On 11/19/05, Roel Schroeven <[EMAIL PROTECTED]> wrote:
> Sven Neumann wrote:
> > The Lanczos implementation in CVS is buggy and unless someone shows up
> > who fixes it, it will most likely end up being removed before the 2.4
> > release.
> I might take a shot at fixing that, if I can find enough time to do it.
> Is it bug 305928 you mean? On first sight that is more about the
> interaction between Lanczos and the perspective tool than about Lanczos
> itself.

The bug in the perspective tool is that the lanczos algorithm uses a
different offset from bilinear/bicubic/adaptive supersampling. The
discontinuity that can be observed in e.g.
http://pippin.gimp.org/tmp/buggy.png is the threshold around the 1:1
resampling. The point where the code in the transform tool chooses
between interpolation(lanczos) and decimation(adaptive supersampling).

The problem with using bilinear/bicubic/lanczos for decimation is
(apart from being
wrong in the first place) is that you end up throwing away pixels. For
bicubic a destination pixels uses information from a 4x4 source window
in the original image.
This is wrong. If you for instance scale to 10% of the original image
size. You need to include data from at least a 10x10px window.

/Øyvind K.
«The future is already here. It's just not very evenly distributed»
                                                 -- William Gibson
http://pippin.gimp.org/                            http://ffii.org/
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