Hi Andrzej, I think this code is optimized for the case that you use fillArray() once and calculate the interpolation with multiple different combinations of qx + qy. When you use e.g. a 3'' hgt file with dem-dists=9936,... this happens rarely. When you use the same file with dem-dists=2000 this algo is probable faster if we detect the cases where we can reuse the class and the filled array, but such a value doesn't make sense. Maybe it is also faster for a value like 4971 (9942/2), I did not try that because I thought that one should try to be close to the hgt res. See also http://www.paulinternet.nl/?page=bicubic where I found the simple code that is now used. It also offers a variant similar to the one from Ken Perlin. I'll try later if this code really improves throughput when users specifies small values for dem-dists. If that is not much faster I'd prefer to keep the rather simple code that is used now and rather tell the user that the dem-dists value is too small. To be honest, I am not sure if any value that is much smaller than the hgt resolution makes sense, as it just blows up DEM with interpolated values, but maybe I don't understand the math enough. Gerd ________________________________________ Von: mkgmap-dev <mkgmap-dev-bounces@lists.mkgmap.org.uk> im Auftrag von Andrzej Popowski <popej@poczta.onet.pl> Gesendet: Dienstag, 23. Januar 2018 13:22:01 An: mkgmap-dev@lists.mkgmap.org.uk Betreff: Re: [mkgmap-dev] DEM: performance with bicubic interpolation Hi Gerd, there are different kind of bicubic interpolations. I'm not good at this math, I think the previous version was actually bicubic spline interpolation. See other possibilities here: http://mrl.nyu.edu/~perlin/cubic/Cubic_java.html I don't know, which type of spline or cubic interpolation is best for DEM. Maybe differences aren't big, but I think it is better to have good interpolation than fast one. You compile a map once but then many people can use it. It shouldn't be difficult to include an option like: --dem-interp=.. -- Best regards, Andrzej _______________________________________________ mkgmap-dev mailing list mkgmap-dev@lists.mkgmap.org.uk http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev