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RE: Is the px unit equivalent to user units?

From: Doug Schepers <doug@schepers.cc>
Date: Mon, 2 Jan 2006 23:44:59 -0500
To: "'Jonathan Watt'" <jonathan.watt@strath.ac.uk>, <www-svg@w3.org>
Message-Id: <20060103044500.740ED5FFE2@filch.dreamhost.com>

Hi, Jonathan-

Jonathan Watt wrote:
[...]
| 'One px unit is defined to be equal to one user unit. Thus, a 
| length of "5px" is the same as a length of "5".' 
[...]
| But a little further on you find the sentence: 'Note that use 
| of px units or any other absolute unit identifiers can cause 
| inconsistent visual results on different viewing environments 
| since the size of "1px" may map to a different 
| number of user units on different systems'.
| 
| How can you reconcile these statements? Is the latter 
| statement a mistake, or is there a reason for it? 

I think that it's a bit of a sticky thing to wrap one's head around. As best
as I can make out, neither statement is not always correct. If you have
specified px units on an element, the size of the device screen in
combination with the initial viewBox and the zoom level will almost
certainly mean that the device pixels are not the actual measurement of the
element's geometry. So, equating user units to pixels (or even using pixels
as a unit) is almost certainly semantically wrong, and most likely literally
wrong as well.

This may be a legacy from CSS's insistence on unit values, or a carryover
from some earlier mechanism or scenario discussed in the SVG WG, but I don't
think that they are particularly relevant to the current realities of SVG.
In fact, I much prefer the way SVG Tiny 1.2 approaches the problem, by
moving all unit measurements to the root. The only place I see this falling
down is in relative units such as percentages, em, and ex, which may still
be useful. I would like to see the 'px' unit deprecated in SVG, in favor of
'uu' (user units) or 'csu' (coordinate space units).

Would a real SVG WG member care to chime in?


| Is the advice I'm giving sound or not?

I would personally prefer the reverse advice, if it's practical. Since SVG
Tiny 1.2 will not allow CSS units, it would be better to advise people not
to use CSS at all, except possibly in the case of stylesheets (which you do
state on your page).

Regards-
Doug

doug.schepers@vectoreal.com
www.vectoreal.com ...for scalable solutions.
 
Received on Tuesday, 3 January 2006 04:45:16 GMT

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