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Re: [css21][css3][svg] SVG and unit-less length values

From: Chris Lilley <chris@w3.org>
Date: Thu, 12 Aug 2010 23:47:36 +0200
Message-ID: <781036552.20100812234736@w3.org>
To: "Tab Atkins Jr." <jackalmage@gmail.com>
CC: "Dr. Olaf Hoffmann" <Dr.O.Hoffmann@gmx.de>, public-fx@w3.org, <www-style@w3.org>
On Thursday, August 12, 2010, 10:58:35 PM, Tab wrote:

TAJ> 2010/8/7 Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>:
Fantasai said:
>>>Going forward, if SVG needs coordinate units in addition to CSS lengths
>>>for SVG 2.0, we could define an actual unit identifier for them. Maybe
>>>"cu" for "coordinate system units". 

That would be massively incompatible with all deployed content. Co-ordinate pairs are used *all over the place* in SVG.

>>> Unit identifiers are an important
>>>extension point in CSS syntax; we should take advantage of that.

>> This unit is already defined, it is called 'px'. This corresponds to one
>> unit in the local coordinate system and is implicated, if a length value
>> has no unit identifier.

And in the absence of any other transform, or zoom.

TAJ> This is not at all compatible with CSS.  'px' has a very specific
TAJ> meaning, and it is not dependent on the local coordinate system.
TAJ> Modulo visual-warping effects like zoom: and some transform: values,

In other words, modulo the local to global coordinate system transform of svg, ...

TAJ> 1px is 1/96 of 1in.

Now that the CSS WG has (mostly) settled on that definition (several times) this year, yes. Previously, it was some tiny fraction of a degree (originally, the angle subtended by one device pixel on Hakon's old Sun monitor at a reading distance of one Hakon arm. Honestly.)

TAJ> The LCS-dependent lengths that SVG employs are a different unit than 'px'.

Agreed. Slapping a unit identifier onto SVG lengths and coordinates really doesn't help here except for making everyting more verbose and incompatible.

 Chris Lilley                    mailto:chris@w3.org
 Technical Director, Interaction Domain
 W3C Graphics Activity Lead
 Co-Chair, W3C Hypertext CG
Received on Thursday, 12 August 2010 21:49:24 UTC

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