[whatwg] Canvas: clarification of compositing operations needed

Leonardo Dutra leodutra.br at gmail.com
Thu Jul 29 07:17:52 PDT 2010

People have a very strong feeling about Firefox. This is beautiful, but not
good to it. Firefox is now, over the Windows 7 and Mac, the slowest browser.
Just pick some JavaScripts and test. XUL is the most hard way for builting
an extension and Firefox eats CPU when we run any game built using HTML

But this is not the point.

*"**Composite **A** within the clipping region over the current canvas
bitmap using the current composition operator."*
The word "clipping" is the point. I am very sad that input type="color" does
not cite the need of a color picker. But This part, is completely bright.

That's why Microsoft agree with Chrome, Chrome with Safari (they don't agree
sometimes) and Opera presents that way too.

A hug.

2010/7/29 David Flanagan <david at davidflanagan.com>

> James Robinson wrote:
>> On Wed, Jul 28, 2010 at 2:46 PM, Tab Atkins Jr. <jackalmage at gmail.com<mailto:
>> jackalmage at gmail.com>> wrote:
>>    On Wed, Jul 28, 2010 at 2:43 PM, David Flanagan
>>    <david at davidflanagan.com <mailto: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.
>> I don't think your attachment made it through.
>> https://developer.mozilla.org/samples/canvas-tutorial/6_1_canvas_composite.htmlshows some of the differences, although it does not cover all cases.
> You didn't miss much.  My example was very similar to the one you link to.
>> The spec is certainly clear but that does not make the behavior it
>> specifies good.  I find the spec's behavior pretty bizarre and Microsoft has
>> expressed a preference for the Safari/Chrome interpretation:
>> http://lists.w3.org/Archives/Public/public-canvas-api/2010AprJun/0046.html- although that thread did not get much discussion.
> Thanks for that link.  The thread you reference refers back to an earlier
> thread on this list.  My apologies for not finding it and reading it before
> posting again.
> For example, I think
>> drawing a 20x20 image into a 500x500 canvas without scaling with a
>> globalCompositeOperation of 'copy' should result in only the 20x20 region
>> being cleared out, not the entire canvas.
> Yikes!  It hadn't occurred to me that copy should behave that way.  But
> you're right that that is what the spec requires.  Opera does it that way.
>  Firefox, thankfully, does not.
> Perhaps independently of the debate over infinite bitmap vs. shape extents,
> we can agree that "copy" is a special value that means "do not perform
> compositing"
>        David
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