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Re: Spec anomaly - opacity

From: Daniel Weck <daniel.weck@gmail.com>
Date: Tue, 9 Dec 2008 00:56:04 +0000
Cc: "public-tt@w3.org" <public-tt@w3.org>
Message-Id: <99FC3025-0169-4FCE-900C-CBCD572636CA@gmail.com>
To: Sean Hayes <Sean.Hayes@microsoft.com>


On 8 Dec 2008, at 23:05, Sean Hayes 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.


I disagree with the "completely untestable" statement.

For media and region opacity, the SMIL 3.0 specification uses  
percentage values in the range 0-100% or numbers in the range 0.0-1.0,  
with 100% or 1.0 meaning fully opaque.

http://www.w3.org/TR/SMIL3/smil-extended-media-object.html#smilMediaNS-MediaOpacity

The SMIL 3.0 test-suite includes tests for basic opacity as well as  
for more complex chromaKey processing. Testing a reference  
implementation does not consist in asserting every possible values,  
even in a discrete scenario. It is about evaluating a set of  
judiciously chosen parameters, such as a couple of random values as  
well as edge cases that can be potentially misinterpreted by the  
specification readers/implementors.

In fact, the test-suite is not meant to test implementations of the  
standard, it is meant to prove (to a certain extent, as with any  
tests) that the specification is clear enough to be implementable  
consistently without ambiguity. At least that is my understanding of  
the role of the test-suite. In the case of SMIL 3.0, the test-suite  
coverage satisfies the new features brought into since 2.1.

Here's 2 examples of tests (may require member access):

http://www.w3.org/2007/SMIL30/testsuite/New-SMIL30/files/Layout/ColorQuad_NoText.png
http://www.w3.org/2007/SMIL30/testsuite/New-SMIL30/files/Layout/ColorQuad_WithText.png
http://www.w3.org/2007/SMIL30/testsuite/New-SMIL30/files/Layout/layout-background-opacity-region-override.txt

http://www.w3.org/2007/SMIL30/testsuite/New-SMIL30/files/Layout/ColorGrid.png
http://www.w3.org/2007/SMIL30/testsuite/New-SMIL30/files/Layout/layout-background-opacity-region.txt

More to see here:

http://www.w3.org/2007/SMIL30/testsuite/New-SMIL30/SMILtestsuite.html

Regards, Dan
Received on Tuesday, 9 December 2008 00:56:46 GMT

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