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From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Sun, 28 Oct 2007 22:56:55 -0500
To: w3c-wai-gl@w3.org
Message-id: <00f701c819df$bf7338e0$1b6fa8c0@NC84301>
Several Edits have been suggested for the Photosensitive Epilepsy Provision
Definition of  General Flash and Red Flash Thresholds 


Here they are:  Suggest that this be put into the 26th draft for widespread



1)       Define saturated Red

2)       Add 'at typical viewing distance'

3)       We have a guidance number for those doing web or software products
on desktops/ laptops.  But we do not have anything for other products such
as phones. So add information on size of image for non-pc displays.

4)       Change 'image sequences' to 'visual sequences' 

5)       Change "less than 3" to "less than or equal to 3"

6)       Clarified that the area must be contiguous but can be irregular 

7)       Clarified that the area must fall with in any 10 degree visual

8)       Consistency edits.

9)       Reformatted (rearranged) to make it clearer that there are two
different and independent flash tests

10)   Moved note 1 to end where it makes more sense - and is easier to find.
And more salient.

11)   Graham Harding recommended that we change 50 hz to 60 hz for US.   We
don't need to restrict to European here and at 50 HZ almost half the
population is susceptible - where as at 60 hz it is 15 percent.  He advised
that we recommend over 75hz. (maybe as advisory?)  For harmonization I left
it at 50 hz but added an advisory.


Below is a revised definition.  


There may be some touch ups later but this language addresses the bulk of
the comments that have come in on the list regarding this provision. 




General Flash and Red Flash Thresholds 

A sequence of flashes or rapidly changing visual sequences where all three
of the following are true: 

1.      There are more than three General Flashes and / or more than three
Red Flashes within any one-second period; and 

2.      The flashing is below 50 Hz; and

3.      The combined area of flashes occurring concurrently are occupy area
of 0.006 steradians or more within  any 10 degrees  of visual field (central
vision) on the screen ( 25% of central 10 degrees of visual field)  at
typical viewing distance, and


*         A General flash is defined as a pair of opposing changes in
relative luminance of 10% or more and the relative luminance of the darker
image is below 0.80; where an "a pair of opposing changes" is an increase
followed by a decrease, or a decrease followed by an increase, and 

*         A Red Flash is defined as any pair of opposing transitions
involving a saturated red where, for either or both states involved in each
transition, R/(R+ G + B) >= 0.8, and the change in the value of (R-G-B)x320
is > 20  ( negative values of (R-G-B)x320 are set to zero).   R, G, B values
range from 0-1 as specified in "relative luminance" definition. 

Exception: The flashing is a fine balanced alternating pattern such a white
noise or a checkerboard-like pattern with 'squares' smaller than 0.5 degree
on a side (area less than .000125 steradians).

Note 1: For general Web content, using a 341 x 256 pixel rectangle anywhere
on the displayed screen area when the content is viewed at 1024 x 768 pixels
will provide a good estimate of a 10 degree visual field for standard screen
sizes and viewing distances  (e.g. 15-17 inch screen at 22-24 inches).
(Higher resolutions yield smaller and safer images so it is lower
resolutions that are used to define the thresholds.) 

Note 2: Tools are available that will carry out analysis from video screen

Note 3: No tool is necessary to evaluate for this condition if flashing is
less than or equal to 3  in any one  second or greater than 50 hz. Content
automatically passes (see #1 and #2 above) 

Note 4:  50 hz is used to coincide with the AC line frequency in Europe and
other countries.  However almost half of the population is susceptible to 50
Hz flashing whereas only 15 % are susceptible to 60 hz.    75 Hz or higher
is recommended where possible. 


Received on Monday, 29 October 2007 03:57:09 UTC

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