W3C home > Mailing lists > Public > www-style@w3.org > January 2010

Re: Making pt a non-physical unit

From: Boris Zbarsky <bzbarsky@MIT.EDU>
Date: Mon, 11 Jan 2010 00:56:40 -0500
Message-ID: <4B4ABD98.7000909@mit.edu>
To: Alex Mogilevsky <alexmog@microsoft.com>
CC: "L. David Baron" <dbaron@dbaron.org>, "robert@ocallahan.org" <robert@ocallahan.org>, www-style <www-style@w3.org>
On 1/11/10 12:38 AM, Alex Mogilevsky wrote:

 > In IE definition of units, 1pt is exactly 1/72 of an inch, as well as 
 > 1mm is 1/254 of an inch. 1px is a unit that used to be more
 > complicated but I believe it is now 1/96 of an inch (in any mode or
 > zoom).

When you say "inch" here, do you mean a physical inch (as measured with 
a ruler on the screen)?  Or a logical inch (defined how?).

I guess this is two questions, really:

1)  Is it the case that CSS '72pt', when measured on screen with a ruler
     I buy at my nearest back-to-school sale, will measure equal to 1
     inch on that ruler?
2)  If not, does the length it will measure equal to depend on anything
     other than the actual physical screen and the resolution the screen
     is being driven at?  In particular, does it depend on the Windows
     "large fonts" settings or the like?

> At some point in this thread there was a notion that CSS2.1 would
> have to change to agree with pt being always proportional to px. Does
> it?

Since at the moment a CSS 2.1 'pt' is defined as an actual physical 'pt' 
and a 'px' is defined as a length subtending a particular angle of arc, 
the answer seems to be "yes" if the question was whether the spec would 
need to change if it were decided that the spec should say that 'pt' is 
always proportional to 'px'.

If the question was whether the spec should say that such a 
proportionality exists, then that is precisely the topic of the 
discussion, right?

Received on Monday, 11 January 2010 05:57:16 UTC

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