- From: Greg Kraus <greg_kraus@ncsu.edu>
- Date: Tue, 12 Aug 2014 11:15:50 -0400
- To: Jim Tobias <tobias@inclusive.com>
- Cc: w3c-wai-ig@w3.org
Hi Jim, I do think there are some unknowns about how to accurately determine what is enough contrast in a given situation. There are times I've zoomed in on a font that appeared to be light blue, but when I looked at the pixel level there wasn't a single light blue pixel in there. It was just a composite of lots of different colors to produce the light blue effect. This has been especially true when a thin font is anti-aliased with a colored background and none of the involved colors are a pure white or pure black. So at the end of the day, I'm not sure how to accurately determine on a pixel-by-pixel basis how a accessible a font color like this is. I originally developed the tool to deal with text over top of CSS3 gradients, images, or any type of background that wasn't a single color. It also happens that if a font has an anti-aliased edge, the algorithm can give you some useful information as long as the font is thick enough that there is a "core" part of the font that is basically a consistent color. Just a note, for the pixel comparison calculations, it does use the formulas in WCAG 2. Greg -- Greg Kraus University IT Accessibility Coordinator NC State University 919.513.4087 gdkraus@ncsu.edu http://go.ncsu.edu/itaccess On Mon, Aug 11, 2014 at 9:45 AM, Jim Tobias <tobias@inclusive.com> wrote: > Very interesting, Greg -- thanks for making that tool. > > I'm not a vision scientist, but something tells me we need their input to make sure that our tools reflect that actual user needs. I think a vision scientist was involved in the original contrast algorithm, and maybe we're ready for a second pass, that looks at how low vision works above the pixel and multi-pixel level. > > *** > Jim Tobias > Inclusive Technologies > +1.908.907.2387 v/sms > skype jimtobias > > >> -----Original Message----- >> From: Greg Kraus [mailto:greg_kraus@ncsu.edu] >> Sent: Monday, August 11, 2014 8:11 AM >> To: w3c-wai-ig@w3.org >> Subject: Re: measuring contrast ratio and Windows Clear Type >> >> I developed a tool to deal with issues like this. It's a Chrome >> extension called Color Contrast Analyzer. >> >> https://chrome.google.com/webstore/detail/color-contrast-analyzer/dagdlcijhfbmgkjokkjicnnfimlebcll?hl=en >> >> It takes a screen shot and compares a pixel with every pixel around >> it. You can set how wide of a radius you want to search around each >> pixel in order to deal with issues like anti-aliasing. The resulting >> image shows each pixel that has an adjacent pixel with enough contrast >> with the original pixel. In the end, the tool creates outlines in the >> page of what I call "contrast borders" that show where the contrast >> exceeds the given WCAG conformance level. >> >> It lets you choose between AA and AAA, and also for standard sized >> text or medium-bold/larger text. The tool does require you to know >> what size text you are analyzing because it is simply doing pixel >> analysis. It has no idea what it is actually analyzing. >> >> Another result of this is it will analyze contrast in images too. >> While contrast in images is not a WCAG requirement, it comes in handy >> when you need to analyze text in a picture, which is a WCAG >> requirement. >> >> Greg >> -- >> Greg Kraus >> University IT Accessibility Coordinator >> NC State University >> 919.513.4087 >> gdkraus@ncsu.edu >> >> On Mon, Aug 11, 2014 at 6:24 AM, Marc Haunschild <mh@zadi.de> wrote: >> > Hi Jan, >> > >> > I can just describe my practice: when I measure contrast with cca I try to >> > find a quite dark pixel (on light background) - normally this has less >> > contrast than the colors defined in the CSS. >> > >> > If the contrast is not high enough, I recommend to change the colors. >> > >> > Of course this is not for official testing (like in BITV-Test), but for real >> > life accessibility I think it is a good compromise. >> > >> > Reasons: On the one hand a user is able to change the Clear Type value >> > himself or to use a bigger font, on the other hand the author of the web >> > site improves the accessbility a little bit more than WCAG aks him to do. >> > >> > To me accessibilty is a matter of fairness and to me it seems to be fair, >> > the way I measure contrast. So I do it this way. ;-) >> > >> > Marc >> > >> > >> > Am 11.08.14 12:05, schrieb Joe Chidzik: >> > >> >> Whenever I've used the CCA with IE, fonts appear anti-aliased, meaning it >> >> is not clear which pixel to pick for the actual text colour. Firefox does >> >> not exhibit this problem for me, and so I've always used Firefox when >> >> measuring contrast with the CCA just to be sure I'm selecting the colours >> >> specified by the CSS. >> >> >> >> I don't know if it's possible to detect (via CSS\JavaScript) if a user has >> >> clear type enabled or not, but in my view, measuring the contrast via the >> >> values provided in the CSS should be all that it is required to check >> >> adherence to the success criteria. >> >> >> >> Joe >> >> >> >> >> >>> -----Original Message----- >> >>> From: Jan Eric Hellbusch [mailto:hellbusch@2bweb.de] >> >>> Sent: 11 August 2014 10:45 >> >>> To: w3c-wai-ig@w3.org >> >>> Subject: measuring contrast ratio and Windows Clear Type >> >>> >> >>> Hi, >> >>> >> >>> I was giving a seminar the other day on Web accessibility testing and was >> >>> asked >> >>> by one of the participants how I measure contrast when clear type is >> >>> activated. I >> >>> was caught by surprise, because I had switched it off when setting my >> >>> Windows 7 >> >>> laptop up a couple of years ago and I never gave it a second thought. >> >>> Since then I >> >>> have been measuring contrast ratios with clear type switched off. >> >>> >> >>> I have been trying to find more detailed information on the precise >> >>> differences in >> >>> measuring contrast ratios with and without clear type on Windows, but I >> >>> obviously >> >>> have the wrong search terms. Perhaps someone on this list can point me >> >>> out in >> >>> the right direction? >> >>> >> >>> * Using CCA there is the possibility of grabing several pixels at a time. >> >>> Does that solve the problem of measuring a correct contrast ratio? >> >>> * Or do I have to switch clear type off or (in Firefox) set >> >>> gfx.content.azure.enabled >> >>> to false? >> >>> >> >>> I would greatly appreciate a hint or a resource. >> >>> >> >>> Thanks, >> >>> Jan >> >>> >> >>> -- >> >>> Jan Eric Hellbusch >> >>> Tel.: +49 (231) 33005825 oder +49 (163) 3369925 >> >>> Accessibility-Beratung: http://2bweb.de >> >>> Blog: www.chemnitzer-14.de >> >>> Bücher, Artikel: www.barrierefreies-webdesign.de >> >>> >> >>> >> >>> >> >> >> >> >> > >> > >> > >> > -- >> > Mit freundlichen Grüßen, >> > >> > i. A. Marc Haunschild >> > Zugängliche Anwendungsentwicklung und Qualitätskontrolle >> > ________________________________________ >> > Referat 414 / Abteilung 4 >> > Bundesanstalt für Landwirtschaft und Ernährung >> > Deichmanns Aue 29, 53179 Bonn >> > Telefon: +49 (0)228 996845-7324 >> > Fax: +49 (0)228 6845-3101 >> > E-Mail: marc.haunschild@ble.de >> > Internet: www.ble.de ( http://www.ble.de/ ) >> > >
Received on Tuesday, 12 August 2014 15:16:20 UTC