[gdal-dev] gdal_polygonize and polygon edges

[Jeff Lacoste]

Thank you Frank ! Very much appreciated. I'll give 'Simplify' option a try.

Jeff


On Tue, Feb 19, 2013 at 3:25 PM, Frank Warmerdam <warmerdam at pobox.com>wrote:

> Jeff,
>
> I believe this is exposed in ogr2ogr using the -simplify argument:
>
> -simplify tolerance:
> (starting with GDAL 1.9.0) distance tolerance for simplification.
> Note: the algorithm used preserves topology per feature, in particular
> for polygon geometries, but not for a whole layer.
>
> Best regards,
> Frank
>
> On Tue, Feb 19, 2013 at 12:12 PM, Jeff Lacoste
> <jefflacostegdal at gmail.com> wrote:
> > 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
> >
> >
>
>
>
> --
>
> ---------------------------------------+--------------------------------------
> 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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/gdal-dev/attachments/20130219/24890e7b/attachment.html>