geography_substring()

LWGEOM * geography_substring	(	const LWLINE *	lwline,
		const SPHEROID *	s,
		double	from,
		double	to,
		double	tolerance
	)

Return the part of a line between two fractional locations.

Definition at line 104 of file lwgeodetic_measures.c.

{
        POINTARRAY *dpa;
        POINTARRAY *ipa = lwline->points;
        LWGEOM *lwresult;
        POINT4D pt;
        POINT4D p1, p2;
        GEOGRAPHIC_POINT g1, g2;
        int nsegs, i;
        double length, slength, tlength;
        int state = 0; /* 0 = before, 1 = inside */
        uint32_t srid = lwline->srid;
 
        /*
        * Create a dynamic pointarray with an initial capacity
        * equal to full copy of input points
        */
        dpa = ptarray_construct_empty(
                (char) FLAGS_GET_Z(ipa->flags),
                (char) FLAGS_GET_M(ipa->flags),
                ipa->npoints);
 
        /* Compute total line length */
        length = ptarray_length_spheroid(ipa, s);
 
        /* Get 'from' and 'to' lengths */
        from = length * from;
        to = length * to;
        tlength = 0;
        getPoint4d_p(ipa, 0, &p1);
        geographic_point_init(p1.x, p1.y, &g1);
        nsegs = ipa->npoints - 1;
        for (i = 0; i < nsegs; i++)
        {
                double dseg;
                getPoint4d_p(ipa, (uint32_t) i+1, &p2);
                geographic_point_init(p2.x, p2.y, &g2);
 
                /* Find the length of this segment */
                /* Special sphere case */
                if ( s->a == s->b )
                        slength = s->radius * sphere_distance(&g1, &g2);
                /* Spheroid case */
                else
                        slength = spheroid_distance(&g1, &g2, s);
 
                /*  We are before requested start. */
                if (state == 0) /* before */
                {
                        if (fabs ( from - ( tlength + slength ) ) <= tolerance)
                        {
                                /* Second point is our start */
                                ptarray_append_point(dpa, &p2, LW_FALSE);
                                state = 1; /* we're inside now */
                                goto END;
                        }
                        else if (fabs(from - tlength) <= tolerance)
                        {
                                /*  First point is our start */
                                ptarray_append_point(dpa, &p1, LW_FALSE);
                                /*
                                 * We're inside now, but will check
                                 * 'to' point as well
                                 */
                                state = 1;
                        }
                        /*
                        * Didn't reach the 'from' point,
                        * nothing to do
                        */
                        else if (from > tlength + slength)
                        {
                                goto END;
                        }
                        else  /* tlength < from < tlength+slength */
                        {
                                /* Our start is between first and second point */
                                dseg = (from - tlength) / slength;
                                interpolate_point4d_spheroid(&p1, &p2, &pt, s, dseg);
                                ptarray_append_point(dpa, &pt, LW_FALSE);
                                /* We're inside now, but will check 'to' point as well */
                                state = 1;
                        }
            }
 
            if (state == 1) /* inside */
            {
                        /*  'to' point is our second point. */
                        if (fabs(to - ( tlength + slength ) ) <= tolerance )
                        {
                                ptarray_append_point(dpa, &p2, LW_FALSE);
                                break; /* substring complete */
                        }
                        /*
                        * 'to' point is our first point.
                        * (should only happen if 'to' is 0)
                        */
                        else if (fabs(to - tlength) <= tolerance)
                        {
                                ptarray_append_point(dpa, &p1, LW_FALSE);
                                break; /* substring complete */
                        }
                        /*
                        * Didn't reach the 'end' point,
                        * just copy second point
                        */
                        else if (to > tlength + slength)
                        {
                                ptarray_append_point(dpa, &p2, LW_FALSE);
                                goto END;
                        }
                        /*
                        * 'to' point falls on this segment
                        * Interpolate and break.
                        */
                        else if (to < tlength + slength )
                        {
                                dseg = (to - tlength) / slength;
                                interpolate_point4d_spheroid(&p1, &p2, &pt, s, dseg);
                                ptarray_append_point(dpa, &pt, LW_FALSE);
                                break;
                        }
            }
 
        END:
                tlength += slength;
                memcpy(&p1, &p2, sizeof(POINT4D));
                geographic_point_init(p1.x, p1.y, &g1);
        }
 
        if (dpa->npoints <= 1) {
                lwresult = lwpoint_as_lwgeom(lwpoint_construct(srid, NULL, dpa));
        }
        else {
                lwresult = lwline_as_lwgeom(lwline_construct(srid, NULL, dpa));
        }
 
        return lwresult;
}

References POINTARRAY::flags, FLAGS_GET_M, FLAGS_GET_Z, geographic_point_init(), getPoint4d_p(), interpolate_point4d_spheroid(), LW_FALSE, lwline_as_lwgeom(), lwline_construct(), lwpoint_as_lwgeom(), lwpoint_construct(), POINTARRAY::npoints, LWLINE::points, ptarray_append_point(), ptarray_construct_empty(), ptarray_length_spheroid(), s, sphere_distance(), spheroid_distance(), LWLINE::srid, POINT4D::x, and POINT4D::y.

Referenced by geography_line_substring(), and test_geography_substring().

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