vertex simplification

Stephen Woodbridge woodbri at SWOODBRIDGE.COM
Sat Oct 27 10:32:00 EDT 2007

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.


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

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