[gdal-dev] Interpolation when using the WMS format

[Peter Hopfgartner]

Hello Evan,

I've tried this on your suggestion. What happens is, that all labels are 
now much smaller, even harder to read then before. Basically, it seems 
that tiles from a higher zoom levels are taken. This would be 
reasonably, if there would be no labels on the tiles. But in the case of 
the basemap.at tiles, labels are drawn on the tiles.
When working with tiles, even more if the tiles are from a third party, 
the available options are much reduced, compared to WMS request. When 
the raster information is lost due to a lower quality interpolation, 
it's gone forever in the processing pipeline.
I did not have the time to look at the code yet (and probably I do not 
have enough C++ experience either), I'll retry during the weekend.

Regards,

Peter

On 09/18/2014 10:30 PM, Even Rouault wrote:
> 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


-- 
Peter Hopfgartner
R3 GIS
web  : www.r3-gis.com