[whatwg] Canvas Proposal: aliasClipping property

Oliver Hunt oliver at apple.com
Fri Oct 16 21:37:11 PDT 2009

On Oct 16, 2009, at 8:10 PM, Robert O'Callahan wrote:

> On Sat, Oct 17, 2009 at 4:01 AM, Philip Taylor <excors+whatwg 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.

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

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