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

Re: Making pt a non-physical unit

From: David Singer <singer@apple.com>
Date: Wed, 13 Jan 2010 16:01:11 -0800
Message-Id: <1ED68CB0-3F06-43A6-A26D-40CC2DB74451@apple.com>
To: www-style <www-style@w3.org>
So, to summarize, I think a number of us have asked a slightly different question from the one in the subject, as 'physical unit' is not always desirable or well-defined.  As I understand it, webkit and others define that there are 72 points to 96 pixels (4:3).  

I think the suggestion on the table is:

A CSS pixel is defined in section 4.3.2 of <http://www.w3.org/TR/CSS21/syndata.html#length-units>, as a square containing an integer number of device pixels that, when viewed with a zoom setting of 1:1, subtends an angle at the eye most closely matching the angle subtended at the eye by 1/96 of an inch viewed from a distance of 28 inches.  (What happens if device pixels are non-square, as happens on TV?  Is it an integer number in the horizontal direction?)

We then define that a CSS inch is exactly 96 CSS pixels, that there are 72 CSS points to the CSS inch, 2.54 CSS mm, and so on. All the physical units relate on this notional viewing surface.

We don't give access to true dimensions on the display surface (because not all displays have a well-defined surface, and for electronic and paper billboards the numbers would be really large, and so on). We don't give access to true pixels (because it's useless unless you either or both of the number of true pixels per true inch, or true pixels per CSS inch).

I don't know if my colleagues agree, but I think this works for any display system - paper, screen, projected, etc.  It allows manufacturers to adjust based on expected or actual viewing distance.  Zoom is a separated question (yes, if the user asks for a 2:1 zoom, then things should look twice as large as at 1:1 zoom, surprise).

The question is...does this 'work'?

David Singer
Multimedia and Software Standards, Apple Inc.
Received on Thursday, 14 January 2010 00:01:46 UTC

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