lwpoly_to_points()

LWMPOINT* lwpoly_to_points	(	const LWPOLY *	poly,
		uint32_t	npoints,
		int32_t	seed
	)

Definition at line 1670 of file liblwgeom/lwgeom_geos.c.

{
        double area, bbox_area, bbox_width, bbox_height;
        GBOX bbox;
        const LWGEOM* lwgeom = (LWGEOM*)lwpoly;
        uint32_t sample_npoints, sample_sqrt, sample_width, sample_height;
        double sample_cell_size;
        uint32_t i, j, n;
        uint32_t iterations = 0;
        uint32_t npoints_generated = 0;
        uint32_t npoints_tested = 0;
        GEOSGeometry* g;
        const GEOSPreparedGeometry* gprep;
        GEOSGeometry* gpt;
        GEOSCoordSequence* gseq;
        LWMPOINT* mpt;
        int32_t srid = lwgeom_get_srid(lwgeom);
        int done = 0;
        int* cells;
        const size_t size = 2 * sizeof(int);
        char tmp[2 * sizeof(int)];
        const size_t stride = 2 * sizeof(int);
 
        if (lwgeom_get_type(lwgeom) != POLYGONTYPE)
        {
                lwerror("%s: only polygons supported", __func__);
                return NULL;
        }
 
        if (npoints == 0 || lwgeom_is_empty(lwgeom)) return NULL;
 
        if (!lwpoly->bbox)
                lwgeom_calculate_gbox(lwgeom, &bbox);
        else
                bbox = *(lwpoly->bbox);
 
        area = lwpoly_area(lwpoly);
        bbox_width = bbox.xmax - bbox.xmin;
        bbox_height = bbox.ymax - bbox.ymin;
        bbox_area = bbox_width * bbox_height;
 
        if (area == 0.0 || bbox_area == 0.0)
        {
                lwerror("%s: zero area input polygon, TBD", __func__);
                return NULL;
        }
 
        /* Gross up our test set a bit (but not too much) to increase
         * odds of getting coverage in one pass */
        sample_npoints = npoints * FP_MIN(bbox_area / area, 10000.0);
 
        /* We're going to generate points using a sample grid as described
         * http://lin-ear-th-inking.blogspot.ca/2010/05/more-random-points-in-jts.html to try and get a more uniform
         * "random" set of points. So we have to figure out how to stick a grid into our box */
        sample_sqrt = lround(sqrt(sample_npoints));
        if (sample_sqrt == 0)
                sample_sqrt = 1;
 
        /* Calculate the grids we're going to randomize within */
        if (bbox_width > bbox_height)
        {
                sample_width = sample_sqrt;
                sample_height = ceil((double)sample_npoints / (double)sample_width);
                sample_cell_size = bbox_width / sample_width;
        }
        else
        {
                sample_height = sample_sqrt;
                sample_width = ceil((double)sample_npoints / (double)sample_height);
                sample_cell_size = bbox_height / sample_height;
        }
 
        /* Prepare the polygon for fast true/false testing */
        initGEOS(lwnotice, lwgeom_geos_error);
        g = (GEOSGeometry*)LWGEOM2GEOS(lwgeom, 0);
        if (!g)
        {
                lwerror("%s: Geometry could not be converted to GEOS: %s", __func__, lwgeom_geos_errmsg);
                return NULL;
        }
        gprep = GEOSPrepare(g);
 
        /* Get an empty multi-point ready to return */
        mpt = lwmpoint_construct_empty(srid, 0, 0);
 
        /* Initiate random number generator.
         * Repeatable numbers are generated with seed values >= 1.
         * When seed is zero and has not previously been set, it is based on
         * Unix time (seconds) and process ID. */
        lwrandom_set_seed(seed);
 
        /* Now we fill in an array of cells, and then shuffle that array, */
        /* so we can visit the cells in random order to avoid visual ugliness */
        /* caused by visiting them sequentially */
        cells = lwalloc(2 * sizeof(int) * sample_height * sample_width);
        for (i = 0; i < sample_width; i++)
        {
                for (j = 0; j < sample_height; j++)
                {
                        cells[2 * (i * sample_height + j)] = i;
                        cells[2 * (i * sample_height + j) + 1] = j;
                }
        }
 
        /* Fisher-Yates shuffle */
        n = sample_height * sample_width;
        if (n > 1)
        {
                for (i = n - 1; i > 0; i--)
                {
                        size_t j = (size_t)(lwrandom_uniform() * (i + 1));
 
                        memcpy(tmp, (char *)cells + j * stride, size);
                        memcpy((char *)cells + j * stride, (char *)cells + i * stride, size);
                        memcpy((char *)cells + i * stride, tmp, size);
                }
        }
 
        /* Start testing points */
        while (npoints_generated < npoints)
        {
                iterations++;
                for (i = 0; i < sample_width * sample_height; i++)
                {
                        int contains = 0;
                        double y = bbox.ymin + cells[2 * i] * sample_cell_size;
                        double x = bbox.xmin + cells[2 * i + 1] * sample_cell_size;
                        x += lwrandom_uniform() * sample_cell_size;
                        y += lwrandom_uniform() * sample_cell_size;
                        if (x >= bbox.xmax || y >= bbox.ymax) continue;
 
                        gseq = GEOSCoordSeq_create(1, 2);
#if POSTGIS_GEOS_VERSION < 30800
                        GEOSCoordSeq_setX(gseq, 0, x);
                        GEOSCoordSeq_setY(gseq, 0, y);
#else
                        GEOSCoordSeq_setXY(gseq, 0, x, y);
#endif
                        gpt = GEOSGeom_createPoint(gseq);
 
                        contains = GEOSPreparedIntersects(gprep, gpt);
 
                        GEOSGeom_destroy(gpt);
 
                        if (contains == 2)
                        {
                                GEOSPreparedGeom_destroy(gprep);
                                GEOSGeom_destroy(g);
                                lwerror("%s: GEOS exception on PreparedContains: %s", __func__, lwgeom_geos_errmsg);
                                return NULL;
                        }
                        if (contains)
                        {
                                npoints_generated++;
                                mpt = lwmpoint_add_lwpoint(mpt, lwpoint_make2d(srid, x, y));
                                if (npoints_generated == npoints)
                                {
                                        done = 1;
                                        break;
                                }
                        }
 
                        /* Short-circuit check for ctrl-c occasionally */
                        npoints_tested++;
                        if (npoints_tested % 10000 == 0)
                                LW_ON_INTERRUPT(GEOSPreparedGeom_destroy(gprep); GEOSGeom_destroy(g); return NULL);
 
                        if (done) break;
                }
                if (done || iterations > 100) break;
        }
 
        GEOSPreparedGeom_destroy(gprep);
        GEOSGeom_destroy(g);
        lwfree(cells);
 
        return mpt;
}

References LWPOLY::bbox, contains(), FP_MIN, LW_ON_INTERRUPT, lwalloc(), lwerror(), lwfree(), LWGEOM2GEOS(), lwgeom_calculate_gbox(), lwgeom_geos_errmsg, lwgeom_geos_error(), lwgeom_get_srid(), lwgeom_get_type(), lwgeom_is_empty(), lwmpoint_add_lwpoint(), lwmpoint_construct_empty(), lwnotice(), lwpoint_make2d(), lwpoly_area(), lwrandom_set_seed(), lwrandom_uniform(), POLYGONTYPE, pixval::x, GBOX::xmax, GBOX::xmin, pixval::y, GBOX::ymax, and GBOX::ymin.

Referenced by lwgeom_to_points(), and lwmpoly_to_points().

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