[whatwg] [Canvas] Behavior on non-invertable CTM
Rik Cabanier
cabanier at gmail.com
Mon Mar 17 09:59:33 PDT 2014
On Mon, Mar 17, 2014 at 8:45 AM, Justin Novosad <junov at google.com> wrote:
> On Mon, Mar 17, 2014 at 11:35 AM, Dirk Schulze <dschulze at adobe.com> wrote:
>
> >
> > > Hmmm, I gave this a bit more thought... To apply the construction
> > > algorithm in transformed space, the ellipse parameters (radiusX,
> radiusY,
> > > rotation) would have to be transformed. Transforming the parameters
> would
> > > be intractable under a projective transform (e.g. perspective), but
> since
> > > we are limitted to affine transforms, it can be done. Now, in the case
> > of
> > > a non-invertible CTM, we would end up with radiusX or radiusY or both
> > equal
> > > to zero. And what happens when you have that? Your arcTo just turned
> > into
> > > lineTo(x1, y1). Tada!
> >
> > Why does radiusX or radiusY need to be zero? Because you define it that
> > way for a non-invertible matrix? That makes sense for scale(0,0). What
> > about infinity or NaN? If Ian didn't update the spec then this is still
> > undefined and therefore up to the UA to decide.
> >
> >
> Oh yeah, I was totally forgetting about singularities caused by non-finite
> values. Could we just the same agree to resolve that case by treating
> arcTo as lineTo(x1, y1) in the case of a non-invertible CTM? Or do you
> think there is a more logical thing to do?
>
Make a clean cut and define that drawing operators are ignored when there's
a non-invertible matrix.
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