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Re: question on spec

From: Wendell Piez <wapiez@mulberrytech.com>
Date: Wed, 27 Jun 2001 16:47:19 +0100
Message-Id: <5.0.2.1.0.20010627163000.0217da40@earthlink.net>
To: "Jon Ferraiolo" <jferraio@Adobe.COM>
Cc: www-svg@w3.org
Looking again at the spec, one point I made seems actually to have a wrinkle:

At 03:58 PM 6/27/01, I wrote:
>'absolute' units are defined relative to px (and thus to context), and so 
>are not really "absolute"

They are not really "absolute" because they are subject to transforms; but 
the ratio between (say) 1in and 1px, apart from transforms, is 
system-dependent, with a *presumed* relation to actual pixel size, so if no 
transforms happen to apply, '1in' may actually be equal to one inch.

Ian Tinsdale's remarks seem to me to be quite to the point. I think my 
approach will be to recommend:

1. Users set up their initial viewport height and width on the assumption 
that there may be a correspondence between "user units" and pixels (but 
test it if you need to be sure). From that point, transforms apply as per 
the spec, so the actual size of user units is controllable. The notation 
'px' should not be taken to be significant of anything in particular: it's 
a user unit.

2. Within an SVG instance, users be aware of the difference between scaling 
with user units, which are subject to transforms, and percentage values 
(relative to the viewport), which are not (cf. SVG 7.10), and avoid 
"absolute" units of measure unless special provision is made both to 
understand their relation to actual rendering devices, and to account for 
and manage the effects of transformations.

I need an informed, intelligent and defensible position on this stuff 
because I'm putting together a course....

Thanks again,
Wendell



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Wendell Piez                            mailto:wapiez@mulberrytech.com
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Received on Wednesday, 27 June 2001 16:43:45 GMT

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