Re: Alternative pixel-based regtest checker

[Phil Holmes]

----- Original Message ----- 
From: "Werner LEMBERG" <address@hidden>
To: <address@hidden>
Cc: <address@hidden>; <address@hidden>; <address@hidden>
Sent: Sunday, March 03, 2013 7:06 AM
Subject: Re: Alternative pixel-based regtest checker


> > 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

I did some more looking at this last night, but didn't have time to post the 
results.  There were lots of differences in a couple of PNG files which 
appeared identical visually.  I actually converted them to bitmaps and found 
there were six pixels different, to the tune of 1/255 in brightness - so 
they were effectively the same.  I assume the PNGs were so different because 
of the way the compression algorithm works.  It's certainly not meta data in 
these files - most comparisons show 2 files created by different lilypond 
versions to be byte-identical.

I've done some more work on the comparison and will post an update once it's 
worth demonstrating.  Not sure when that will be - rehearsing and college 
work is pressing.

--
Phil Holmes