Hi Martin, Thanks for the input.
The simplest case I can come up with is something like this: POLYGON((0 0, 0 4, 4 4, 4 0, 0 0), (1 1, 1 1.5, 1.5 1.5, 1 1)). When we call DouglasPeuckerSimplifier.Simplify with a tolerance of 1, the hole will collapse, and in the DPTransformer specialization of GeometryTransformer, the transformPolygon method will be called, and internall the "isAllValidLinearRings" will be false, causing "factory.buildGeometry(components)" (line 229) to be invoked on a list of the LinearRing and the LineString. This results in a GeometryCollection, of course, which seems to be the right thing to do at this level of GeometryTransformer. Back up in the DPTransform, this result is passed along to Buffer(0.0) before I might be able to inspect it and perhaps weld together the linear ring and discard the line string, and Buffer dutifully discards both components and returns an empty geometry. Thanks for taking a look! -rory On Tue, Dec 2, 2008 at 2:19 PM, Martin Davis <[EMAIL PROTECTED]>wrote: > Rory, > > The conversion of the original polygon shell to a LinearRing sounds like a > bug. I think what *should* happen is that the Polygon shell should be > retained in a Polygon, which will then be processed correctly by buffer(0). > > Can you post your failure case? > > The buffering behaviour on LinearRings is correct - LinearRings are linear > features, and buffer is designed to always return a polygon. There is no > way to validly represent a line as a polygon, so they are removed from the > output. The root of the problem is the bug, not the behaviour of buffer. > > Martin > > Rory Plaire wrote: > >> Hi all, >> I'm running a DP simplification on a multipolygon. At least one of these >> polygons has a hole which degenerates into a linestring when simplifying. >> DPTransform then creates a GeometryCollection containing the shell as a >> LinearRing and the hole as a LineString. The DP transform then performs a >> Buffer(0.0) to create a valid geometry. The buffer discards both the >> degenerate hole (this is fine - it's ok if the hole disappears) as well as >> the LinearRing shell, since it is treated the same as a LineString in the >> Buffer(0.0) operation, and is also discarded. >> I've been considering the best approach to how to get the DP to preserve >> the shell. It seems that both Buffer and DP collude here to kill my polygon >> (DP creates the GeometryCollection w/o converting the LinearRing back into a >> Polygon, and Buffer doesn't treat LinearRing differently than LineString), >> but are just relying on nominal heuristics to compute the geometry. >> Mostly, I'm curious why DP doesn't try to preserve polygons in this case, >> and perhaps why Buffer doesn't treat a LinearRing as bufferable. I'd go >> ahead and make whatever changes I'd need to compute the geometries like I >> want, but I feel I'm missing something, and perhaps my approach would >> reflect that ignorance. >> Anyone have insights they'd like to share? >> Thanks! >> ------------------------------------------------------------------------ >> >> _______________________________________________ >> jts-devel mailing list >> [email protected] >> http://lists.refractions.net/mailman/listinfo/jts-devel >> >> > > -- > Martin Davis > Senior Technical Architect > Refractions Research, Inc. > (250) 383-3022 > > _______________________________________________ > jts-devel mailing list > [email protected] > http://lists.refractions.net/mailman/listinfo/jts-devel >
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