[whatwg] Canvas: clarification of compositing operations needed

James Robinson jamesr at google.com
Wed Jul 28 15:10:10 PDT 2010

On Wed, Jul 28, 2010 at 2:46 PM, Tab Atkins Jr. <jackalmage at gmail.com>wrote:

> On Wed, Jul 28, 2010 at 2:43 PM, David Flanagan <david at davidflanagan.com>
> wrote:
> > Firefox and Chrome disagree about the implementation of the
> > destination-atop, source-in, destination-in, and source-out compositing
> > operators.  Test code is attached.

I don't think your attachment made it through.
some of the differences, although it does not cover all cases.

> > Chrome doesn't touch any destination pixels that are not underneath the
> > source pixels.  Firefox, on the other hand, treats the entire canvas
> (inside
> > the clipping region) as the destination and if you use the destination-in
> > operator, for example, will erase any pixels outside of whatever you are
> > drawing.
> >
> > I suspect, based on the reference to an "infinite transparent black
> bitmap"
> > in Drawing model that Firefox gets this right and Chrome gets
> it
> > wrong, but it would be nice to have that confirmed.
> >
> > I suggest clarifying Compositing to mention that the
> compositing
> > operation takes place on all pixels within the clipping region, and that
> > some compositing operators clear large portions of the canvas.
> The spec is completely clear on this matter - Firefox is right,
> Chrome/Safari are wrong.  They do it wrongly because that's how
> CoreGraphics, their graphics library, does things natively.

The spec is certainly clear but that does not make the behavior it specifies
good.  I find the spec's behavior pretty bizarre and Microsoft has expressed
a preference for the Safari/Chrome interpretation:
http://lists.w3.org/Archives/Public/public-canvas-api/2010AprJun/0046.html -
although that thread did not get much discussion.  For example, I think
drawing a 20x20 image into a 500x500 canvas without scaling with a
globalCompositeOperation of 'copy' should result in only the 20x20 region
being cleared out, not the entire canvas.

In informal discussions I got the impression that most folks would be happy
to standardize on something closer to the Safari/Chrome model if it could be
specified exactly.  In particular, there has to be a precise definition of
what region the compositing operation should apply in.

- James

> ~TJ
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