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Re: Aspect ratios in SVG: Numbers or integers?

From: Chris Lilley <chris@w3.org>
Date: Fri, 7 May 2010 18:57:09 +0200
Message-ID: <1337732187.20100507185709@w3.org>
To: "Tab Atkins Jr." <jackalmage@gmail.com>
CC: public-fx@w3.org
On Friday, May 7, 2010, 6:22:03 PM, Tab wrote:

TAJ> In the CSS list we're discussing an aspect-ratio property which would
TAJ> hold a box to a particular width/height ratio.  Currently the proposed
TAJ> syntax is for the property to take a single <number> or two
TAJ> <integer>s.

TAJ> Someone on the list pointed out that he thought SVG allowed aspect
TAJ> ratios to be specified with two numbers (that is, something like "8 by
TAJ> 4.5" rather than "16 by 9").  He couldn't find a reference when I
TAJ> asked for it, though.

Look at the definition of the viewBox attribute
http://dev.w3.org/SVG/profiles/1.1F2/master/coords.html#ViewBoxAttribute

"The value of the ‘viewBox’ attribute is a list of four numbers <min-x>, <min-y>, <width>  and <height>, separated by whitespace and/or a comma"

where number is defined here
http://dev.w3.org/SVG/profiles/1.1F2/master/types.html#DataTypeNumber


TAJ> So, in similar situations where you specify an aspect ratio as two
TAJ> numbers, does SVG have any prior art on whether the two should be
TAJ> integers or numbers?  I'd like it to match if so.

Two numbers. Note that currently, the definition of number in SVG depends on whether it has to disallow scientific notation to fit in with CSS grammar, or not. So in a stylesheet, its

number ::= integer
           | [+-]? [0-9]* "." [0-9]+

while in an attribute (such as viewBox), its

number ::= integer ([Ee] integer)?
           | [+-]? [0-9]* "." [0-9]+ ([Ee] integer)?



Its also worth noting that viewBox is establishing a coordinate system, not just an aspect ratio. So

0 0 16 9
0 0 16000 9000

establish different coordinate systems, but the same aspect ratio.

-- 
 Chris Lilley                    mailto:chris@w3.org
 Technical Director, Interaction Domain
 W3C Graphics Activity Lead
 Co-Chair, W3C Hypertext CG
Received on Friday, 7 May 2010 16:57:29 GMT

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