I want to warp Geotiff image of shaded relief (RGBA) in projection EPSG:8353 to the projection EPSG:3857 to use it in Mapnik web tiled map. I want to use lanczos resampling, but the result image looks "jagged", like nearest resampling has been used. I am using this command:
gdalwarp \ -of GTiff -overwrite -r lanczos \ -tr 1.194328566968441 1.194328566968441 -tap \ -t_srs EPSG:3857 shading-sjtsk.tif shading-web.tif
The target resolution in georeferenced units is set to be the same as zoom level 17 in web mercator projection of tiled map.
Please could you advise me, why the lanczos resampling is not used? I tried also other methods (like cubic), but image always seems to be indistinguishable from nearest.