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OCTOBER FLASH EDITS - REVISED

From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Tue, 30 Oct 2007 20:35:49 -0500
To: w3c-wai-gl@w3.org
Message-id: <00a601c81b5e$5e804700$a8aba8c0@NC84301>
 

Based on some feedback - here are a few edits 

 

-          Item # 3 in the main definition had wrong/missing words 

-          The definition of saturated Red was moved to a NOTE:   (from the
definition of Red Flash)

-          The exception was adjusted to a known safe level.  (we may be
able to increase it later but need to stay safe and this keeps the exception
which is useful for screen resetting).   

 

 

If the doc hasn't been locked these could go in.   Otherwise they can go in
Sandbox. 

 

 

(EWG - if you decided to make the changes in the draft then only the
following lines need to be changed) 

 

-          Delete the old Line 6 in the rationale 

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

-          A Red Flash is defined as any pair of opposing transitions
involving a saturated red.

-          Exception: Flashing that is a fine, balanced, alternating pattern
such a white noise or a checkerboard pattern with 'squares' smaller than 0.1
degree on a side does not violate the thresholds.

-          Note 2: The current working definition of 'opposing transitions
involving a saturated red" is 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) for
both transitions.   R, G, B values range from 0-1 as specified in "relative
luminance" definition.  (Harding and Binnie 2002) 

-          Note 4: No tool is necessary to evaluate for this condition if
flashing is less than or equal to 3 flashes in any one  second or greater
than 50 hz. Content automatically passes (see #1 and #2 above)

-          Note 5:  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.

 

 

 

 


Gregg
 -- ------------------------------ 
Gregg C Vanderheiden Ph.D. 

 

 

Several Edits have been suggested for the Photosensitive Epilepsy Provision
Definition of  General Flash and Red Flash Thresholds by TEITAC members.

 

Here they are:  Suggest that this be put into the 26th draft for widespread
review.    Sorry for delay.  Took a bit to assemble them all. 

 

 

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 fall with in any 10 degree visual
field

7)       Consistency edits.

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

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

10)   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 occupy an area
of 0.006 steradians or more of any 10 degrees of visual field on the screen
( 25% of 10 degrees of visual field)  at typical viewing distance,

where:  

*         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. 

Exception: Flashing that is a fine, balanced, alternating pattern such a
white noise or a checkerboard pattern with 'squares' smaller than 0.1 degree
on a side does not violate the thresholds.

Note 1: For general software or 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: The current working definition of 'opposing transitions involving a
saturated red" is 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) for both
transitions.   R, G, B values range from 0-1 as specified in "relative
luminance" definition.  (Harding and Binnie 2002)

Note 3: Tools are available that will carry out analysis from video screen
capture. 

Note 4: No tool is necessary to evaluate for this condition if flashing is
less than or equal to 3 flashes in any one  second or greater than 50 hz.
Content automatically passes (see #1 and #2 above) 

Note 5:  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 Wednesday, 31 October 2007 01:36:11 GMT

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