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Re: Proper handling of SVG and rgba()

From: Dean Jackson <dino@apple.com>
Date: Wed, 2 Jun 2010 05:50:05 -0700
Cc: public-fx@w3.org
Message-Id: <9B5B77D4-E37D-4C11-80F3-FFC7C451A68F@apple.com>
To: Jonathan Kliegman <kliegs@chromium.org>
My personal take is that rgba should be supported in SVG content. I'd be interested to hear arguments against it (*).

The alpha in the color should be added to the start of the list of alpha blends used in the final compositing (which is a list of multiplications). eg.

fill: rgba(0, 0, 0, 0.5);
fill-opacity: 0.6;
opacity: 0.7;

... would end up (in the absence of other styles) painting a 0.5 * 0.6 * 0.7 = 0.21 opaque black. 

(*) I argued against it myself when CSS was adding it back in the day, because I preferred the way SVG had separated opacity - it allows you to animate a fill/stroke opacity independently of color. But rgba is well-loved now, and it would have been impractical to add -opacity properties for every place that CSS could use a color.


On 01/06/2010, at 8:43 AM, Jonathan Kliegman wrote:

> I was looking at Webkit, specifiaclly bug 16183 (https://bugs.webkit.org/show_bug.cgi?id=16183) - SVG doesn't support rgba() colors.  I was planning on fixing this but after reading the SVG spec at http://www.w3.org/TR/SVG11/ which states that SVG supports CSS2 color specifications I'm unsure if this should be fixed or not. rgba() was introduced with CSS3 so is technically not supported by SVG.  
> Looking at other implementations Chrome and webkit-head both render a rgba() color as pure black.  Safari 4.0.5 I've seen render it as either black or missing.  Firefox does render it properly.  Batiq fails to render and gives an error: "The attribute "stroke" represents an invalid CSS value ("rgba(0, 255, 0, 0.5)")."  
> Looking at this bug from an end user perspective it would be good to add this feature in as its likely that people will build sites that aren't strictly compliant and would want things to just work.  However I'm always wary of making changes that go against a published standard as that path can lead to ugliness, divergent standards and future headaches.  
> What I'm looking for from the community is feedback on what the right thing to do here with respect to webkit is.  Is it worth the convenience to the user to add support for rgba() to the SVG code? 
> -Jon
Received on Wednesday, 2 June 2010 12:50:39 UTC

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