[gdal-dev] Interpolation when using the WMS format

[Even Rouault]

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/google3857/
> ${z}/${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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