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zlib filtering PNGs etc

Posted: 2007-02-01T06:05:47-07:00
by zx80user
I am writing some code to improve the compression of PNGs and I am having some difficulties with PNGs that have been edited with the GIMP (as JPEGs) and then transformed into PNGs on the fly by Perl Magick.

Essentially, if I turn on Z_FILTERED in the compression code the PNGs will not display, though a casual look over them in a hex editor reveals no problem.

If Z_DEFAULT is used then there is no problem... sorry to be vague about this but I really don't know why this happens and wonder if anyone could give me some clues to what might be up?

Posted: 2007-02-01T12:39:52-07:00
by glennrp
Please provide a sample image. You could email it to or post a URL.

A couple of urls

Posted: 2007-02-01T13:14:33-07:00
by zx80user
Generating PNG with Z_DEFAULT_STRATEGY ... pl?index=4

Generating PNG with Z_FILTERED

Posted: 2007-02-01T13:46:53-07:00
by zx80user
I may have broken the links now because I've been fiddling with the code, inserting break points etc...