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.roua...@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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