Martin Davis a écrit : >Michaël Michaud wrote: > > >>... about the second point, as you know SFS a no one, you may have an >>answer to the following question : >>why polylines may be made of lines sharing their boundaries >>and multipolygons cannot be made of polygons sharing their edges (if >>that's what "shells can touch at any finite number of points" means) >> >> >> >That *is* what I meant by "shells can touch at any finite number of points". > >I can't see into the minds of the standards authors, but speaking as a >developer who has implemented the standard, the restriction on how >polygon shells touch makes for nicer topological assumptions, and hence >simpler code. With this rule you can assume that all edges in polygon >geometries are truly on the topological boundary - i.e that they divide >the polygon interior from the exterior. Without this assumption, you >have to basically do a "dissolve" to remove "interior" edges before >doing any further processing. > >Polylines don't offer quite such a strong topological distinction (they >don't divide the world into interior and exterior), so there aren't any >unpleasant implications to just letting their linework "go anywhere it >wants to". > >There are other assumptions which SFS could have made to make processing >simpler - such as enforcing an orientation for polygon rings. But I >guess they wanted to open the model up as much as reasonable, so as not >to make a whole bunch of geometry invalid. Although then they could >have allowed "inverted" polygons (self-touching at a point - ArcSDE >style) as well. > >M > > All that makes sense to me. Thanks for the detailed answer.
Michaël > > > > > > ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://ad.doubleclick.net/clk;164216239;13503038;w?http://sf.net/marketplace _______________________________________________ Jump-pilot-devel mailing list Jump-pilot-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel
