[cairo] Questions and optimizations in cairo-arc

[Jason Dorje Short]

Behdad Esfahbod wrote:
> On Tue, 26 Jul 2005, Jason Dorje Short wrote:

>>>This comes from PostScript's arct operator. The two points actually
>>>form two tangent lines---one from the current point to (x1,y1) and one
>>>from (x1,y1) to (x2,y2). These are not points on the spline at all.
>>>
>>>See the PostScript Language Reference Manual for a more detailed
>>>explanation.
>>
>>That is very unintuitive since in most cases neither the current nor
>>final (x2,y2) point will be on the arc (and of course the tangent point
>>(x1,y1) won't be on the arc except in degenerate cases).  This at least
>>needs some heavy commenting.
> 
>>From a use-case point of view though it's quite intuitive.  It's
> like drawing a line to (x1,y1) and then (x2,y2), except that it
> draws a nice arc instead of the sharp edge.

But if that's what you want it would be much more intuitive to draw the
interior arc of the oval uniquely defined by the two tangents, rather
than a line plus the arc of the circle uniquely defined by the tangents
*and* the radius.

-jason