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Re: measuring contrast ratio and Windows Clear Type

From: John Hudson <john@tiro.ca>
Date: Tue, 12 Aug 2014 10:58:46 -0700
Message-ID: <53EA55D6.3010801@tiro.ca>
To: w3c-wai-ig@w3.org
Greg Kraus wrote:

	It also happens that if a font has an anti-aliased
	edge, the algorithm can give you some useful information
	as long as the font is thick enough that there is a "core"
	part of the font that is basically a consistent color.

This gets to the core of the matter, if you'll excuse the pun. In 
general, accessibility requirements regarding figure/ground contrast 
levels presume consistent stroke density. The paradigm of consistent 
stroke density is, of course, print, in which the same density of ink 
and edge sharpness apply regardless of the thickness of individual 
strokes or letter features. In antialiased text on screen, there is 
always a loss of consistency in edge sharpness and stroke density. In 
subpixel rendering, such as ClearType, this is further complicated by 
the introduction of chromatic antialiasing, and exacerbated further in 
more recent CT environments (DirectWrite, as used in recent versions of 
IE) by subpixel (fractional) spacing, which results in variable 
rendering of the same glyphs depending on location.*

If one is genuinely concerned about text accessibility -- as may be 
distinct from simply checking a box to say your site is conformant with 
a possibly inadequate accessibility specification --, then what matters 
is stroke density across all the salient features of a glyph displayed 
on a given device at a given size. ClearType has tended to perform 
better in this regard than some other subpixel antialiasing technologies 
-- notably Apple's Quartz rendering -- because stroke density was 
identified as a priority by Microsoft's Advanced Reading Technologies 
group. As screen resolutions increase, this difference becomes less 
critical, but if you compare even GDI ClearType with Quartz in 96ppi 
environments you will observe how the latter tends to lose density in 
thin strokes such as the crossbar of the lowercase e, while ClearType 
preserves it.

Because text rendering is device, resolution and browser specific -- as 
well as dependent on reading distance and visual acuity --, I'm really 
not sure what it means for a site to claim to meet accessibility 
requirements for text, other than by making all text quite large and 
avoiding light weight fonts.

JH


* It should be noted that there are multiple versions of ClearType, each 
using slightly different colour filtering techniques, asymmetric 
scaling, and full or fractional pixel spacing. The Win8 Metro 
environment also introduced a fractionally spaced greyscale rendering, 
which was adopted to address processing speed concerns around subpixel 
chromatic antialiasing in the dynamic text environment.



-- 

Tiro Typeworks        www.tiro.com
Gulf Islands, BC      tiro@tiro.com

If stung by another man's bee, one must calculate the
extent of the injury, but also, if one swatted it in the
process, subtract the replacement value of the bee.
— Mediaeval Irish legalism
Received on Tuesday, 12 August 2014 17:59:15 UTC

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