plugins/org.eclipse.actf.visualization.doc.user/docs/problemsLv.html - gerrit/actf/org.eclipse.actf.visualization - Git at Google


            
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            <TITLE>Accessibility and usability problems for users with low vision</TITLE>

            
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            <H1>Accessibility and usability problems for users with low vision</H1>

            
            <P>

            
            Use the Low Vision View to simulate weak eyesight, color vision deficiencies, cataracts, and their arbitrary combinations.

            
            This simulation helps see how users with low vision experience a webpage.

            
            The first four sections that follow describe the low-vision-related problems that ACTF can detect and how to address these problems.

            
            (Problems are listed in the Problem pane. Clicking on the line that describes a problem highlights the corresponding element.)

            
            The final section describes the severity values of the problems appearing in the Line View pane of the low vision mode.

            
            </P>


            <H2>Fixed-size font problems</H2>

            
            <P>

            
            Text with fixed-size fonts that cannot be enlarged by browser options presents a problem for those with vision problems.

            
            This problem occurs when a webpage uses a fixed size value, such as &quot;8pt&quot; or &quot;5mm,&quot; as a font size.

            
            This problem can be addressed by using absolute-size keywords (for example, &quot;large&quot;), relative-size keywords (for example, &quot;larger&quot;), or percentage values as font sizes.

            
            (For information on how to set font sizes, see <A href="http://www.w3.org/TR/CSS21/fonts.html#font-size-props">15.7 Font size: The 'font-size' property</A>.)

            
            Another solution is to provide a menu option that allows readers to select their own font sizes.

            
            </P>


            <H2>Small character problems</H2>

            
            <P>

            
            For some people with weak eyesight, some characters may be too small to read even when browser options permit them to use the largest font size available.

            
            When that happens, the reader needs additional assistive technologies.

            
            Small characters should not be used for layout purposes.

            
            </P>


            <H2>Text color problems</H2>

            
            <P>

            
            People with color vision deficiencies and cataracts can see colors, but they perceive them differently than people with normal vision.

            
            People with color vision deficiencies may have difficulties in reading texts even if their foreground and background colors are different.

            
            Such problems can be addressed by choosing sharply contrasting colors for the foreground and background.

            
            </P>


            <H2>Image color problems</H2>

            
            <P>

            
            People with color vision deficiencies and cataracts can see colors, but they perceive them differently than people with normal vision.

            
            If color alone conveys certain information, people with color vision deficiencies and cataracts will not perceive the information.

            
            Any information conveyed with color must be available without colors.

            
            For example, in a line graph, not only colors but also types of lines, such as solid, dotted and wavy lines, should be used to distinguish among the quantities of different items.

            
            </P>


            <H2>Severity values</H2>

            
            <P>

            
            Each problem in the low vision mode has its own severity value, which indicates the seriousness of the problem and is rated on a scale that ranges from 0 to 100.

            
            The higher the value, the more serious the problem.

            
            </P>

            
            <P>

            
            The severity values of font-related problems (fixed-size font problems or small character problems) are 25.

            
            If an element has both a fixed-size font problem and a small character problem, these problems are combined and shown in one line in the Line View pane.

            
            In this case, the severity value becomes 50.

            
            </P>

            
            <P>

            
            The severity values of color-related problem are evaluated using the <A href="http://www.w3.org/TR/AERT#color-contrast">algorithm described in &quot;Techniques For Accessibility Evaluation And Repair Tools&quot;</A> issued by the World Wide Web Consortium.

