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Re: [CSS21] 4.3.2 Lengths (reference pixel?)

From: David Singer <singer@apple.com>
Date: Thu, 9 Dec 2010 11:48:24 -0600
Cc: www-style@w3.org
Message-Id: <857F4C6B-D54C-4945-8924-DCE1FB806560@apple.com>
To: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>
I don't understand your question, and I don't think you understand what is defined here.

In the cases where there is no 'display surface' to measure on, or when it is viewed at distances that mean that some other size is appropriate (e.g. when a normal web page is shown on an electronic billboard), this set of relationships is used.  Did you read the section where it talks about choosing an anchor, that measurements are either anchored in physical lengths on the page, or by the reference pixel?

The only thing I'd maybe complain about here is that we appear to define that an inch is exactly 2.54 cm, whereas I think that's not a relationship for us to define, merely inform about.  But since it's right, it hardly matters.

On Dec 9, 2010, at 11:31 , Dr. Olaf Hoffmann wrote:

> Hello,
> 
> as far as I understand this, after the changes, 
> the sections about the reference pixel are misleading.
> 
> It is defined now:
> "
> in: inches  1in is equal to 2.54cm.
> cm: centimeters
> mm: millimeters
> pt: points  the points used by CSS are equal to 1/72nd of 1in. 
> pc: picas  1pc is equal to 12pt.
> px: pixel units  1px is equal to 0.75pt.
> "
> And because the size of a mm or cm is precisely known by measurements
> of a second and the defintion of the speed of light,
> a px results in (127/480) mm.
> A standard unit like a second (and therefore derived units like
> meter, mm, cm etc) are of high cultural and scientific importance
> and it is as well important for techniques and trade to be able to
> rely on the assumption, that all people follow the same definition
> of such international standard units (to avoid catastrophes and
> cheating). Therefore there cannot be another definition for 
> international standard units. 
> (It is of course possible to propose a high precision method and
> measurement of 1s to improve the current definition, but this does
> not happen here obviously).
> 
> 
> Therefore the section about the reference pixel and the image
> about it is misleading now or in contradiction with the new definition
> above, because there can be only one definition of a px.
> I propose to remove this part about the reference pixel therefore.
> 
> Olaf
> 

David Singer
Multimedia and Software Standards, Apple Inc.
Received on Thursday, 9 December 2010 17:48:58 GMT

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