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Re: Inconsistent behaviour of globalCompositeOperation property

From: Charles Pritchard <chuck@jumis.com>
Date: Tue, 25 May 2010 14:52:26 -0700
Message-ID: <4BFC469A.40601@jumis.com>
To: Jatinder Mann <jmann@microsoft.com>
CC: "public-canvas-api@w3.org" <public-canvas-api@w3.org>
This was revised last year, and will be adjusted appropriately in a 
future version of Firefox, per Robert O'Callahan.

I believe this is the end of the thread which addressed the issue:
http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2009-October/023706.html

It's quite subtle, but it's standardized in the drawing-model section.


-Charles

On 5/25/2010 11:56 AM, Jatinder Mann wrote:
>
> As we were reviewing the globalCompositeOperation property [1] of the 
> Canvas 2D Context, we have noticed that there is a large inconsistency 
> with the way web browsers implement this property.
>
> Firefox 3.6.3 and Opera 10.5 have a similar behaviour that matches the 
> current version of the Canvas 2D Context spec. Chrome 4.1 and Safari 
> 4.0.5 behave similarly to each other, albeit differently from the 
> spec. Today, web developers cannot reliably use this property in 
> designing their webpages due to this interoperability issue.
>
> We consider that the Safari implementation of the Drawing Model makes 
> more sense from a web developer point of view. Most of the Canvas APIs 
> are applied to the next primitive being drawn. However, the clipping 
> behavior, as defined in the current Drawing Model [2], affects the 
> current contents of the Canvas outside of the current primitive being 
> drawn. The Safari Drawing Model calls for the clipping region to only 
> affect the current primitive being drawn.
>
> We recommend updating the spec to match the Safari Drawing Model. We 
> would like to discuss this area in more detail.
>
> Thank you,
>
> Jatinder
>
> [1] http://dev.w3.org/html5/2dcontext/#compositing
>
> [2] http://dev.w3.org/html5/2dcontext/#drawing-model
>
> Jatinder Mann | Microsoft Program Manager | jmann@microsoft.com 
> <mailto:jmann@microsoft.com>
>
Received on Tuesday, 25 May 2010 21:53:13 UTC

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