Hi :) gg@xxxxxxxxxxx wrote: > Comparing Lanczos 3% old vs patched: lefthand building roof has bad moire > effects that totally obscure underlying detail. Both sets of trees have > much less obvious staircasing in the current code. There is an overall > impression of sharpness in the new code but this seems really to be just > high contrast artifacts with a lack of intermediate tones. I think these are aliasing artifacts caused by high-frequency components in the original image - unless you take steps to remove frequencies higher than the target sample rate before resampling a signal, aliasing will result. And as you noted, it affects my code too. Reducing that effect required some form of low-pass filtering before scaling - to remove the high frequency components which can't be represented in the lower-resolution image. Here's another version of the 3% reduction image, with a 33 radius (100% / 3%) gaussian blur applied before the reduction: http://www.blackfiveservices.co.uk/3PercentPreBlur.png I also note that my original 3% version was one pixel narrower than Sven's, so here it is again: http://www.blackfiveservices.co.uk/3Percent.png > Interestingly the blackfive code (thanks for sending the that algo > Alistair) seems even harsher but does give some impression of sharpness by > apparently accentuating edges. I suspect that's just the result of a "cleaner", single-stage reduction with the aliasing artifacts on top. > If this is considered from an analytical , data processing perspective I > can't imaginge what the frequency responce of this multipass approach must > look like. Chances are it would be low-pass to some degree, so arguably a beneficial side-effect, even if a designed filter would be an improvement! All the best, -- Alastair M. Robinson _______________________________________________ Gimp-developer mailing list Gimp-developer@xxxxxxxxxxxxxxxxxxxxxx https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
