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Re: Circular paths

From: Alex Danilo <alex@abbra.com>
Date: Tue, 08 Mar 2011 14:09:05 +1100
Message-Id: <53YPHL.B5MFDI26OLEN@abbra.com>
To: Cameron McCormack <cam@mcc.id.au>
Cc: "Dr. Olaf Hoffmann" <Dr.O.Hoffmann@gmx.de>, www-svg@w3.org
Hi Cam,

	Hmm, I just read that thread which I'd completely ignored.
Some comments:
1) We implemented elliptic arcs a while ago for full compatibility
   and we get two circles, which is what David expected.
2) The talk about normalization relates to how many Beziers to
   use to approximate the arc. This is simply wrong.

	The entire point of having arcs in the path syntax is to have
accurate curves for things like rivers in maps, etc, etc. We do not
use Beziers in our implementation. Just sin() and cos() as should be
used for an arc. A Bezier approximation will always result in error.

	SVG already defines how accurate the rendering should be, i.e.
within one device pixel. We flatten our arcs to guarantee the chords
are within one device pixel as we also do for beziers.

	ASV erroneously used Beziers to approximate elliptic arcs
due to the fact their underlying library only supported beziers and
the elliptic arc syntax was introduced as a result of a Canadian
member arguing successfully for its usefulness. ASV also as a result
ends up sticking markers at the ends of the approximated bezier
segments instead of the arcs, which is clearly bad.

	I don't believe anything more needs to be specified/normalized.
If you sit on top of a library that doesn't support arcs, then extra
effort needs to made to ensure any bezier approximation produces the
correct result.


--Original Message--:
>Coming back to this old thread…
>Dirk Schulze:
>> > > That reminds me, that it would be great to have rules how to
>> > > normalize an arc :-)
>Cameron McCormack: 
>> > What kind of rules do you want to see? Just that arcs are normalized
>> > to, say, 1 or more cubic Béziers? Or a specific number of them?
>Dr. Olaf Hoffmann:
>> Them number of segments depend on the required accuracy.
>> […]
>While we won’t be defining this for SVG 1.1 Second Edition, I’ve created
>an issue so that we don’t forget to consider it for SVG 2:
>  http://www.w3.org/Graphics/SVG/WG/track/issues/2409
>Cameron McCormack ≝ http://mcc.id.au/
Received on Tuesday, 8 March 2011 03:09:47 UTC

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