[Gimp-developer] patch for scale-region.c

[Alastair M. Robinson]

Hi :)

gg at catking.net 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