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Re: Graphics contrast - critical principle

From: Alastair Campbell <acampbell@nomensa.com>
Date: Thu, 8 Dec 2016 14:55:35 +0000
To: Detlev Fischer <detlev.fischer@testkreis.de>, "james.nurthen@oracle.com" <james.nurthen@oracle.com>
CC: "w3c-wai-gl@w3.org" <w3c-wai-gl@w3.org>
Message-ID: <FA3100FB-BE93-43CC-9E9F-87C4B36ADC5C@nomensa.com>
Hi Detlev,

It is an interesting point, just to clarify though a pie chart (with ‘thick’ slices over 3px wide/high) would need 3:1.

Even so, in the higher-contrast third example pie chart, none of the slices have more than 2:1 contrast against each other.
https://www.w3.org/WAI/GL/low-vision-a11y-tf/wiki/Informational_Graphic_Contrast_(Minimum)#Pie_Charts 

Unless you go light – dark – light – dark, you wouldn’t have sufficient contrast without another method. And then you’d have to use even numbered pie charts!

What you could do is have some sort of border between them. All of the slices would have sufficient contrast with a 3px black or white border between them, and with careful colour choice that could with a 1px border.

Using a gestalt approach could help refine what a ‘graphical element’ is (although Bruce thinks we should not define that), but I can’t see how it would help from a contrast testing point of view.

Thanks,

-Alastair


On 08/12/2016, 12:54, "Detlev Fischer" <detlev.fischer@testkreis.de> wrote:

    Hi Alastair,
    
    I'm just wondering whether the gestalt nature of the pie chart (and similar effects of arrangements) might mitigate the need for 4.5:1 for *all* elements. If you had a pie with four wedges and the fourth was low contrast, all four having a labelled legend explaining what colour is what, the fourth wedge with insufficent contrast would be quite easy to identify by force of exclusion (it's the one remaining if you take away the three others which DO meet 4,5:1 against background AND the other two in the set). This is of course not scalable but I wonder if gestalt / shape might have to be taken into account somehow. For me, it seems to make a difference whether you have some arbitrary shape against a background or an ensemble with an overall gestalt (pies, bars etc.) where both gestalt and order of elements may help in associating the correct label. If you think this makes it just more confusing or see it as a thin end of a wedge, by all means disregard this comment.
    
    Best,
    Detlev
    
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    Detlev Fischer
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    Alastair Campbell schrieb am 07.12.2016 13:25:
    
    > HI James,
    > 
    > As a quick follow up, I’ve added a section to that page that works through
    > making a pie-chart accessible, and which of the guidelines each example would
    > pass/fail:
    > https://alastairc.ac/tests/graphics-contrast-explanation.html#piechart 
    > 
    > This time I started with a default Excel pie chart, and made adjustments so
    > that each one demos a different approach.
    > 
    > Hopefully that is much clearer.
    > 
    > Cheers,
    > 
    > -Alastair
    > 
    > 
    
    

Received on Thursday, 8 December 2016 14:56:12 UTC

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