
                return LINE_MULTICROSS_END_RIGHT;
 
        if ( cross_left - cross_right == 0 && first_cross == SEG_CROSS_LEFT )
                return LINE_MULTICROSS_END_SAME_FIRST_LEFT;
 
        if ( cross_left - cross_right == 0 && first_cross == SEG_CROSS_RIGHT )
                return LINE_MULTICROSS_END_SAME_FIRST_RIGHT;
 
        return LINE_NO_CROSS;
 
}

References getPoint2d_cp(), LINE_CROSS_LEFT, LINE_CROSS_RIGHT, LINE_MULTICROSS_END_LEFT, LINE_MULTICROSS_END_RIGHT, LINE_MULTICROSS_END_SAME_FIRST_LEFT, LINE_MULTICROSS_END_SAME_FIRST_RIGHT, LINE_NO_CROSS, lw_segment_intersects(), LWDEBUG, LWDEBUGF, lwfree(), lwgeom_to_ewkt(), POINTARRAY::npoints, LWLINE::points, ptarray_length_2d(), SEG_COLINEAR, SEG_CROSS_LEFT, SEG_CROSS_RIGHT, POINT2D::x, and POINT2D::y.

Referenced by ST_LineCrossingDirection(), test_lwline_crossing_bugs(), test_lwline_crossing_long_lines(), and test_lwline_crossing_short_lines().

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