On Wed, 2008-10-29 at 13:07 +0100, Claus Berghammer (Bugzilla) wrote:

> Benchmarking GIMP Scaledown Performance:
> Scale layer from 5000x5000px -> 2500x2500px:

This particular case (downscaling by 50%) is broken in GIMP 2.6.0 and
2.6.1. A workaround is in SVN and will be in the 2.6.2 release.  You
can't compare the quality without this fix.

> As Sven Neumann already mentioned (see
> http://bugzilla.gnome.org/show_bug.cgi?id=557950), the upscaling could
> be made faster easily.

Not the upscaling that you benchmarked here. What can be made faster
easily is the case that you presented in the bug report (upscaling by a
factor of 10). This is done in multiple passes right now. And I can't
think of a good reason why this is not done in a single step. So we
could apply the patch attached to that bug report.

For GIMP 2.8 we should at least review the downscaling code. We need
proper decimation code. The current approach that uses interpolation is
inherently broken.

> But I think, that there is room for performance improvements in
> downscaling too...

I've spent quite a few time optimizing this code before the 2.6 release.
The original patch was up to eight times slower than what we have now. I
doubt that you can further speed it up much without changing the
underlying algorithm.


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