[whatwg] Canvas Proposal: aliasClipping property

Robert O'Callahan robert at ocallahan.org
Fri Oct 16 20:10:56 PDT 2009

On Sat, Oct 17, 2009 at 4:01 AM, Philip Taylor
<excors+whatwg at gmail.com<excors%2Bwhatwg at gmail.com>
> wrote:

> Yes, mostly.
> http://philip.html5.org/tests/canvas/suite/tests/index.2d.composite.uncovered.html
> has relevant tests, matching what I believed the spec said - on
> Windows, Opera 10 passes them all, Firefox 3.5 passes all except
> 'copy' (https://bugzilla.mozilla.org/show_bug.cgi?id=366283), Safari 4
> and Chrome 3 fail them all.
> (Looking at the spec quickly now, I don't see anything that actually
> states this explicitly - the only reference to infinite transparent
> black bitmaps is when drawing shadows. But
> http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#drawing-model
> is phrased in terms of rendering shapes onto an image, then
> compositing the image within the clipping region, so I believe it is
> meant to work as I said (and definitely not by compositing only within
> the extent of the shape drawn onto the image).)

Yes, I think that's pretty clear as written.

I think there is a reasonable argument that the spec should be changed so
that compositing happens only within the shape. (In cairo terminology, all
operators should be bounded.) Perhaps that's what Safari and Chrome
developers want.

"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
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