Le vendredi 28 octobre 2016 10:37:27, Rutger a écrit : > While on the subject, i'm not aware of the implementation details of GDAL's > bilinear algorithm**. But normally you go from four source points/pixels to > a new one. How is this handled in extreme downsampling cases like Travis is > doing?
All resampling methods (except Gauss but that should likely be fixed, and nearest neighbour of course) since GDAL 2.0 take into account all the needed source pixels. For a bilinear interpolation without resampling (or with oversampling), the weight function (in 1D) is : 1 - |x| if |x| <= 1 0 otherwise where x is the distance of the center of the target pixel (considered in the dimensions of the source image) to the center of the source pixel considered When downsampling by a factor of N, it is generalized as: (1 - |x/N|)/N if |x| <= N 0 otherwise So you have 2*N pixels taken into account So when downsampling by a factor of 20, it is (2*20)^2 = 1600 source pixels taken into account for bilinear. Example, let's consider test.asc being: ncols 6 nrows 6 xllcorner 0 yllcorner 0 cellsize 0 0 0 0 0 0 0 0 100 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 $ gdal_translate test.asc /vsistdout/ -of aaigrid -r bilinear -outsize 50% 50% ncols 3 nrows 3 xllcorner 0.000000000000 yllcorner 0.000000000000 cellsize 0.000000000000 18 5 0 5 2 0 0 0 0 This is the same result you'd got by using imagemagick "convert -filter triangle -resize 50% in.tif out.tif" Whereas average (after https://trac.osgeo.org/gdal/changeset/35991, so this email took me all the morning lol) will return: $ gdal_translate test.asc /vsistdout/ -of aaigrid -r average -outsize 50% 50% ncols 3 nrows 3 xllcorner 0.000000000000 yllcorner 0.000000000000 cellsize 0.000000000000 25 0 0 0 0 0 0 0 0 Even -- Spatialys - Geospatial professional services http://www.spatialys.com _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
