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: Fri, 15 Jan 2010 18:01:04 -0800
Message-Id: <F6CF0A51-3CB2-473B-B8C3-677205465D6A@apple.com>
To: www-style Group <www-style@w3.org>
So, are we leaning towards something like the following?

* * * 

All CSS units have a fixed relationship to each other:  there are 96 CSS pixels to the CSS inch, 72 CSS points to the CSS inch, 2.54 CSS mm to the CSS inch, and so on.

It is recommended (required?) that a CSS pixel be an integer number of device pixels, particularly if the ratio of CSS pixel to device pixel size is small.

A CSS pixel normally subtends an angle of XXX (insert CSS 2.1 text here) at the eye when viewed at a normal arm's length distance.

Manufacturers may choose how these units map to their device, and may also offer users control over the mapping.

In print media intended to be read 'at hand', the CSS inch should generally map to a true inch on the printed surface.  Printers generally have a higher resolution than 96 dpi, so a CS pixel is therefore typically larger than a device pixel.

In screen media, a CSS pixel is often (recommended to be?) one or more device pixels, subtending the angle given above at the eye when viewed from the expected viewing distance.  For 96 dpi computer displays read at a normal viewing distance, both CSS pixel and CSS inch may map to their true counterparts.  For small high-resolution displays intended for 'close up' viewing, this means that a CSS pixel may map to a device pixel and that a CSS inch may be smaller than a physical inch on the screen, and for large displays intended for distant viewing, a CSS inch may be larger than a CSS inch.

In both printed and electronic 'billboard' displays, a CSS inch may be much larger, therefore, than a physical inch, so that a CSS pixel subtends the expected angle at the eye for the typical viewer.

* * * *

Does this give enough discretion to manufacturers and enough information and predictability to designers?  Note that if you are designing for a known display, you know its mapping, and you're ok (as long as you can use units large or small enough for the effect you want).

David Singer
Multimedia and Software Standards, Apple Inc.
Received on Saturday, 16 January 2010 02:01:39 UTC

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