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

From: fantasai <fantasai.lists@inkedblade.net>
Date: Thu, 17 Jun 2010 13:06:13 -0700
Message-ID: <4C1A8035.6070401@inkedblade.net>
To: David Singer <singer@apple.com>
CC: "www-style@w3.org" <www-style@w3.org>
On 06/17/2010 01:38 AM, David Singer wrote:
> 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/

Ah, sorry, I forgot to account for the <del>s in that paragraph. It
should read

   | Absolute length units are mainly useful when the output environment
   | is known.

>> | The absolute units are:
>
> add ", and always have these fixed relationships in CSS"?

Added.

>> |
>> |    * 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)

Ok, here's the new paragraph:

  | For a CSS device, these dimensions are either anchored (i) by
  | relating the physical units to their physical measurements, or
  | (ii) by relating the pixel unit to the <i>reference pixel</i>.
  | For print media and similar high-resolution devices, the anchor
  | unit should be one of the standard physical units (inches,
  | centimeters, etc). For lower-resolution devices, and devices
  | with unusual viewing distances, it is recommended instead that
  | the anchor 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.

>> | 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"

But the px unit still exists here. The point here is that
in higher-resolution devices a px unit is not the same as a
device pixel. I've removed the resolution from the example;
it should be consistent now.

http://fantasai.inkedblade.net/style/specs/css2.1/px-unit

~fantasai
Received on Thursday, 17 June 2010 20:06:47 GMT

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