[whatwg] [Canvas] Behavior on non-invertable CTM
junov at google.com
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>
>> > > 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
>> > > be intractable under a projective transform (e.g. perspective), but
>> > > we are limitted to affine transforms, it can be done. Now, in the
>> > 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...
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