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RE: Is the px unit equivalent to user units?

From: Jon Ferraiolo <jonf@adobe.com>
Date: Wed, 4 Jan 2006 11:00:18 -0800
Message-ID: <6ECA24BE410D994496A2AE995367C5C857616E@namail3.corp.adobe.com>
To: "Jonathan Watt" <jonathan.watt@strath.ac.uk>, <www-svg@w3.org>, "Doug Schepers" <doug@schepers.cc>

Jonathan,

In response to your question:
-----------
I'm asking is a length of "5px" is always the same as a length of "5"?
-----------

The anwser depends on whether you are talking about Tiny or Full, and
whether you are talking about the width/height attributes on the root
'svg' element or other length values within SVG Full.

In SVG Tiny, the only place where "5px" is allowed is on the 'width' and
height' attributes on the root 'svg' element. Assuming a web browser
context, "5px" said that the intrinsic size of the graphic is 5px, where
the definition of px units is the same as the CSS definition of px
units. Thus, if you have an html:object tag, the SVG graphic should
appear as the same size on the screen no matter if the html:object
specifies "5px" or if it specified nothing but uses the instrinsic "5px"
from the root 'svg' element.

In SVG Full 1.1, CSS absolute units are allowed in many other places for
length values, but there is a simple algorithm to convert from absolute
units to user units via a simple multiple operation. Thus, "1px" is
equivalent to "1" (i.e., 1 user unit), "1pt" is equivalent to "1.25",
and "1in" is equivalent to "90". (See
http://www.w3.org/TR/SVG11/coords.html#Units). The basic idea is that
all lengths (except for the intrinsic size from width/height on the root
'svg' element) can be quickly converted into user units.

(Note: I believe the Tiny 1.2 Last Call draft as it stands is confusing
in this area. It includes the conversion rules from CSS absolute units
to user units within section 7.11, but as far as I know there are no
features in Tiny 1.2 that would ever invoke these conversion rules.)

Jon

-----Original Message-----
From: www-svg-request@w3.org [mailto:www-svg-request@w3.org] On Behalf
Of Jonathan Watt
Sent: Wednesday, January 04, 2006 9:47 AM
To: www-svg@w3.org; Doug Schepers
Subject: Re: Is the px unit equivalent to user units?


Doug Schepers wrote:
> 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.

Sure, that's expected because despite its name the, px unit isn't a
measure of 
device pixels. It's not what I'm asking though. I'm asking is a length
of "5px" 
is always the same as a length of "5"?

> So, equating user units to pixels (or even using pixels
> as a unit) is almost certainly semantically wrong,

Well the same applies to all the other units. Anyway, I'm not interested
in the 
semantics right now, I'm only interested in the question I posed at the
start of 
this thread.

> and most likely literally
> wrong as well.

I'd be grateful if the WG could give a concrete answer to my question.
As you 
know I appreciate you answers, but a "most likely" is not so helpful.

> 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.

If so I'd appreciate a WG member making a statement to the effect that
the 
latter paragraph is wrong and will be removed at some point.

> In fact, I much prefer the way SVG Tiny 1.2 approaches the problem, by
> moving all unit measurements to the root.

Okay, but the question at hand is of pressing importance to make sure
the advice 
being given today isn't going to give people problems later. In this
thread, I'm 
not so interested in what might happen in the future with SVG Tiny 1.2.

> 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).

First of all I don't see any point in making px == user units == 
uu/csu/whatever. If px is always equal to user units then we have what
we need. 
px isn't a measure of device pixels.

Secondly I think you know my opinion on deprecating things.
Implementations 
still have to implement whatever you deprecate to be compatible with 
pre-existing content, so that isn't a great option.

> | Is the advice I'm giving sound or not?
> 
> I would personally prefer the reverse advice, if it's practical.

The advice I'm giving is for people to use px for lengths in CSS
wherever they 
would have omitted a unit (in the belief that px is always equivalent to
user 
units). The opposite advice is not practical because then the CSS is in
error in 
Firefox and I guess in any other future multi-namespace 
CSS-supporting-and-conforming UA.

> 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).

Thanks for your reply Doug.

Jonathan
Received on Wednesday, 4 January 2006 18:59:24 GMT

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