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filterRes

From: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>
Date: Thu, 9 Jul 2009 12:14:09 +0200
To: www-svg@w3.org
Message-Id: <200907091214.09818.Dr.O.Hoffmann@gmx.de>
Hello,


I think, the wording of the definition of the
filter attribute filterRes is a little bit
confusing (both in SVG 1.1 and the 1.2 draft).
http://www.w3.org/TR/2007/WD-SVGFilter12-20070501/#FilterEffectsRegion
http://www.w3.org/TR/SVG11/filters.html#FilterEffectsRegion

It is noted:
"
filterRes (which has the form x-pixels [y-pixels]) indicates the width and
height of the intermediate images in pixels. If not provided, then a
reasonable default resolution appropriate for the target device will be 
used. (For displays, an appropriate display resolution, preferably the current
display's pixel resolution, is the default. For printing, an appropriate
common printer resolution, such as 400dpi, is the default.)
"

Looking on the behaviour of Opera and the adobe plugin, meanwhile I'm pretty
sure, that the value are one or two numbers representing lengths and no
resolution. Related to this area a resolution is typically measured in
'something per length' with 'something' indicating, how many distinguishable
fractions one length unit can contain. Indeed therefore a resolution can
be a number without a unit, but then it does not represent a width or height.
However width and height are only indicated in the first sentence, the others
speak about a resolution, what is not a length but something like the
mentioned dots per inch.
This does not fit together and especially a number presenting a resolution
cannot be the default for numbers representing lengths. But lengths can be
calculated, if a resolution is known and one characteristic size.
As far as I understand this, the filter region has some width and height in 
device pixels (dots) and at the same time the filter region has some width
and height in user/filter coordinate units.
A viewer can now determine a resolution for example in device pixels 
per user/filter coordinate unit.
And the default value for filterRes is the size of the filter region in device
pixels.
Is this correct?

If yes, I think, the wording should be clarified slightly. 
If not, it should be clarified even more, to ensure that author
get the meaning right and use it in the intended way.


Best wishes

Olaf
Received on Thursday, 9 July 2009 10:23:32 GMT

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