https://bugzilla.wikimedia.org/show_bug.cgi?id=24857
--- Comment #7 from Tim Starling <tstarl...@wikimedia.org> 2010-08-20 08:06:05
UTC ---
The issue is an arbitrary change in SharpenImageChannel() made some time
between ImageMagick 6.4.9 and 6.5.1. The relevant code changed from this:
alpha=exp(-((double) u*u+v*v)/(2.0*MagickSigma*MagickSigma));
kernel[i]=(double) (-alpha/(2.0*MagickPI*MagickSigma*MagickSigma));
if ((width < 3) || (u != 0) || (v != 0))
normalize+=kernel[i];
To this:
alpha=exp(-((double) u*u+v*v)/(2.0*MagickSigma*MagickSigma));
kernel[i]=(double) (-alpha/(2.0*MagickPI*MagickSigma*MagickSigma));
normalize+=kernel[i];
The result is that for a given sigma, images look more blurry in the new
convert. Based on brief testing, to reproduce a result similar to the old 0x0.4
requires a sharpen parameter of approximately 0x0.8 in the new ImageMagick.
Both -unsharp and -sharpen exist in both versions, and have completely
different implementations. Unsharp does not appear to have changed
significantly, so it might be a candidate for version-independent operation in
the next MediaWiki release. But for now I will just change our configuration to
use 0x0.8 instead of 0x0.4.
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