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Re: Alternative pixel-based regtest checker
From: |
Werner LEMBERG |
Subject: |
Re: Alternative pixel-based regtest checker |
Date: |
Sun, 03 Mar 2013 08:06:39 +0100 (CET) |
> I know from my Windows pixel-based checker that, generally, the PNG
> files contain the same image, version after version (there are a few
> oddities with some text, which I'm used to). I would have believed
> that identical images would only be produced by identical PNG files?
Usually, picture container formats like PNG also contain metadata like
the date of image creation or a comment. This makes PNGs different
even in case the contained images are identical.
Additionally, it's possible that identical images are stored in
different formats, for example, a B/W image can be stored either as a
1-bit bitmap or an 8-bit pixmap.
However, you could convert PNG images temporarily to the PGM format
to compute a hashsum: PGM doesn't have metadata.
http://en.wikipedia.org/wiki/Netpbm_format
The program to do this is `pngtopnm', probably followed by a call to
`pgmtopgm' to assure that B/W images are handled as grayscale images
too:
pngtopnm image.png \
| pgmtopgm \
| md5sum -b > image.md5
Werner
Re: Alternative pixel-based regtest checker, Phil Holmes, 2013/03/03
Re: Alternative pixel-based regtest checker, Julien Rioux, 2013/03/09