# [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,
> > > 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.

```