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Re: Elliptical Arc as basic shape

From: Chris Lilley <chris@w3.org>
Date: Thu, 26 Aug 2004 11:07:27 +0200
Message-ID: <1198083076.20040826110727@w3.org>
To: Dean Jackson <dean@w3.org>
Cc: Philippe Lhoste <PhiLho@GMX.net>, "SVG (www) list" <www-svg@w3.org>

On Thursday, August 26, 2004, 8:03:30 AM, Dean wrote:

DJ> On Mon 23 Aug 2004, Philippe Lhoste wrote:

>> - There is already an arc primitive in the path tag (but also polyline
>> and polygons can be made with path, and are more redundant, IMO);

DJ> Unfortunately implementors hate the arc command in path. That's
DJ> why it isn't supported in Tiny.

>> - They need too many parameters (stated in the message above, seems to
>> be a strange argument...);

DJ> That's one reason why implementors and authors hate it :)

But that reason is different for an elliptical arc primitive. The reason
the elliptical arc command in path is complex is that, as with all other
path commands, it starts with a current point and ends by creating a new
current point for the next path command.

This would not apply to an elliptical arc basic shape, which would more
likely use a formulation in terms of center, major and minor radii,
start and end sweep angles, probably a rotation. Which is closer to the
more usual formulation in graphic libraries.

>> - Because (seems to be the main reason).

DJ> Not really.

Well, "because adding any new thing has a cost so there needs to be a
reason to overcome the starting inertia", perhaps.

>> Pros:
>> - It would be more consistent (this is still a basic shape, found in 
>> many graphic libraries);

DJ> Right.

>> - Such a primitive would be an opportunity to implement arcs specified
>> with center, radii and angles. They are quite missing in the spec. I 
>> understand the reason why arcs in path are defined the way they are, but
>> it would be nice to have an alternative way, more natural in some cases
>> (piecharts come to mind, but not only), and defining arc as a basic 
>> shape would be that opportunity:
>> - Because (people want it!).
>> <arc cx="100" cy="100" rx="20" ry="30"
>>      x-axis-rotation="0" start-angle="30" end-angle="60"/>

OK, you had much the same formulation as I did. Perhaps one more param
for closing the arc - one line (chord) or two (center).

>> Fill if needed, stroke only the arc (perhaps need a flag to stroke radii?)
>> Doesn't seems overcomplicated...
>> So, is the answer still the same, or may it be planned for SVG 1.3? (I
>> understand 1.2 is almost frozen, feature-wise)

DJ> Well, 1.2 is pretty much frozen. It was considered for 1.2, but
DJ> I think it will have to wait for 1.3. 

>> Or should it be definitively put away because stuff like XBL (which I
>> don't know yet) would allow this (with or without scripting?)?

DJ> XBL was one reason why we didn't increase the number of basic
DJ> shapes in SVG 1.2. Someone has already implemented arc in RCC (before
DJ> XBL was around).

 Chris Lilley                    mailto:chris@w3.org
 Chair, W3C SVG Working Group
 Member, W3C Technical Architecture Group
Received on Thursday, 26 August 2004 09:07:28 UTC

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