On mardi 19 septembre 2017 15:19:26 CEST Roarke Gaskill wrote: > Hi, > > I am seeing an issue where after I create an overview the pixels at the > lower zoom_level are slightly shifted as compared to if I don't create an > overview. > > You can reproduce by running the following and comparing overview1.tif with > overview2.tif. They don't quite line up. You will find overview1.tif and > overview3.tif do line up. > > https://eoimages.gsfc.nasa.gov/images/imagerecords/57000/57752/land_shallow_ > topo_8192.tif gdal_translate -a_srs EPSG:4326 -a_ullr -180 -90 180 90 > land_shallow_topo_8192.tif marble.tif > > gdal_translate -of GPKG marble.tif marble.gpkg -co > TILING_SCHEME=GoogleMapsCompatible > gdal_translate -r nearest -of GTIFF -tr 156543.0339280410 156543.0339280410 > marble.gpkg overview1.tif > gdaladdo -r near marble.gpkg > gdal_translate -r nearest -of GTIFF -tr 156543.0339280410 156543.0339280410 > marble.gpkg overview2.tif > gdalwarp -r near -of GTIFF -tr 156543.0339280410 156543.0339280410 > marble.gpkg overview3.tif > > > Why would the pixels shift because of the overview creation?
I could reproduce. The issue specifically comes from nearest resampling. My suspicion is that the choice of the source pixel for a target pixel is slightly different in the algorithm that does overview building than the one used by gdal_translate (that's not the same code) If you use -r bilinear (or cubic, lanczos), you won't get this effect since all source pixels contributing to the target pixel are taken into account (and actually gdal_translate -r something_not_near uses the same code as overview building) > > > Thanks, > Roarke -- Spatialys - Geospatial professional services http://www.spatialys.com
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