[whatwg] CanvasRenderingContext2D with addPath, currentPath

[Rik Cabanier]

On Mon, Nov 4, 2013 at 7:29 PM, Rik Cabanier <cabanier at gmail.com> wrote:

>
>
> On Mon, Nov 4, 2013 at 5:21 PM, Robert O'Callahan <robert at ocallahan.org>wrote:
>
>> If you return a path in user-space, what do you get if you call
>> getCurrentPath with a singular transform?
>>   ctx.moveTo(0,0);
>>   ctx.lineTo(1,1);
>>   ctx.scale(0,0);
>>   var p = ctx.getCurrentPath();
>>
>
> I mixed up my terms :-)
> getCurrentPath should return the path in device coordinates (not user).
>
> However, for your example, I'm unsure what the right solution is. The
> canvas specification is silent on what the behavior is for non-invertible
> matrices.
> I think setting scale(0,0) or another matrix operation that is not
> reversible, should remove drawing operations from the state because:
> - how would you stroke with such a matrix?
> - how do patterns operate? the same for gradient fills.
> - how would you pass this to the underlying graphics library?
> - certain operators such as 'arc' rely on doing the transform in reverse.
>
> I seem to remember that for SVG we decided that non-invertible matrices
> don't draw either.
>

After pondering this some more and looking at the different
implementations, I propose the following:
if the user sets a non-invertible matrix, the canvas context should be in a
state that ignores all path drawing operations, stroke/fill calls and all
other ctm operations (apart from setTransform). setCurrentPath is also
ignored and getCurrentPath should return an empty path.
If the ctm becomes invertible again (from a setTransform or a restore),
drawing operations pick up again with the currentPoint that was active when
the non-invertible matrix was set.

I could be convinced that set/getCurrentPath should still work...

so If I expand your example a bit:

ctx.moveTo(0,0);
ctx.lineTo(1,1);
ctx.scale(0,0);
var p = ctx.getCurrentPath(); // return empty path

ctx.stroke(); // nothing happens

ctx.setTransform(1,0,0,1,0,0);
p = ctx.getCurrentPath(); // return path with moveto/lineto
ctx.stroke(); // draw line


This would match what WebKit and Blink are doing.
Firefox gets in a bad state as soon as you set a non-invertible matrix but
we could fix that ;-)
IE keeps drawing and even strokes when scale(0,0) is set.

It would be nice to have interop...