# [whatwg] [Canvas] Behavior on non-invertable CTM

Mon Mar 17 10:18:48 PDT 2014

```On Mon, Mar 17, 2014 at 12:59 PM, Rik Cabanier <cabanier at gmail.com> wrote:

>
>
>
> 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.
>
> I could totally go for that, but you are talking about going back on the
spec of a feature that has shipped, as opposed to clarifying edges cases.
Maybe that would be fine in this case though...

```