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Precision of <canvas> rendering (was: Re: Formal definition of HTML5)

From: Henri Sivonen <hsivonen@iki.fi>
Date: Tue, 17 Apr 2007 23:57:43 +0300
Message-Id: <C879A066-C0F2-47FC-BA0D-A8292F10DA31@iki.fi>
Cc: public-html@w3.org
To: Joe D'Andrea <jdandrea@gmail.com>

On Apr 16, 2007, at 22:15, Joe D'Andrea wrote:

> My takeaway thus far:
> <canvas> should not define pixel-perfect rendering, nor does it.

<canvas> is really about providing a JavaScript API for the PDF 1.4  
imaging model in a way that maps sanely to the kind of C libraries— 
Quartz 2D in particular—that would be suitable for implementing the  
painting part of a PDF 1.4 viewer. I'd expect the spec to give room  
for using such libraries without making onerous requirements about  
the details of Bézier tesselation or the details of anti-aliasing.  
For example, it should be permissible to use Quartz 2D, Cairo or WPF  
as the rendering library with whatever anti-aliasing algorithms that  
they provide.

Thus, for a given CSS pixels to device pixels ratio, implementations  
should (subject minor to Bézier tesselation rounding errors) have an  
agreement about which shapes participate in which pixels, but when a  
pixel is not fully filled by a given shape, the exact degree of anti- 
aliasing shading should be allowed to fudge in an implementation- 
specific way.

Henri Sivonen
Received on Tuesday, 17 April 2007 20:57:48 UTC

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