[Gimp-developer] need help with bug #464466 (downscaling quality)

[gg at catking.net]

On Fri, 01 Aug 2008 13:51:02 +0200, David Gowers <00ai99 at gmail.com> wrote:

>  Pixel art ('block graphics'?) is a
> pathological case for most scaling algorithyms, and no algorithym old
> or new performed very well on upscaling image #8.
> David


yes, this is a good test for scaling alorithms. I had not really  
apreciated the nature of this image since, although pixel art, it does  
have predominantly degraded edges so it is not quite the sort of block  
graphic I was refering to. However, picking out some specific areas, it  
does give some good information on the merits of different algorithms for  
this kind of image.

Viewing the comparitive for 133% at 200% (in Opera) there is a noticable  
difference between cubic and lanczos.
http://svenfoo.org/scaletest/133Z8g.png
http://svenfoo.org/scaletest/133C8g.png

opening these two in adjacent tabs in opera and flipping allows direct  
comparison. This difference is quite marked.

Comparing directly to the original at 266% with scaled image at 200% there  
is an impressive restoration of detail lost to pixelisation in the orginal.

The eyebrow, hair and eye are well enhanced as it the hand on the blue  
figure. The staircasing on the knee and thigh of the yellow figure are  
also well smoothed out, though not removed completely.

All parts of this image seem to have been noticably enhanced , I think  
it's a good demonstration of the lanczos code.

Although downscaling is probably the more critical test (and the case that  
was needing most improvement) it may be informative to add another  
enlargement to some oddball ratio (say 3.14159) to see how the detail  
comes out when the result is several times bigger.

BTW , has this code been ported to the rotation operation? IIRC, that was  
a separte file that closely duplicated the scaling code. Maybe this would  
be a good point to unify the two.

In any case I'm very pleased to see the lanczos results on this image.  
Very impressive.

regards.



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