From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Thu, 23 Mar 2006 01:57:15 -0600
To: <w3c-wai-gl@w3.org>
Message-ID: <012601c64e4f\$6987d3d0\$ee8cfea9@NC6000BAK>
```I am at CSUN and someone came up to me and pointed out something

Currently all 3 to 50 hz flashing is outlawed in WCAG 1.0

This means that even a pixel or a small 20 pixel icon (say of a disk drive
to show when it is busy) would cause screen to fail.  But it shouldn't .

However the new criterion allows very provocative stimuli as long as it is
less than 40% of 10 degrees of viewing angle. (or 341 x 256 pixels to be
exact).

The problem is that if viewers view the pages enlarged, then  341 x 256
pixels of a regular image will be blown up to more than that number of
pixels on the screen enlarger.

We had thought of this earlier but couldn't think of a solution.  When
talking to this person I thought about the 'simple test' that we came up
with and added as a sufficient technique.  Namely   There is no area that
flashes more than 3 times in a one second period.  For technical reasons
this is easier to conform to than 'less than 3 hz'.  Easier to measure as
well.

The idea is to put this at a lower level and make it apply to a much smaller
portion of the screen.  It could be "any pixel or pixels".    That would
still eliminate the disc drive - unless it just was limited to 3 flashes in
any second.   But it would be at the lower level of conformance (level 2 or
3).

Or we could allow something more than one pixel - but still only a few.
If we figured 8x magnification, 341 x 256 becomes  42 X 32.   If we figure
16x magnification it becomes 21 x 16 pixels.

So we could just say at level 2 or 3

No more than three flashes within a one-second period where the combined
area of flashes occurring concurrently (but not necessarily contiguously)
occupies more than one quarter of any 21 x 16 pixel rectangle anywhere on
the displayed screen area when the content is viewed at 1024 by 768 pixels

I know it is late in the process - but I would like to see if we would like
to propose the following

<proposed>

2.3.2 Content does not violate the general flash threshold or the red flash
threshold at 16x magnification [How to meet 2.3.2]

Definition

16x magnification

Magnification where any 21 x 16 pixel area would be enlarged to 336 x 256
pixels

</proposed>

The how to meet documents and techniques would be identical to our current
guidelines except that the 16x magnification  (or 21 x 16 pixel) numbers
would apply.

Again - ANY single, double or triple flash of any size automatically passes.
It is only when you go beyond that that you trigger anything.

We could do this at Level 3.

Gregg

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

Gregg C Vanderheiden Ph.D.
Professor - Depts of Ind. Engr. & BioMed Engr.
Director - Trace R & D Center
< <http://trace.wisc.edu/> http://trace.wisc.edu/> FAX 608/262-8848
For a list of our list discussions  <http://trace.wisc.edu/lists/>
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```
Received on Thursday, 23 March 2006 07:57:59 GMT

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