Re: [proposal] use xz compression for Debian package by default
On Tue, Aug 28, 2012 at 12:56:47PM +0200, Vincent Lefevre wrote:
> Before wondering whether PNG files should have an additional
> compression level, is there any reason why a better PNG compression
> isn't used in the first place? For instance, "optipng -o9" tries
> various parameters and keeps the best one.
optipng can improve only earlier stages of PNG format (ARGB->paletted, pixel
filters), its deflate implementation is pretty bad. You'd want to use it
together with advpng (package advancecomp) which attacks the deflate stage
optipng -o4 -i0 -fix $* && advpng -z4 $*
(Optimizations above -o4 affect deflate only, advpng is scared by interlaced
images and files with junk after PNG data.)
As per my tests, other combinations of PNG optimizers give worse results,
and in some cases (PNGOUT, fortunately not in Debian) even destroy images.
It might be possible to compress files even better by tossing away dubious
chunks added by some editors (Adobe stuff is especially notorious), but
that can be argued to be data loss of sorts.
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