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Re: Spec anomaly - opacity

From: Glenn A. Adams <gadams@xfsi.com>
Date: Tue, 09 Dec 2008 12:04:12 +0800
To: Sean Hayes <Sean.Hayes@microsoft.com>, Public TTWG List <public-tt@w3.org>
Message-ID: <C5640F3C.6D6F%gadams@xfsi.com>


We simply refer to the CSS3 Color Module for the value space of opacity and
its semantics. I think we do not want to vary from this, particularly since
other specs, such as SVG, also use these values. DFXP also supports
specifying alpha in RGBA tuples, in which case the value range of [0-255]
applies. There is a straightforward transformation between alpha (A) and
opacity (O) that is easily testable, i.e.,

A = FLOOR ( O * 255 )

and

O = FLOAT ( A ) / 255.0

Glenn

On 12/9/08 7:05 AM, "Sean Hayes" <Sean.Hayes@microsoft.com> wrote:

> 
> I notice that opacity is specified as a float value, implying an inordinate
> (and completely untestable) set of gradations of alpha; while colour alpha is
> specified as only 255 steps. This seems completely skewed to me.
> 
> I would recommend that we align opacity and colour alpha by specifying <alpha>
> expressions as component-value.
> 
> Whether we wish also to anticipate 16bit colour is a separate question we
> probably ought to look at.
> 
> Sean Hayes
> Media Accessibility Strategist
> Accessibility Business Unit
> Microsoft
> 
> Office:  +44 118 909 5867,
> Mobile: +44 7875 091385
> 
Received on Tuesday, 9 December 2008 04:04:55 GMT

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