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Re: question on spec

From: Wendell Piez <wapiez@mulberrytech.com>
Date: Wed, 27 Jun 2001 15:58:45 +0100
Message-Id: <>
To: "Jon Ferraiolo" <jferraio@Adobe.COM>
Cc: www-svg@w3.org
Hi Jon,

To extend your example, if we had

<svg width="90px" height="90px" viewBox="0 0 900 900">
    <rect x="0" y="0" width="450px" height="450px" fill="blue"/>
    <rect x="50%" y="50%" width="5in" height="5in" fill="green"/>

The first rectangle (which happens to be square) will be in the upper-left 
corner and occupy one-quarter the area of the initial (also square) viewport.

The second rectangle will be the same size, but will appear at the lower 
right of the viewport. This is because the 'in' unit is also subject to the 
transformation (which is why it's a bad idea to use it except in special 
circumstances). 1in is always 90px, whatever 'px' happens to be.

Correct? (Both the Adobe viewer and Batik show it like this.)

Would the following statement be better than my last attempt?

"An unlabelled measure, e.g. width='100', gives user units, which are 
equivalent to 'px', are initially system-dependent, and may also be 
affected by operations that change the working scale such as viewBox 
transformations. Since 'absolute' units such as 'in' and 'cm' are actually 
defined relative to user units, they cannot be assumed to correspond to 
'real' inches and centimeters except when (a) no transformations are being 
applied, and (b) an untransformed 'px' unit (user unit) happens actually to 
be 1/90 of an inch (regardless of actual pixel size).

I think it may seem more complex than it actually is. The bottom line as it 
stands seems to be: 'px' is always the same as user units; both px and user 
units are relative to context (being not only system-dependent, inasmuch as 
they are initially 'negotiated', but also subject to transformations); 
'absolute' units are defined relative to px (and thus to context), and so 
are not really "absolute".

Thanks again for your patience on this, and sorry to be such trouble.


Wendell Piez                            mailto:wapiez@mulberrytech.com
Mulberry Technologies, Inc.                http://www.mulberrytech.com
17 West Jefferson Street                    Direct Phone: 301/315-9635
Suite 207                                          Phone: 301/315-9631
Rockville, MD  20850                                 Fax: 301/315-8285
   Mulberry Technologies: A Consultancy Specializing in SGML and XML
Received on Wednesday, 27 June 2001 15:55:45 UTC

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