ST_AsMVTGeom document internals of transform step

git-svn-id: http://svn.osgeo.org/postgis/trunk@15801 b70326c6-7e19-0410-871a-916f4a2858ee

diff --git a/postgis/mvt.c b/postgis/mvt.c
index 0ef1401ae7b0e4aec25c94ba8ac34074398cdcd1..0924e4ab1e2e0ce5633fc3340994f6ca4845dc3e 100644 (file)
--- a/postgis/mvt.c
+++ b/postgis/mvt.c
@@ -706,40 +706,44 @@ LWGEOM *mvt_geom(const LWGEOM *lwgeom, const GBOX *gbox, uint32_t extent, uint32
                }
        }
 
+       /* if no clip output deep clone original to avoid mutation */
        if (lwgeom_out == NULL)
                lwgeom_out = lwgeom_clone_deep(lwgeom);
 
+       /* transform to tile coordinate space */
        memset(&affine, 0, sizeof(affine));
        affine.afac = fx;
        affine.efac = fy;
        affine.ifac = 1;
        affine.xoff = -gbox->xmin * fx;
        affine.yoff = -gbox->ymax * fy;
-
        lwgeom_affine(lwgeom_out, &affine);
 
+       /* snap to integer precision, removing duplicate points */
        memset(&grid, 0, sizeof(gridspec));
        grid.ipx = 0;
        grid.ipy = 0;
        grid.xsize = 1;
        grid.ysize = 1;
-
        lwgeom_out = lwgeom_grid(lwgeom_out, &grid);
 
        if (lwgeom_out == NULL || lwgeom_is_empty(lwgeom_out))
                return NULL;
 
+       /* if polygon(s) make valid and force clockwise as per MVT spec */
        if (lwgeom_out->type == POLYGONTYPE ||
                lwgeom_out->type == MULTIPOLYGONTYPE) {
                lwgeom_out = lwgeom_make_valid(lwgeom_out);
                lwgeom_force_clockwise(lwgeom_out);
        }
 
+       /* if geometry collection extract highest dimensional geometry type */
        if (lwgeom_out->type == COLLECTIONTYPE) {
                LWCOLLECTION *lwcoll = lwgeom_as_lwcollection(lwgeom_out);
                lwgeom_out = lwcollection_as_lwgeom(
                        lwcollection_extract(lwcoll, max_type(lwcoll)));
                lwgeom_out = lwgeom_homogenize(lwgeom_out);
+               /* if polygon(s) make valid and force clockwise as per MVT spec */
                if (lwgeom_out->type == POLYGONTYPE ||
                        lwgeom_out->type == MULTIPOLYGONTYPE) {
                        lwgeom_out = lwgeom_make_valid(lwgeom_out);