[gdal-dev] Interpolation when using the WMS format

[Jukka Rahkonen]

Peter Hopfgartner <peter.hopfgartner <at> r3-gis.com> writes:

> 
> On 09/18/2014 11:03 PM, Jukka Rahkonen wrote:
> > Peter Hopfgartner <peter.hopfgartner <at> r3-gis.com> writes:
> >
> >> Salut Evan,
> >>
> >> thanks for you recipe. Unfortunately, what I am really interested in is to
> > use this from MapServer. Could
> >> you give me a pointer in the source where the resampling takes place?
> > Hi,
> Hi Jukka
> > Why don't you test with Mapserver then before starting to code?
> You are certainly right that starting with MapServer is a more 
> reasonable approach. Anyway, sometimes a look at the code might help to 
> better understand the problem. (black box vs. white box)
> >   If you use
> > Mapserver as WMS GDAL can request correct resolution directly and there
> > should be no need at all for resampling.
> This is true for WMS layers. Unfortunately, with tiles this is 
> different. I guess (and I think to have seen this in the code), that 
> GDAL is looking for the best matching resolution for this particular 
> tile server, fetches the necessary tiles and then resamples the tiles to 
> the final pixmap, which, if not by incredible chance, has a lower 
> resolution then the cpomplete set of fetched tiles. My guess is, that in 
> this resampling the labels get unreadable to to nearest point 
> interpolation, which is really fast but delivers a lower quality then 
> e.g. bilinear interpolation.
> >   Most Mapservers are also configured
> > to use png as one of the outputformats which should lead to better quality
> > than the jpeg compressed tiles you have been playing with.
> Even here the, I have to follow the rules set by the service provider, 
> as specified in http://www.basemap.at/wmts/1.0.0/WMTSCapabilities.xml , 
> which limits me to JPEG. I have to take what I get.

Hi,

You are naturally right about the tiles and it is good if you will work and
make GDAL better. I reacted when you wrote "what I am really interested in
is to use this from MapServer". I believe that your improvements will have
no effect on what you said you are really interested in but if you have
strong enough will to make the world of open source GIS better anyway we all
appreciate you.

-Jukka Rahkonen-