W3C home > Mailing lists > Public > www-svg@w3.org > February 2012

Re: path data attribute lacks units!

From: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>
Date: Tue, 14 Feb 2012 11:14:26 +0100
To: www-svg@w3.org
Message-Id: <201202141114.27401.Dr.O.Hoffmann@gmx.de>
Tab Atkins Jr.:
> Let's not rehash this discussion. 

Seems to be useful, if someone still wants to know how to
create files or path data with (absolute) units.
It would be misleading to conceal practical and theoretical
problems of this approach. 

> Dr. Hoffman 

n

> doesn't understand the  
> reasoning behind the definition of the css units, but all the browsers do,

Well, it is easy to understand and at least I and the implementors do:
Because typical implementations failed to meet the definition in CSS 2.0,
presumably because it is/was not always simple or possible to present a
good approximation on some devices or operating systems for absolute
units, those units were obfuscated in CSS 2.1 ;o)
Or vice versa, if one starts with an international standard unit like mm,
the unit px becomes redefined as a derivation of a standard unit, 
but currently viewers don't do it this way - especially because for 
most screen applications of CSS this consistent interpretation would 
not be very useful - and of course, the reason why they did not manage
to display a cm on screen as a cm did not change just with CSS 2.1.

> and there's no theoretical merit to his complaints. 

As scientist, well more experimental than theory, I know exactly
what units and uncertainties of measurements are - 
and what causes problems. And it is not very difficult to understand,
that those practical problems for authors and bugs in
implementations for SVG result finally from the  contradictions in
CSS 2.1 respectively the bugs in implementations, CSS 2.1 was
tried to be aligned with, what obviously cannot work with already
defined absolute units like mm, cm etc 
(ex contradictione sequitur quodlibet ;o)

And indeed it is more a practical problem. Obviously it turned
out, that typically absolute units were not implemented correctly,
but some authors used them nevertheless - obviously some of
them not with the intention, that this will display correctly in the future,
if the implementations problems are solved.
And some people had the impression that it is more important, 
that those authors can rely on the wrong implementation (in some viewers) 
than to depreciate/remove those units from CSS (for screens or in general), 
to concede, that they do not gain in practice what they promise, or to fix the
bugs in operation systems and viewers.
An if SVG aligns with CSS 2.1 in the future or already now in real existing
implementations, the problem is relevant here as well, especially because 
for SVG there are much more applications for absolute units than for
CSS2.1+(X)HTML on screens.


> As you point out, Doug, 
> printers have the necessary resolution and information to faithfully render
> units like 'in'. If they currently fail to, that's merely an implementation
> bug.

Indeed, here I agree, all this is based on implementation bugs.
But if one cannot rely on this and there is a risk for SVG to align with
CSS 2.1 in the future even in theory, for authors this simply means, 
that absolute units are of no use, at least if the documents are 
presented somehow or if the interpretations try to follow the
CSS 2.1 recommendation.


Olaf
Received on Tuesday, 14 February 2012 10:14:56 GMT

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