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Re: Clarification of duration of flashing or flickering (Success Criteria 2.3.1)

From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Wed, 28 Oct 2009 00:18:49 -0400
To: Phill Jenkins <pjenkins@us.ibm.com>
Cc: w3c-wai-ig@w3.org, public-comments-wcag20@w3.org
Message-id: <DCE2A9D1-881D-4ADF-8DFF-BAEF5138E8E8@trace.wisc.edu>

On Oct 27, 2009, at 11:42 PM, Phill Jenkins wrote:

> Is the duration of the flashing or flickering (i.e., more than one  
> second of time) a factor in determining accessibility compliance to  
> WCAG 2.0
>         See Success Criteria 2.3.1 http://www.w3.org/TR/WCAG20/ 
> #seizure
>         "... do not contain anything that flashes more than three  
> times in any one second period..."
> In my special case example, there is a slightly less than 1 second  
> duration of 3 flashes in an Adobe Flash image at the top of a draft  
> Shanghai EXPO 2010 page.
> WARNING - currently it flashes 3 times for slightly less than one  
> second, then repeats several seconds later.
>         see http://fr.expo2010.cn/indextest.htm
> Assuming the developer adds a capability to stop the repeating Adobe  
> Flash movie, is it still a problem because it flashes 3 times,  
> regardless of the duration (e.g., 1 second, 10 seconds, etc.)?  Or  
> does it need to flash more that 3 times a second of time for more  
> that 1 second of duration?

if it flashes MORE THAN 3 times in less than a second -- then it  
flashed 3 times in a second.   so it fails.

but since yours only flashed three time in any one second period - it  
would PASS.    even if it repeats later it passes unless the repeat is  
soon enough that there are more than 3 flashed in any one second period

four flashes alone are enough if they occur within a 1 second window.

(in the television code it would also fail if it ALMOST flashed four  
times a second  but continued to almost fail for a long time.  We did  
not carry that into WCAG).

but more than three flashes in less than a second fails all by  
itself.  it is enough to trigger seizures.

> The "Understanding Guidelines 2.3" further mentions: "likely to  
> cause seizure when viewed even for a second or two."
>         see http://www.w3.org/TR/UNDERSTANDING-WCAG20/seizure.html

In the phrase  "likely to cause seizure when viewed even for a second  
or two."     the operative word here is "for a second".     the "or  
two"  should probably be removed since it implies that more than one  
second may be needed.   For some people that is probably true.  but  
not for others.    so the sentence should probably just say "likely to  
cause seizure when viewed even for a second"  since that is  
accurate.   We will look at this in the group.

> metrics to consider: Frequency, Duration, and Size (the text only  
> really address frequency and size, not duration)

The metric does include duration.  it says   more than 3 flashes in  
any one second period.    the duration is one second.   the frequency  
is not stated.  it is a count and a duration.   the size and the  
intensity are also both specified.

>         frequency:         (or interval) more than 3 flashes per  
> second of time? (3 to 50 Hz)
>         duration:        more than one second duration of time? if  
> it flashes more than 3, but for less than 1 second, is it a problem?
duration is any 1 second period.
>         size:                341 x 256 pixel block or larger - see http://www.w3.org/TR/WCAG20/#general-thresholddef
> Since my example only flashes 3 times and not more than 3, and it  
> soon may change to not repeat - Guidelines 2.3 says "more than  
> three", then I think I do not have a problem, but I'm not sure -  
> hence the question to the list.  Your informed opinions welcomed.

You are correct.  you just pass.

> I also believe the text and explanation can be simplified.  For  
> example:
> 1. by addressing the three different metrics; frequency, duration,  
> and size.

these are confusing.  frequency assumes the flashes are equally  
spaced.   so you can't use that term.

duration is specified.   any one second period.    size is specified  
-  as an area of the retina.

> 2. by addressing the lower and upper limits of each.  For example,  
> if the frequency is 40 times in a second, but only lasts 1 second or  
> less, regardless of size it may be hardly noticeable and not be a  
> problem.

40 times a second may not be noticeable (in fact your flicker  
frequency is lower than that ) but it is still very capable of  
creating a seizure.     In fact 40 times per second is as dangerous as  
10.     (20 is the worst)

> 3. add some more common passing and failing examples.  The two  
> existing ones are good.

yes - I can see this could be helpful.

> Regards,
> Phill Jenkins,

Received on Wednesday, 28 October 2009 04:19:34 UTC

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