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Checkpoint 1.5 Proposal

From: Ben Caldwell <caldwell@trace.wisc.edu>
Date: Wed, 16 Apr 2003 22:09:57 -0600
To: <w3c-wai-gl@w3.org>
Message-ID: <000201c30497$35574370$6501a8c0@BenMobile>

A number of proposals related to Checkpoint 1.5 have been posted to the list
since our last draft. Here is a proposed redraft of this checkpoint for
inclusion in the next draft of WCAG 2.0. It combines a number of
suggestions, proposals and discussions from the working group over the last
few months.

Edits:

1. multiple edits based on comments from Judy Harkins[9]. 
     o  added a decibel level for audio contrast (20db).
     o  reworded example two to say, "There are no sounds or
        background music playing behind the speech track or they are
        more than 20db down." Was, " There are no sounds or
        background music playing behind the narration or they are
        more than 20db down."
     o  reworded second bullet under benefits to read, "Background
        sounds interfere with the ability of someone with a hearing
        impairment to understand speech, and may also interfere with
        the intelligibility of the speech to someone who is in an
        environment with ambient noise." Was, "Individuals with
        hearing impairments that limit their ability to hear all of
        the frequencies of speech can make out the words from the
        sounds they can hear because they are not mixed with   
        residual sounds from the music."

2. incorporated a number of proposals compiled by Andi resulting from
comments on the last public draft. [2]
     o  reworded minimum level success criterion to read, text
        content that is presented over a background image or pattern
        is implemented using mechanisms that allow the user to
        display the text without the background image or pattern."
        Was, "no text content is presented over a background 
        picture, color or pattern that seriously interferes with 
        readability unless the background picture or pattern can be 
        easily removed." [proposal 1]
     o  removed second minimum level success criterion, "prepared
        audio presentations do not have background sounds that
        seriously interfere with foreground auditory content."
        [proposal 1]
     o  reworded second level success 
        criterion to read, "when text content is presented over a 
        background image or pattern, the text is easily readable 
        when the page is viewed in 256 grayscale." Was, "content 
        that might create a problem has been run through a simulator 
        for major types of color blindness and has been found to 
        remain easily readable"
     o  revised level 3 audio success criterion to read, "audio
        content does not contain background sounds OR the background
        sounds are at least 20 db lower than the foreground audio
        content." Was, "background sounds are at least ?? db lower
        than foreground content." [proposal 4A]
     o  revised level 3 contrast success criterion to read, "text
        content is not presented over a background image or pattern
        OR the text is easily readable when the page is viewed in 
        256 grayscale." Was, "there are no background pictures or
        patterns behind foreground content"
     o  @@ include reviewer's note and consider presentation here
        (also check on source of this) for proposal 4b added new
        level 3 success criterion [proposal 4B]


Revised Checkpoint:

Checkpoint 1.5 Ensure that foreground content is easily differentiable from
background for both auditory and visual presentations.

Success criteria

You will have successfully met Checkpoint 1.5 at the Minimum Level if:

    1. text content that is presented over a background image or pattern
       is implemented using mechanisms that allow the user to display the
       text without the background image or pattern.

You will have successfully met Checkpoint 1.5 at Level 2 if:

     * when text content is presented over a background image or pattern,
       the text is easily readable when the page is viewed in 256
       grayscale.

You will have successfully met Checkpoint 1.5 at Level 3 if:

    1. text content is not presented over a background image or pattern
       OR the text is easily readable when the page is viewed in 256
       grayscale.
    2. audio content does not contain background sounds OR the background
       sounds are at least 20 db lower than the foreground audio content.
    3. text content is not presented over a background image or color OR
       the colors used for the text and background or background image
       pass the following test (from Techniques For Accessibility
       Evaluation And Repair Tools[3], dated April 26, 2000):
          + Two colors provide good color visibility if the brightness
            difference and the color difference between the two colors
            are greater than a set range.
          + Color brightness is determined by the following formula:
            ((Red value X 299) + (Green value X 587) + (Blue value X
            114)) / 1000 Note: This algorithm is taken from a formula for
            converting RGB values to YIQ values. This brightness value
            gives a perceived brightness for a color.
          + Color difference is determined by the following formula:
            (maximum (Red value 1, Red value 2) - minimum (Red value 1,
            Red value 2)) + (maximum (Green value 1, Green value 2) -
            minimum (Green value 1, Green value 2)) + (maximum (Blue
            value 1, Blue value 2) - minimum (Blue value 1, Blue value
            2))
          + The range for color brightness difference is 125. The range
            for color difference is 500.
       Reviewer's Note: There is some question about whether this
       algorithm yields content with sufficient contrast. The working
       group is currently in the process of researching a number of
       methods for determining sufficient contrast and color visibility.

Please let me know if you have questions, comments or concerns or if there's
anything I've missed.

Thanks,

-Ben

References:

[1] http://lists.w3.org/Archives/Public/w3c-wai-gl/2003JanMar/0279.html
[2] http://lists.w3.org/Archives/Public/w3c-wai-gl/2003JanMar/0146.html
[3] http://www.w3.org/TR/AERT
Received on Wednesday, 16 April 2003 23:10:40 GMT

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