On Jun 27, 2011, at 12:14 AM, Flo Ledermann wrote: > > > On 06/27/2011 07:07 AM, Dane Springmeyer wrote: >> >> On Jun 26, 2011, at 10:48 PM, Magnus Bäck wrote: >> >>> So, I converted that GPX file into a shapefile with gpx2shp >>> before feeding it to Mapnik. As I didn't see any crossing lines >>> when viewing either the GPX file nor the shapefile I've assumed >>> it was somehow related to Mapnik. Also, the lines are (in most >>> cases) a little bit too perpendicular and of equal length to >>> indicate a problem with the data itself. >> >> Yep, rendering artifact, that I can replicate. Your hunch was right. Not >> looked closely yet as to the cause, but likely the high density of nodes >> triggers those sharp spikes in some part of AGG. > > Just as additional info: this is not a bug, although it can lead to > extreme looking cases like the above. It's the way line drawing works > with noisy data, which will always contain small but sharp spikes. The > data points will "jump" left and right of the actual route, producing > very sharp angles perpendicular to the track. Depending on the line join > setting, these are exaggerated to sharp triangles sticking out of the track. > > See http://www.carto.net/papers/svg/samples/stroking.shtml#lineJoin fpr > an illustration. Under the heading of stroke-miterlimit you find an > example of such a sharp edge case, producing a long, sharp triangular > spike. (This is page is for SVG, but the same will be true of pretty > much any graphics package) > > The solution is, as Dane said, to set an appropriate line join mode > other than "miter", or to set an appropriate miter limit if the renderer > supports it. (This doesn't seem to be supported/exposed by mapnik)
Flo, nice spot. Perhaps you could code up a patch to support setting mitre-limit ? It appears it is hardcoded currently on line 140: http://trac.mapnik.org/browser/trunk/src/agg/process_line_symbolizer.cpp#L140 Dane > > Best, > > Flo > > >> >> The fix, however, is easy, and likely something want you want anyway for >> smoother a looking line given the high density of node. >> >> Add these two extra options to the LineSymbolizer: >> >> stroke-linejoin="round" stroke-linecap="round" >> >> That should fix things up nicely. Though your pre-processing solution is >> great too of course because things will draw faster that way. >> >> Dane > _______________________________________________ > Mapnik-users mailing list > [email protected] > https://lists.berlios.de/mailman/listinfo/mapnik-users _______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
