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CHI-WEB  October 2003, Week 1

CHI-WEB October 2003, Week 1

Subject:

SUMMARY: colour contrast algorithm

From:

Dey Alexander <[log in to unmask]>

Reply-To:

Dey Alexander <[log in to unmask]>

Date:

Sun, 5 Oct 2003 19:54:37 +1000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (113 lines)

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
algorithm.

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?

Responses
---------

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
http://joeclark.org/book/sashay/serialization/Chapter09.html

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".

Resources
---------

Testing The Readability Of Web Page Colors
(this paper uses the W3C algorithm)
http://www.aprompt.ca/WebPageColors.html

Visibility check application (beta, based on the W3C algorithm)
http://aprompt.snow.utoronto.ca/ColorVisibilityProgram.html

Colour Contrast Analyser (based on the W3C algorithm)
http://www.juicystudio.com/services/colourcontrast.asp

Color contrast verification tool (based on the W3C algorith, but lowers
the threshold for colour difference)
http://h10014.www1.hp.com/accessibility/color_tool.html

Colour contrast
http://www.lighthouse.org/color_contrast.htm

Colourblind web page filter
http://colorfilter.wickline.org/

Considering the colour blind
http://webtechniques.com/archives/2000/08/newman/

Colour tester
http://www.tesspub.com/colours.html

Vischeck
http://www.vischeck.com/

What do colourblind people see? (collection of java applets)
http://www.tsi.enst.fr/~brettel/colourblindness.html


Cheers,
Dey

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