            
            For example, if a page includes two colors, R<SUB>1</SUB>, G<SUB>1</SUB>, B<SUB>1</SUB> and R<SUB>2</SUB>, G<SUB>2</SUB>, B<SUB>2</SUB>, where each of the RGB values ranges from 0 to 255, then two types of difference values, the brightness difference and the hue difference between the two colors are calculated as follows:

            
            <PRE>

            
            The brightness difference = |(299R<SUB>1</SUB> + 587G<SUB>1</SUB> + 114B<SUB>1</SUB>)/1000 - (299R<SUB>2</SUB> + 587G<SUB>2</SUB> + 114B<SUB>2</SUB>)/1000|<BR>

            
            The hue difference = |R<SUB>1</SUB>-R<SUB>2</SUB>| + |G<SUB>1</SUB>-G<SUB>2</SUB>| + |B<SUB>1</SUB>-B<SUB>2</SUB>|

            
            </PRE>

            
            The threshold values are set to 125 and 500 for the brightness difference and the hue difference, respectively.

            
            If at least one of the difference values is greater than or equal to its corresponding threshold value, the two colors are judged to be different enough to provide good color visibility.

            
            Otherwise, the severity values for both brightness and hue are computed as follows:

            
            <OL>

            
            <LI>If a difference value is equal to its corresponding threshold value, the severity value is 0.

            
            <LI>If the difference value is equal to 0, the severity value is 100.

            
            <LI>If the difference value is intermediate between the threshold value and 0, the severity value is the corresponding fraction between 0 and 100.

            
            </OL>

            
            The smaller of the two severity values (for brightness and hue) becomes the severity value of the problem.

            
            </P>

            
            <P>

            
            The severity values depend on the parameters you set.

            
            For a text element, ACTF tests its foreground and background colors in the Simulation View.

            
            For an image, ACTF tests colors of every combination of large connected components in the simulated image whose colors provide good color visibility in the original image.

            
            </P>

            
            <P>

            
            The overall rating of a webpage is based on the total of the severity values of all of the problems detected in the page.

            
            </P>


            <H1 class="related">Related topics</H1>

            
            <UL class="related">

            
            <LI><A href="checkLv.html">Checking the accessibility and usability of a document in the Low Vision View</A>

            
            <LI><A href="problemsBlind.html">Accessibility and usability problems for blind users</A>

            
            <LI><A href="compliance.html">Compliance and accessibility guidelines for Web content</A>

            
            </UL>


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	<HEAD>
	<META http-equiv="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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	<meta name="copyright" content="Copyright (c) IBM Corporation and others 2007. This page is made available under license. For full details see the LEGAL in the documentation book that contains this page." >
	<TITLE>Accessibility and usability problems for users with low vision</TITLE>
	<LINK href="base.css" rel="stylesheet" type="text/css" class="head">
	</HEAD>
	<BODY>
	<H1>Accessibility and usability problems for users with low vision</H1>
	<P>
	Use the Low Vision View to simulate weak eyesight, color vision deficiencies, cataracts, and their arbitrary combinations.
	This simulation helps see how users with low vision experience a webpage.
	The first four sections that follow describe the low-vision-related problems that ACTF can detect and how to address these problems.
	(Problems are listed in the Problem pane. Clicking on the line that describes a problem highlights the corresponding element.)
	The final section describes the severity values of the problems appearing in the Line View pane of the low vision mode.
	</P>

	<H2>Fixed-size font problems</H2>
	<P>
	Text with fixed-size fonts that cannot be enlarged by browser options presents a problem for those with vision problems.
	This problem occurs when a webpage uses a fixed size value, such as "8pt" or "5mm," as a font size.
	This problem can be addressed by using absolute-size keywords (for example, "large"), relative-size keywords (for example, "larger"), or percentage values as font sizes.
	(For information on how to set font sizes, see <A href="http://www.w3.org/TR/CSS21/fonts.html#font-size-props">15.7 Font size: The 'font-size' property</A>.)
	Another solution is to provide a menu option that allows readers to select their own font sizes.
	</P>

	<H2>Small character problems</H2>
	<P>
	For some people with weak eyesight, some characters may be too small to read even when browser options permit them to use the largest font size available.
	When that happens, the reader needs additional assistive technologies.
	Small characters should not be used for layout purposes.
	</P>

