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List:       gimp-user
Subject:    Re: [Gimp-user] incremental scaling
From:       David Hodson <hodsond () ozemail ! com ! au>
Date:       2005-02-20 12:23:07
Message-ID: 4218812B.5020909 () ozemail ! com ! au
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 > David Hodson wrote:
 >
 >> Alexander Rabtchevich wrote:
 >>
 >>> It is known incremental scaling produces better results than one-step
 >>> scaling.
 >>
 >> Only if your scaling algoroithm is not very good. Better would be
 >> to fix any problems in the scaling algorithm, and scale in a
 >> single pass.

(Sorry, I accidentally replied direct to Alexander first time...)

Alexander Rabtchevich wrote:
> I've downsampled from 1562x1562 (crop) to 64x64 (avatar). And the
> incremental scaling produced definetely better results. GIMP 2.2.3,
> Windows, cubic scaling.

The problem with scaling down by a large factor is that you need
to average out a large number of pixels to get an accurate result.
As far as I know, the Gimp rescale function uses the same algorithm
for scaling either up or down, so if you're using cubic interpolation
it will only sample 16 pixels.

The simplest solution is to blur the image before scaling down.
I'd try with a radius about the same as the scaling factor, but
that's just a guess.

-- 
David Hodson  --  this night wounds time
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