Re: [svgwg] Add circular arc path command 'R'. (#767)

Le dim. 10 mai 2020 à 11:47, tatarize <notifications@github.com> a écrit :

> Yeah, but there's a lot of usefulness to on-curve points. You're going to
> need 3 points to define a circular arc regardless, even if one of those
> points isn't on the arc. So you might as well go with a pretty easy way of
> writing those. You often be able to know sort of where you want the arc to
> go and can give a point around the right area. They are also pretty
> intuitive.
>
> You want to arc from here to here, all you need to give me then is a
> single extra point that also exists in your arc.
>
No you just need an extra point which is *off* the curve, only to define
the two tangeants intersecting at that off-curve point.

This is really intuitive, easy to compute, and makes life simpler to define
a bounding area (this extra point is also sufficient for drawing the full
ellipse, which is the reunion of the two arcs: the small arc passing the
nearest from this third point, the long arc passing the furthest from it.

This also works when the ellipse is degenerate (when the third control
point is aligned with the starting and ending points on curve, so that the
two tangeants at the starting and ending points are equal: in that case,
just draw a straight segment connecting the three points: this degenerate
case may occur because of some external transform matrix projecting the
ellipse onto a straight line).


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Received on Tuesday, 12 May 2020 10:20:12 UTC