vertex simplification

Julien-Samuel Lacroix jlacroix at MAPGEARS.COM
Mon Oct 29 17:26:26 EDT 2007


Just to say that few months back I added two functions in mapgeos.c to 
simplify a shape object via mapscript. They used GEOSSimplify and 
GEOSTopologyPreserveSimplify. Is that something that would be useful for 
you? I could commit it if you want.


Stephen Woodbridge wrote:
> thomas bonfort wrote:
>> hi all,
>> I'm looking into adding some vertex simplification at the image level,
>> for the time being limited to the agg renderer. I've got two questions
>> I'd like to have feedback on:
>> * it's very easy to add a local simplification, by removing vertexes
>> in a linestring that are closer than some threshold to their preceding
>> vertex. I'm getting nice speedups by mimicking what's currently (more
>> or less implicitely) done with gd, by removing vertexes that fall in
>> the same pixel than its predecessor (of course these speedups would
>> only happen for complex line layers. I'm currently testing with the
>> canadian road data mentionned in
>> I
>> know agg is all about subpixel accuracy and such, but I'd think this
>> behaviour could be enabled by default ( as in this case the maximum
>> error for a vertex is less than a pixel)
>> * is anyone interested in having a global and tunable vertex
>> simplification functionality. I don't think it would be too much work
>> work to implement a douglas-peuker line generalisation algorithm, that
>> could be turned on and tunable with a new mapfile keyword. That's
>> basically doing what postgis' SIMPLIFY can already do, except this
>> would be done in pixel coordinates, and therefore would not have to be
>> tuned w.r.t. scale.
> Thomas,
> This sounds like a very cool idea. I think the value in this is that is 
> speeds things up, so if implementing douglas-peuker does not improve on 
> what you have already tried, then I'm not sure of the value. I think the 
> implementing the same for polygons which can have as many or more 
> vertices as polylines would be a huge win also.
> -Steve W

Julien-Samuel Lacroix

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