[whatwg] Canvas Proposal: aliasClipping property

[Philip Taylor]

On Fri, Oct 16, 2009 at 2:25 PM, Robert O'Callahan <robert at ocallahan.org> wrote:
> On Sat, Oct 17, 2009 at 1:06 AM, Philip Taylor <excors+whatwg at gmail.com>
> wrote:
>>
>> I think the spec is clear on this (at least when I last looked; not
>> sure if it's changed since then). Image A is infinite and filled with
>> transparent black, then you draw the shape onto it (with no
>> compositing yet), and then you composite the whole of image A (using
>> globalCompositeOperation) on top of the current canvas bitmap. With
>> some composite operations that's a different result than if you only
>> composited pixels within the extent of the shapes you drew onto image
>> A.
>
>
> Ah, so you mean Firefox is right in this case?

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).)

-- 
Philip Taylor
excors at gmail.com