W3C home > Mailing lists > Public > png-group@w3.org > October 2005

A little design flaw

From: Dirk Schenkewitz <schenkewitz@docomolab-euro.com>
Date: Fri, 21 Oct 2005 17:05:33 +0200
Message-ID: <435903BD.9000504@docomolab-euro.com>
To: png-group@w3.org

Dear All,

several years ago when I read the PNG specification for the first
time, I saw a little design flaw, regarding the pHYs-tag. The only
"known unit" is metre and the resolution value must be a 4-bit

Now we all know that practically all existing devices have their
resolution defined in "dpi" (dots per inch), while an inch is

That means that it is impossible to specify the resolution value
of a real-world device without losing some precision, because 1m
obviously cannot be divided by 25.4mm without a remainder.

One possible solution: Specify that unit = 2 means the unit is

I'm a bit astonished that over the years nobody seemed to notice
this, at least I did not find any hint in the archives for "dpi"
or "inch" and only one for "metre" which is about spelling.

Is it possible that nobody ever uses this tag?

Best regards
   Dirk Schenkewitz
Received on Saturday, 22 October 2005 06:53:11 UTC

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