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New proposed 2.3

From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Tue, 17 Feb 2004 15:53:56 -0600
To: w3c-wai-gl@w3.org
Message-id: <0HT900GF50U1R0@smtp4.doit.wisc.edu>
Submitted herewith is a new proposal for guideline 2.3   

 

It eliminates the blanket "avoid all flicker" approach and focuses on
content that is more directly tied to and specific to triggers for
photosensitive epilepsy.  

It allows a much broader range of Web content to pass, while focusing on the
important issues.  

It also adds pattern sensitive epilepsy to the coverage - though at level 3
since we do not yet have much information on how this would relate to web
content and how easily it could be otherwise addressed. 

 

The list of bugs addressed by this rewrite are in the email immediately
following. 


 


Guidelines 2.3 


Allow users to avoid content that could cause Photosensitive Epileptic
Seizures.


 


Level 1 Success Criteria for Guideline 2.3


 

1.	Content that violates one or more of the flash and red flash
thresholds (below) is marked in way that the user can access prior to its
appearance. 

 

General Flash Threshold

*        A sequence of flashes or rapidly changing image sequences is not
permitted when both the following occur:  the combined area of flashes
occurring concurrently (but not necessarily contiguously) occupies more than
one quarter of any 355 x 268 pixel rectangle anywhere on the displayed
screen area when the page is viewed at 1024 by 768 pixels and there are more
than three flashes within any one-second period.  For the general flash
threshold - a flash is defined as a pair of opposing changes in luminance
(i.e., an increase in luminance followed by a decrease, or a decrease
followed by an increase) of 20 candelas per rectangle meter (cd.m-2) or more
and where the screen luminance of the darker image is below 160 cd.m-2 .
(See notes 1 and 2)

Red Flash Threshold

*        A transition to or from a saturated red at any luminance level is
not permitted when both of the following occur:  the combined area of
flashes occurring concurrently occupies more than one quarter of any 355 x
268 pixel rectangle anywhere on the displayed screen area when the page is
viewed at 1024 by 768 pixels and there are more than three flashes within
any one-second period.

[Thresholds are based on ITC Guidance Note for Licensees on Flashing Images
and Regular Patterns in Television (Revised and re-issued July 2001)
(www.ofcom.co.uk) as modified by Wisconsin Computer Equivalence Algorithm]

 

 


Level 2 Success Criteria for Guideline 2.3


 

1.	Content does not violate any of the flash or red flash Thresholds
(above).

 


Level 3 Success Criteria for Guideline 2.3


 

1.	Content does not violate any of the spatial pattern Thresholds 

 

Spatial Pattern Thresholds 

*        Clearly discernible stripes consisting of more than five light-dark
pairs in any orientation are not permitted when the stripes are stationary
when the pattern occupies more than 40% of any 355 x 268 pixel rectangle
anywhere on the displayed screen area when the page is viewed at 1024 by 768
pixels.

*        Clearly discernible stripes consisting of more than five light-dark
pairs in any orientation that change direction, oscillate, flash, or reverse
in contrast are not permitted when the pattern occupies more than 25% of any
355 x 268 pixel rectangle anywhere on the displayed screen area when the
page is viewed at 1024 by 768 pixels.

a)      Clearly discernable is defined as stripes where the screen luminance
of the darker bars in the pattern is below 160 cd.m-2 and differs from the
lighter bars by 20 cd.m-2 or more (see notes 1 and 2).

 

Notes:

 

1.      Video waveform luminance is not a direct measure of display screen
brightness.  Not all domestic display devices have the same gamma
characteristic, but a display with a gamma of 2.2 may be assumed for the
purpose of determining electrical measurements made to check compliance with
these guidelines (see Appendix 1).

 

2.      For the purpose of measurements made to check compliance with these
guidelines, pictures are assumed to be displayed in accordance with the
'home viewing environment' described in Recommendation ITU-R BT.500 in which
peak white corresponds to a screen illumination of 200 cd.m-2.

 

3.      NOTE: A free tool will be available from the University of
Wisconsin's Trace Center that will carry out the above analysis on Web pages
by the second quarter of 2004. 

 

 

 




 

DEFINITIONS:

 

marked in way that the user can access prior to its appearance

-         in metadata on page

-         in header (so search engine shows it)

-         on page before it is encountered.

 

Wisconsin Computer Equivalence Formulae

The Wisconsin Computer Equivalence Algorithm is a method for applying the
United Kingdom's ITC "ITC Guidance Note for Licensees on Flashing Images and
Regular Patterns in Television (Revised and re-issued July 2001)"  to
content displayed on a computer screen, such as web pages and other computer
content.  The ITC Guidance Document is based on the assumption that the
television screen occupies the central ten degrees of vision.  This is not
accurate for a screen which is located in front of a person.  The Wisconsin
Algorithm basically carries out the same analysis as the ITC Guidelines
except that is does it on every possible ten degree window for a
prototypical computer display.  For complete details see
www.trace.wisc.edu/docs/peat.

 

 


Gregg

------------------------

Gregg C Vanderheiden Ph.D. 
Professor - Depts of Ind. Engr. & BioMed Engr.
Director - Trace R & D Center 
University of Wisconsin-Madison 
< <http://trace.wisc.edu/> http://trace.wisc.edu/> FAX 608/262-8848  
For a list of our list discussions http://trace.wisc.edu/lists/

 <http://trace.wisc.edu:8080/mailman/listinfo/>  

 

 
Received on Tuesday, 17 February 2004 16:54:17 UTC

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