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Re: [CSS21] Issue 149 - px vs. pt

From: Peter Moulder <peter.moulder@monash.edu>
Date: Thu, 01 Jul 2010 06:37:18 +1000
To: "www-style@w3.org" <www-style@w3.org>
Message-id: <20100630203718.GA24593@bowman.infotech.monash.edu.au>
On Wed, Jun 16, 2010 at 06:32:37PM -0700, fantasai wrote:

>  | The reference pixel is the visual angle of one pixel on a device with
>  | a pixel density of 96dpi and a distance from the reader of an arm's
>  | length. For a nominal arm's length of 28 inches, the visual angle is
>  | therefore about 0.0213 degrees. For reading at arm's length, 1px thus
>  | corresponds to about 0.26 mm (1/96 inch).

Somehow, the existing CSS2.1 text defining a px seems to cause problems for people:
they don't have a good feel for how many px there should be to an inch on paper
or on a handheld computer or on a billboard.  Not just estimating how the
distance compares to the usual distance of a desktop monitor, but even whether
a shorter viewing distance should result in more or less than 96 px per inch.

"Subtending the same the visual angle as a 96dpi pixel does when it's 28in
away" is rather indirect.  Somewhere in the text, I suggest pointing out that
2688 reference px should equal the usual viewing distance (so 672 reference px
is a quarter of the expected viewing distance).

pjrm.
Received on Wednesday, 30 June 2010 20:37:49 GMT

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