	<H2>Text color problems</H2>
	<P>
	People with color vision deficiencies and cataracts can see colors, but they perceive them differently than people with normal vision.
	People with color vision deficiencies may have difficulties in reading texts even if their foreground and background colors are different.
	Such problems can be addressed by choosing sharply contrasting colors for the foreground and background.
	</P>

	<H2>Image color problems</H2>
	<P>
	People with color vision deficiencies and cataracts can see colors, but they perceive them differently than people with normal vision.
	If color alone conveys certain information, people with color vision deficiencies and cataracts will not perceive the information.
	Any information conveyed with color must be available without colors.
	For example, in a line graph, not only colors but also types of lines, such as solid, dotted and wavy lines, should be used to distinguish among the quantities of different items.
	</P>

	<H2>Severity values</H2>
	<P>
	Each problem in the low vision mode has its own severity value, which indicates the seriousness of the problem and is rated on a scale that ranges from 0 to 100.
	The higher the value, the more serious the problem.
	</P>
	<P>
	The severity values of font-related problems (fixed-size font problems or small character problems) are 25.
	If an element has both a fixed-size font problem and a small character problem, these problems are combined and shown in one line in the Line View pane.
	In this case, the severity value becomes 50.
	</P>
	<P>
	The severity values of color-related problem are evaluated using the <A href="http://www.w3.org/TR/AERT#color-contrast">algorithm described in "Techniques For Accessibility Evaluation And Repair Tools"</A> issued by the World Wide Web Consortium.
	For example, if a page includes two colors, R<SUB>1</SUB>, G<SUB>1</SUB>, B<SUB>1</SUB> and R<SUB>2</SUB>, G<SUB>2</SUB>, B<SUB>2</SUB>, where each of the RGB values ranges from 0 to 255, then two types of difference values, the brightness difference and the hue difference between the two colors are calculated as follows:
	<PRE>
	The brightness difference = \|(299R<SUB>1</SUB> + 587G<SUB>1</SUB> + 114B<SUB>1</SUB>)/1000 - (299R<SUB>2</SUB> + 587G<SUB>2</SUB> + 114B<SUB>2</SUB>)/1000\|<BR>
	The hue difference = \|R<SUB>1</SUB>-R<SUB>2</SUB>\| + \|G<SUB>1</SUB>-G<SUB>2</SUB>\| + \|B<SUB>1</SUB>-B<SUB>2</SUB>\|
	</PRE>
	The threshold values are set to 125 and 500 for the brightness difference and the hue difference, respectively.
	If at least one of the difference values is greater than or equal to its corresponding threshold value, the two colors are judged to be different enough to provide good color visibility.
	Otherwise, the severity values for both brightness and hue are computed as follows:
	<OL>
	<LI>If a difference value is equal to its corresponding threshold value, the severity value is 0.
	<LI>If the difference value is equal to 0, the severity value is 100.
	<LI>If the difference value is intermediate between the threshold value and 0, the severity value is the corresponding fraction between 0 and 100.
	</OL>
	The smaller of the two severity values (for brightness and hue) becomes the severity value of the problem.
	</P>
	<P>
	The severity values depend on the parameters you set.
	For a text element, ACTF tests its foreground and background colors in the Simulation View.
	For an image, ACTF tests colors of every combination of large connected components in the simulated image whose colors provide good color visibility in the original image.
	</P>
	<P>
	The overall rating of a webpage is based on the total of the severity values of all of the problems detected in the page.
	</P>

	<H1 class="related">Related topics</H1>
	<UL class="related">
	<LI><A href="checkLv.html">Checking the accessibility and usability of a document in the Low Vision View</A>
	<LI><A href="problemsBlind.html">Accessibility and usability problems for blind users</A>
	<LI><A href="compliance.html">Compliance and accessibility guidelines for Web content</A>
	</UL>

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	<A href="index.html">Go to Index</A>
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