[gdal-dev] gdal_polygonize and polygon edges

[Jeff Lacoste]

Yes visually attractive or smooth polygons is the goal. Thanks again Frank.

Doing a web search about simplification algorithm i found one named
'*Ramer-Douglas–Peucker'
(*http://en.wikipedia.org/wiki/Ramer-Douglas-Peucker_algorithm).
It appears that 'Geos'' library implement this algorithm. Is this algorithm
exposed through OGR ?

Could this algorithm help smoothing a polygon without necessary make the
new nodes too far from the original one ? Or may be there
are other *more* recommended algorithms ?

If any one could suggest a simplification algorithm or had some experience
with smoothing polygons, I appreciate their input.

Thanks

Jeff




On Tue, Feb 19, 2013 at 2:42 PM, Frank Warmerdam <warmerdam at pobox.com>wrote:

> On Tue, Feb 19, 2013 at 11:28 AM, Jeff Lacoste
> <jefflacostegdal at gmail.com> wrote:
> > Hi Frank,
> >
> > Thanks for your quick response. Following the edges of the pixels seems a
> > perfect solution for non continuous grid (ex. land use, etc.) as
> > the boundary between the class is important to keep when constructing the
> > polygon. However for continuous grid (.ex elevations), the boundaries are
> > a bit not clear and not clear cut. When following the pixels edges, the
> > created polygons appear to have the stairs effect and are less visually
> > attractive.
> >
> > I thought of a smoothing the polygons to not have *rough* edges using the
> > current gdal_polygonize by trying to not follow the pixels edges and use
> > instead of the
> > pixel centers. Basically do something similar to what contour generator
> does
> > by treating the raster values as continuous.
>
> Jeff,
>
> Ah, I see, you are looking for visually attractive polygons from
> continuous fields.
>
> I have wondered if it would be reasonable to produce a version of the
> contour generator that actually produces polygon regions.  If we had
> that then applying appropriate simplification to the resulting very
> detailed edges should give something attractive and with reasonable
> information density.  An appropriate simplification algorithm might do
> this in a reasonable way for the existing polygonize output but I
> don't know enough about the simplification algorithms to suggest one.
>
> I don't think aiming for pixel centers in gdal_polygonize would really
> solve the problem.
>
> Best regards,
> --
>
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Software Developer
>
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