@Eric P:
Upscaling image:
500px -> 5000px (bicubic):
Gimp 2.6.1: 35,21 sec
Gimp 2.4.7: 6,9 sec

Downscaling layer:
Image is 5000x5000 px, 2 white layers
Scaling top layer to 2500x2500px (bicubic):
Gimp 2.6.1: 7,85 sec
Gimp 2.4.7: 4,78 sec

@Sven Neumann:
Thanks for your hint on the related bugthread. I will read it carefully to
better understand the whole thing.


Eric P wrote:
> Claus Berghammer wrote:
>> Hello Gimp Users and Developers,
>> This is a follow up of Bug 557950 (which in fact isn't a bug, according
>> to
>> Sven Neumann ;-)
>> http://bugzilla.gnome.org/show_bug.cgi?id=557950
>> As described in the “Bug”, scaling in Gimp 2.6 series is far slower, than
>> it
>> was in 2.4. Sven Neumann commented:
>> “We have completely changed the scaling implementation. The new algorithm
>> is
>> slower for some cases, but that is not a bug.”
>> Since there is no explanation WHY the algorithm was rewritten, I guess 2
>> possible reasons:
>>      1.)The old code did something wrong in some cases
>>      2.)The new code was necessary due to GEGL integration
>> For the first point, I compared scaling results from 2.4 and 2.6, and
>> they
>> are (ignoring some harmless alignment issues) 100% identical (using
>> difference blend mode). I also cannot remember, that in the past years,
>> the
>> scaling routine in Gimp produced noticeable wrong results. (Beside the
>> lanczos interpolation, that didn't work right, when it was introduced)
>> So my question is, isn't it possible, to have both algorithms in Gimp,
>> and
>> let the user decide which one he wants to use? (Option in Scale Dialog) 
>> If it was due to point 2, the GEGL integration, than can we expect a
>> faster
>> version of the new scaling routine? Or will it be automatically faster,
>> when
>> GEGL is integrated more/better?
>> The current situation draws some users (not myself) to not use Gimp 2.6,
>> and
>> stick with 2.4 instead, because the difference in speed is so
>> dramatically.
>> Sincerely, Claus Berghammer
> I'd be curious to see some benchmarks comparing 2.4 and 2.6 in this regard
> so that we know just how dramatically
> different the speed is.
> Eric P.
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