[EMAIL PROTECTED] 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:
I also note that my original 3% version was one pixel narrower than
Sven's, so here it is again:
> 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
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