smoothness of feature borders

[Frank Warmerdam]

On 2/2/06, Jeff de La Beaujardiere <jeff2005 at svs.gsfc.nasa.gov> wrote:
> Eijnden, B. van den (Bart) wrote:
> > just reporting back something which had never struck my eye, but which I
> > noticed when comparing Mapserver's WMS output to Ionic's WMS output, is that
> > the output from Ionic is a lot smoother with respect to feature borders. Is
> > this due to GD?
>
> The Ionic image is anti-aliased, the Mapserv one is not.
> Notice that the file size of the Ionic image is ~3* greater.
>
> Mapserv does not appear to support anti-aliasing, except for
> text and symbols.
> (http://lists.umn.edu/cgi-bin/wa?A2=ind0411&L=mapserver-users&D=0&I=-3&X=638C7737989424DD73&P=75934)
> However, GD supposedly does have anti-aliasing
> (http://www.boutell.com/gd/manual2.0.33.html).

Bart,

Jeff is right that Ionic is antialiasing the polygon edges.

For those with poor eyes like me, I have prepared
zoomed in views of Bart's images to see the effect
up close.

  http://www.gdal.org/tmp/bodem50_close.png
  http://www.gdal.org/tmp/mapserv_close.png

Note that the ionic (bodem50) image has a smeary
effect between abutting green polygons? This is also
an effect (mostly undesirable in my opinion) of the
antialiasing.

That said, it would be nice if we could antialias polygon
edges too.  GD does have some degree of support for it,
though we might need to use polygon edges drawn as
lines and anti-alias those.   But I don't think this ought to
be the default due to size, and imprecision of results (IMHO).

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 Programmer for Rent