On Oct 16, 2009, at 8:10 PM, Robert O'Callahan wrote:
On Sat, Oct 17, 2009 at 4:01 AM, Philip Taylor <excors+wha...@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.
This is the behaviour of the original canvas implementation (and it
makes a degree of sense -- it is possible to fake composition implying
an infinite 0-alpha surrounding when the default composite operator
does not do this, but vice versa is not possible). That said I
suspect we are unable to do anything this anymore :-/
Rob
--Oliver
