[Qgis-user] WMS/GeoWebCache image quality in QGIS

[Ross McDonald]

Hi List,

I am trying to work out why there is a deterioration in image quality 
when I load pre-seeded cached tiles from Geowebcache into QGIS.  I have 
a Geoserver (v2.5) cluster and an external GWC (v1.5) configured to use 
the GS WMS.  GS supplies the tiles to GWC as PNG, building them from a 
number of vector datasets supplied by PostGIS (v2.1).  GS is configured 
to output good quality PNG.  The data is supplied and consumed in 
EPSG:27700 so no reprojection is happening.

If I look at the WMS straight from Geoserver in something like 
OpenLayers or Leaflet then I get nice crisp high quality images.  In 
QGIS I get pretty good results too.  Looking at the tiles from 
Geowebcache in Leaflet/OpenLayers they are crisp and fast and there is 
no problem reading small fonts.  Loading the same tiles into QGIS there 
is a marked deterioration in image quality using the default resampling 
settings of "NN/NN" and oversampling at 2x.  Changing the resampling 
method to "Cubic" and "Average" improves the image quality slightly.  
Upping the oversampling to 5x is starting to get there but the wait 
times make it unusable.  I have set the zoom levels in QGIS to the same 
scales/resolutions as used in GS/GWC.

So what is QGIS doing to the PNG tiles as it loads them from the cache?

I am running both QGIS 2.4 and the development version.

Thanks

Ross