[gdal-dev] Interpolation when using the WMS format

[Even Rouault]

Le jeudi 18 septembre 2014 22:23:06, Peter Hopfgartner a écrit :
> 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?

In gcore/rasterio.cpp : GDALRasterBand::IRasterIO(). I must warn you : quite 
tricky code

But perhaps you don't need code change. In MapServer did you try to define 
PROCESSING "RESAMPLE=BILINEAR" ?

> 
> Thanks,
> 
> Peter
> 
> Am 18.09.2014 um 22:11 schrieb Even Rouault <even.rouault at spatialys.com>:
> > Le jeudi 18 septembre 2014 17:40:11, Peter Hopfgartner a écrit :
> >> Hi, everybody
> >> 
> >> we are using the GDAL WMS driver for getting maps from a public WMTS
> >> service, http://www.basemap.at. Anyway, the resulting image does not
> >> look smooth. In particular, I would guess that the the image was scaled
> >> with something like a nearest neighbor algorithm.
> >> So my question is simply, if I can switch the interpolation method in
> >> some place.
> >> 
> >> The XML file, taken from
> >> http://gisforge.wordpress.com/2012/11/06/basemap-at-wmts-layer-in-qgis/,
> >> is:
> >> 
> >> <GDAL_WMS>
> >> 
> >>  <Service name="TMS">
> >> 
> >> <ServerUrl>http://maps.wien.gv.at/basemap/geolandbasemap/normal/google38
> >> 57/ ${z}/${y}/${x}.jpeg</ServerUrl> </Service>
> >> 
> >>  <DataWindow>
> >>  
> >>   <UpperLeftX>-20037508.34</UpperLeftX>
> >>   <UpperLeftY>20037508.34</UpperLeftY>
> >>   <LowerRightX>20037508.34</LowerRightX>
> >>   <LowerRightY>-20037508.34</LowerRightY>
> >>   <TileLevel>18</TileLevel>
> >>   <TileCountX>1</TileCountX>
> >>   <TileCountY>1</TileCountY>
> >>   <YOrigin>top</YOrigin>
> >>  
> >>  </DataWindow>
> >>  <Projection>EPSG:3857</Projection>
> >>  <BlockSizeX>256</BlockSizeX>
> >>  <BlockSizeY>256</BlockSizeY>
> >>  <BandsCount>3</BandsCount>
> >>  <Cache />
> >> 
> >> </GDAL_WMS>
> >> 
> >> The command is:
> >> 
> >> gdal_translate -srcwin 35500000 23500000 100000 100000 -of JPEG -outsize
> >> 250 250 basemap_at.xml basemap_at.jpg
> > 
> > Peter,
> > 
> > Resampling done by gdal_translate is nearest neighbour, so not always
> > that great.
> > 
> > What you can do is select an outsize that is compatible with the source
> > overviews. In TMS, the ratio between sucessive overviews work with factor
> > of 2, so you should select an outsize of 10000 / 2^N. The closest is
> > 10000 / 256 = 390.625 -> 391
> > 
> > So
> > 
> > gdal_translate -srcwin 35500000 23500000 100000 100000 -outsize
> > 391 391 basemap_at.xml tmp.tif
> > 
> > and then:
> > 
> > gdalwarp tmp.tif tmp2.tif -r lanczos -ts 250 250 (you can try different
> > resampling kernel)
> > gdal_translate tmp2.tif basemap_at.jpg -of JPEG
> > 
> > Best regards,
> > 
> > Even

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