[gdal-dev] Interpolation when using the WMS format

[Even Rouault]

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

> 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.

Did you try tweaking OVERSAMPLE_RATIO ? See
http://mapserver.org/input/raster.html.

Note: the doc is out of sync. The default value is 2.0 (not 2.5).


> 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
>
>


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