W3C home > Mailing lists > Public > w3c-wai-gl@w3.org > October to December 2003

Fwd: SUMMARY: colour contrast algorithm

From: Wendy A Chisholm <wendy@w3.org>
Date: Thu, 09 Oct 2003 19:20:54 -0400
Message-Id: <>
To: w3c-wai-gl@w3.org
Cc: Dey Alexander <dey.alexander@ITS.MONASH.EDU.AU>

With Dey's permission, I am forwarding this to the WCAG WG mailing list.

>Date: Sun, 5 Oct 2003 19:54:37 +1000
>From: Dey Alexander <dey.alexander@ITS.MONASH.EDU.AU>
>Subject: SUMMARY: colour contrast algorithm
>I recently enquired about the validity of the colour contrast testing
>algorithm suggested by the W3C.  I didn't get any direct responses as to
>the validity of the algorithm. Instead, I was referred to other possible
>methods or to resources including tools that are based on the W3C algorithm.
>I contacted the W3C and was advised that they are now considering some
>alternatives--the Brewer Palette and a method suggested by Jim Thatcher
>(see below under "responses").  And just a couple of days ago, a
>colleague referred me to a paper that looked at the validity of the W3C
>All useful responses/resources are provided below.
>My original post
>The W3C's draft techniques document for Accessibility Evaluation and
>Repair Tools at http://www.w3.org/TR/AERT#color-contrast provides an
>algorithm for testing to see if text and background colours provide
>sufficient contrast.
>The document indicates, however, that the algorithm is a "suggested" one
>that is "still open to change". Can anyone shed any light on whether
>this should be considered as a reliable indicator of colour contrast, or
>whether some more authoritative/reliable algorithm exists?
>Dick Penn wrote:
>I have developed a subjective test pattern which might be useful to you
>in dramatizing the effects. It's at
>http://positiveinteraction.com/TestPattern - I'd be interested to know
>whether people with unusual color vision perceive this pattern
>differently. Let me know if it needs any more explanation.
>Janine Purcell wrote:
>Fowler and Stanwick 1995 The GUI Style Guide p. 331.  They give values
>for a gray scale ruler, each gradation 10%.  For sufficient contrast the
>separation should be 30%.
>Gray                     RGB values
>10%                      26,26,26
>20%                      51,51,51
>30%                      79,79,79
>40%                      102, 102, 102
>50%                      128, 128, 128
>60%                      153, 153, 153
>70%                      181, 181, 181
>80%                      204, 204, 204
>90%                      232, 232, 232
>100% black               255, 255, 255
>Brewer palette:
>See Building Accessible Websites - Chapter 9: Type and Colour
>Thatcher method:
>"It is clear that good contrast occurs when two colors are close to
>complementary. 'Closeness' can be calculated as if the R G B colors are
>points in the three-dimensional space: Distance = ( (R1-R2) ^2 + (G1-G2)
>^2 + (B1-B2) ^2) ^1/2".
>See p. 93 in Chapter 4 (Creating Accessible Content) of "Constructing
>Accessible Web Sites".
>Testing The Readability Of Web Page Colors
>(this paper uses the W3C algorithm)
>Visibility check application (beta, based on the W3C algorithm)
>Colour Contrast Analyser (based on the W3C algorithm)
>Color contrast verification tool (based on the W3C algorith, but lowers
>the threshold for colour difference)
>Colour contrast
>Colourblind web page filter
>Considering the colour blind
>Colour tester
>What do colourblind people see? (collection of java applets)
>    --------------------------------------------------------------
>    Tip of the Day: Use the archives to research common questions
>     CHI-WEB: www.sigchi.org/web POSTINGS: mailto:chi-web@acm.org
>              MODERATORS: mailto:chi-web-moderators@acm.org
>       SUBSCRIPTION CHANGES & FAQ:  www.sigchi.org/web/faq.html
>    --------------------------------------------------------------

wendy a chisholm
world wide web consortium
web accessibility initiative
Received on Thursday, 9 October 2003 19:21:18 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 16 January 2018 15:33:46 UTC