The general rule is that if you're walking the border in the order that it's given, then your left hand is inside the area, and your right hand is outside. That allows you to define holes, multipolys, etc in the same format without needing extra flags.
Remember that "polygon" is a very loosely used term here. So it's perfectly legit to enter a "poly" as a single line/border in a clockwise direction. That just means that your "poly" is infinite in the plane and has a single hole in it. So if a mirroring function is just inverting the sign of all the X or Y coordinates and not re-ordering the verticies in each sub-border, then it's a bug. And if it's treating a shape as equivalent to "everything except that shape" then it's also a bug. (Unrelated note: I know the module is "geo <space> poly" as two words, but I can't help but always pronounce it in a way that rhymes with monopoly. Would anyone be up for a game of Mono Poly?) -----Original Message----- From: sqlite-users [mailto:sqlite-users-boun...@mailinglists.sqlite.org] On Behalf Of Graham Hardman Sent: Saturday, November 17, 2018 8:37 PM To: sqlite-users@mailinglists.sqlite.org Subject: [sqlite] geopoly - rules re data entry Hi, The documentation for the geopoly module states that the vertices must be defined in a counter-clockwise order so that the interior of the polygon is on the left of the element as it is drawn. However, I have discovered that a polygon transformed with geopoly_xform can result in a polygon having vertices reading clockwise. For instance, a polygon mirrored about either the x or y axis will produce this effect. The geopoly module allows these transformed polygons to be added to the table. If I then create an exact same polygon shape having counterclockwise order for the vertices and use the geopoly_overlap function to test it against the one with clockwise order I find the result = 4, meaning the polygons are regarded as identical. My question then is, is it really necessary to ensure a counterclockwise order when a polygon shape is entered for the first time in the table. regards, Graham _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
