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Re: Test harness and test suite assumptions wrt anti-aliasing (font smoothing, clearType)

From: Gérard Talbot <css21testsuite@gtalbot.org>
Date: Sat, 17 Mar 2012 04:04:19 -0400
Message-ID: <1f238f54f1c318623e2088d84952970f.squirrel@ed-sh-cp3.entirelydigital.com>
To: "John Daggett" <jdaggett@mozilla.com>
Cc: "Public CSS test suite mailing list" <public-css-testsuite@w3.org>

Le Sam 17 mars 2012 3:24, John Daggett a écrit :
>
> Gérard Talbot wrote:
>
>> > Browsers typically render text with
>> > subpixel antialiasing *and* with subpixel or integer-pixel
>> > positioning. All browsers support subpixel anti-aliasing but only
>> > Firefox/IE9 use subpixel positioning, Webkit/Opera use integer-pixel
>> > positioning. Line metrics may be calculated with precise or rounded
>> > metrics and where exactly the rounding occurs is not defined
>> > precisely.  Under Windows, hinting will also affect these metrics
>> for a
>> > given font size, depending on the font.  All of these factors will
>> > affect the
>> > precise placement.  Only disabling subpixel anti-aliasing will still
>> > leave you with other factors that affect placement.
>>
>> Of course it will leave the other factors that affect placement. Why
>> should we want to have many factors affect precise placement of text
>> when automated checking of tests start? Why not eliminate sources of
>> differential rendering, especially if such sources are entirely
>> outside
>> the realm of CSS spec?
>
> If you're seeing subpixel anti-aliasing affect a test, then it's a
> sign that the text in question isn't placed precisely such that it
> lands on integer pixel boundaries.  So that's either a bug in the
> browser or a bug in the test.  For example, many tests I've seen are
> in subtle ways dependent upon the metrics of the default font when
> they really shouldn't be.


John,

please bear with me.

I loaded

http://www.gtalbot.org/BrowserBugsSection/css21testsuite/absolute-non-replaced-width-017-ref.xht

and

http://www.gtalbot.org/BrowserBugsSection/css21testsuite/absolute-non-replaced-width-017.xhtml

in Firefox 11.0, Opera 11.61 and Safari 5.1.4 (each in 2 distinct,
separate tabs) under Windows XP SP3 and I had anti-aliasing disabled:

Start / Control Panel / Category Appearance and Themes /
Display / Appearance tab / Effects button / and the
checkbox "Use the following method to smooth edges of screen fonts" was
UNchecked.

I saw no difference whatsoever between the test and its reftest. Perfect
identical rendering.

Now, I do

Start / Control Panel / Category Appearance and Themes /
Display / Appearance tab / Effects button / and then check
checkbox "Use the following method to smooth edges of screen fonts" and
"Standard" is viewable.

Now, there is a difference of rendering between the test and its
reftest. I see a difference.

When I have time, I amy convert such test and reftest to text/html just
to see what happens with IE8 and with clearType.

Gérard
-- 
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CSS 2.1 Test suite RC6, March 23rd 2011:
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Received on Saturday, 17 March 2012 08:04:49 GMT

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