From: Mike \ <pomax@nihongoresources.com>

Date: Tue, 10 May 2011 21:17:35 -0700

Message-ID: <4DCA0DDF.8050802@nihongoresources.com>

CC: public-canvas-api@w3.org

Date: Tue, 10 May 2011 21:17:35 -0700

Message-ID: <4DCA0DDF.8050802@nihongoresources.com>

CC: public-canvas-api@w3.org

On 5/10/2011 8:30 PM, Ian Hickson wrote: > What is the use case for a dedicated ellipse API rather than using bezier? A fair question, with a pretty simple two-fold answer: 1) bezier arc approximation is an approximation, using very expensive operations to approximate a very cheap operation instead (and if you want to do it right, there's first the particularly nasty business of working out the math that tells you how many bezier segments you even need to get an approximation with an precision-error lower than ...). That doesn't make a lot of sense. 2) why would people have to reinvent the wheel time and time again just to get an ellipse or elliptical arc? That's just bad API design =) If that question underlines why there is no elliptical arc, then I could of course retort with "good point: why is there a circular arc? you can approximate it with bezier curves too." -- however the math for circular arc approximation is just as hard as the math for elliptical arc approximation. And it'll always be much more expensive to perform than dedicated, actually correct, elliptical/circular arc functions. A lack of implementation desire should not be much of a problem here - implementing the elliptical arc, given that people already implement the circular arc, is not a lot of work. Implementing it as a bezier approximation, on the other hand, is very much a lot of work. It would be a pretty good idea to make sure that only a handful of implementations are needed for an elementary shape, rather than forcing everyone who wants to draw one to reinvent the wheel, and end up with something that isn't actually wheel, but more of a polygon. If you offer an arc (which I think you should for a 2D graphics API, because it makes life for the people using that API much easier, keeping them creatively busy instead of bogged down trying to work out math for what should be elementary drawing operations) then offering an elliptical arc as "main" arc just makes sense. Even if you, as designer can only think of circles, the rest of the world is bigger. An elliptical lets you draw circles, so you get what you want, and it can also draw a literally infinitely more diverse family of shapes, which are essential to many people who aren't you. I actually looked up how to get a message to this list, because I hear everyone around me complain that there's no ellipse command, and that the Canvas API people should do something about it. When asked if they've posted to your list, the responses are invariably "no, I'm not subscribed" or "they won't listen anyway". So I hope to prove the latter wrong by arguing the case for an elliptical arc command. It just makes sense to have one. - MikeReceived on Wednesday, 11 May 2011 04:17:50 UTC

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