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

From: David Singer <singer@apple.com>
Date: Thu, 17 Jun 2010 11:38:24 +0300
Cc: "www-style@w3.org" <www-style@w3.org>
Message-Id: <74F49A26-40CA-4E87-A40E-9B418F632AA5@apple.com>
To: fantasai <fantasai.lists@inkedblade.net>
cool stuff, thanks.  minor notes below.

On Jun 17, 2010, at 4:32 , fantasai wrote:

> I was given an action item to write proposed wording for CSS2.1 Issue 149
>  http://wiki.csswg.org/spec/css2.1#issue-149
> to define a fixed ratio of 4:3 for pt:px and to allow the physical value
> of these units to vary. Since it's a multimedia section and a complicated
> set of changes, I've posted the wording as HTML here:
>  http://fantasai.inkedblade.net/style/specs/css2.1/px-unit
> 
> The new text of the Absolute Units section is posted below:
> 
> | Absolute length units are only mainly useful when the physical

"Absolute units are generally only useful" perhaps?

> | properties of the output medium are output environment is known.

s/are/and/

> | The absolute units are:

add ", and always have these fixed relationships in CSS"?

> |
> |    * in: inches  1 inch is equal to 2.54 centimeters.
> |    * cm: centimeters
> |    * mm: millimeters
> |    * pt: points  the points used by CSS 2.1 are equal to 1/72nd
> |                   of an inch.

"a CSS inch"?

> |    * pc: picas  1 pica is equal to 12 points.
> |    * pt: pixel units  1 pixel unit is equal to 0.75 points.

"px: pixel units" and maybe add "(i.e. 1/96th of a CSS inch)"

I would preface the following with something like "For a CSS device, these dimensions are either anchored (i) by the length units (inch, centimeter, etc.), or (ii) they are anchored by relating the pixel unit to the reference pixel."

> |
> | At a zoom level of 100% and when the resolution of the output medium
> | is known, the absolute units should approximate their physical values.

"the absolute length units (inch, centimeter) should approximate their physical values" (or people will say that they cannot simultaneously match inch and pixel)

> | For print media and similar high-resolution devices, the reference
> | unit should be one of the standard physical units (inches, centimeters,
> | etc). For lower-resolution devices, and devices with unusual viewing

I am not sure 'unusual' adds much here, but I am not sure what other adjective to use!

> | distances, it is recommended instead that the reference unit be the
> | pixel unit: for such devices it is recommended that the pixel unit
> | refer to the whole number of device pixels that best approximates
> | the reference pixel.
> |
> | 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).
> |
> | The image below illustrates the effect of viewing distance on the size
> | of a reference pixel: a reading distance of 71 cm (28 inches) results
> | in a reference pixel of 0.26 mm, while a reading distance of 3.5 m
> | (12 feet) results in a reference pixel of 1.3 mm.
> |                                 [[IMAGE]]
> | This second image illustrates the effect of a device's resolution on
> | the pixel unit: an area of 1px by 1px is covered by a single dot in
> | a low-resolution device (a computer screen), while the same area is

"e.g. a normal computer display"?

> | covered by 16 dots in a higher resolution device (such as a 400 dpi
> | laser printer).

since we previously put print media in the other category (anchored by length units), how about "such as a high-resolution display"


David Singer
Multimedia and Software Standards, Apple Inc.
Received on Thursday, 17 June 2010 08:39:02 GMT

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