[whatwg] Canvas arcTo method

[Rik Cabanier]

On Mon, Aug 20, 2012 at 5:55 PM, Michael Day <mikeday at yeslogic.com> wrote:

> Hi Rik,
>
>  The 'scale(2,1)' set up a different coordinate system. You can rewrite
>> your code from this:
>>
>>     ctx.lineTo(100, 100);
>>     ctx.scale(2, 1);
>>     ctx.arcTo(100, 100, 100, 200, 100);
>>
>> to this:
>>
>>     ctx.scale(2, 1);
>>     *ctx.lineTo(50, 100);*
>>
>>     ctx.arcTo(100, 100, 100, 200, 100);
>>
>
> Right, these will produce the same arc. But how should this be implemented
> in the user agent?
>
> It's almost like it is getting the last point in the previous subpath,
> transforming it by the inverse of the current transformation matrix,
> generating the arc, and then transforming the arc by the matrix.
>

Yes, that is one way of implementing it. This is not specific to arcTo;
this happens with all drawing operators.


>
> Is this what Firefox and Chrome do? There is no hint of this in the spec,
> which is quite ambiguous about how the current transform should affect
> previous subpaths.
>

I don't know. It just depends how they implemented in. They might apply the
CTM to all the coordinates or keep the coordinates and pass them along with
the CTM to the drawing system.