[whatwg] Canvas: clarification of compositing operations needed

[Tab Atkins Jr.]

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.
>
> 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 4.8.11.1.13 Drawing model that Firefox gets this right and Chrome gets it
> wrong, but it would be nice to have that confirmed.
>
> I suggest clarifying 4.8.11.1.3 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.

~TJ