RE: One Simple Infographic that explains Spatial Frequency

Hi Andrew,

For images in emails, I suggest putting in a link to that image on your website (or a website), as a secondary method for people to see it. For LVTF email the images do get stripped, on the AG list they get included at the bottom, I don't know why it's different!

Also, I think the special frequency aspect might provide guidance for a question I have about the new focus indicator SC.

We're currently setting the minimum bar at:

  *   A change in contrast of at least 3:1.
  *   A size in CSS pixels that is at least 1px multiplied by the longest edge.
  *   If the indicator it doesn't contrast with the adjacent color, it should be at least 2px thick.

E.g. these [1] pass, default then focused:
[cid:image002.jpg@01D5840D.39D884B0]
[cid:image008.jpg@01D5840D.39D884B0]
[cid:image010.jpg@01D5840D.39D884B0]

[cid:image012.jpg@01D5840D.39D884B0]
[cid:image013.jpg@01D5840D.39D884B0]

These fail:
[cid:image015.jpg@01D5840D.39D884B0](1px increase next to a non-contrasting color)
[cid:image022.jpg@01D5840D.39D884B0](light grey replaces white)

The problem is that this [2] also passes:
[cid:image025.jpg@01D5840D.39D884B0]

In terms of ratcheting up the requirement, my impression is that it would make more sense to increase the surface area than the contrast requirement.

E.g. A size in CSS pixels that is at least 2px multiplied by the longest edge

So this would pass:
[cid:image028.png@01D5840D.39D884B0]
With a 2px thickness line.

Whereas increasing the contrast to 4.5:1 wouldn't make as much difference?

Cheers,

-Alastair

1] https://alastairc.uk/tests/wcag22-examples/focus-more-visible-2.html <https://alastairc.uk/tests/wcag22-examples/focus-more-visible-2.html%20%0d2>
2<https://alastairc.uk/tests/wcag22-examples/focus-more-visible-2.html%20%0d2>] http://3needs.org/en/testing/code/focus-visibility.html

From: Andrew Somers <me@andysomers.com>
Sent: 11 October 2019 05:28
To: Jim Allan <jimallan@tsbvi.edu>; Wayne Dick <wayneedick@gmail.com>; Jeanne Spellman <jspellman@spellmanconsulting.com>; Shawn Lauriat <lauriat@google.com>; Laura Carlson <laura.lee.carlson@gmail.com>; jjuett@salesforce.com
Cc: public-low-vision-a11y-tf <public-low-vision-a11y-tf@w3.org>
Subject: One Simple Infographic that explains Spatial Frequency

Hello everyone,

I am endeavoring to write shorter emails! Here is a simple infographic:

[cid:image003.png@01D58408.43CE8370]

Assuming a standard viewer distance (CSS reference pixel visual angle):

  *   a stroke width of between 8px to 20px is at the peak of contrast sensitivity, while
  *   a 1px stroke width drops contrast perception down an order of magnatude x2 from peak.

     *   Increasing stroke width (by increasing font size and/or font weight) increases perceived contrast

        *   especially closer to threshold, and
        *   especially at the higher frequencies (over 16cpd).

     *   impairments offset this curve, typically down and to the left.

        *   The implication is that impairments come closer to threshold sooner, and need additional contrast
Please let me know if there are questions/comments.

I do have one question: is there a way to save graphics/attachments for emails to the list archive? I see they are always stripped? Perhaps I should just double post them to the Wiki?

Thank you!!

Andy

Andrew Somers
Address Redacted for List