- From: Patrick H. Lauke <redux@splintered.co.uk>
- Date: Fri, 29 Jul 2016 09:54:26 +0100
- To: w3c-wai-gl@w3.org
On 29/07/2016 09:39, Alastair Campbell wrote: > I guess this is mostly a matter of how it is measured? > > So if designer/developer specifies a foreground and/or background > colour with some transparency, you shouldn’t use the CSS defined > colour to test contrast without accounting for the transparency. (I > know transparency is generally defined with the colour, but that > doesn’t mean a testing tool accounts for it.) If the transparent > colour of a box (the background for text) is on top of a white > background it will be lighter than on a black background, which needs > to be accounted for. Correct. Assuming it's all flat colors (not over a gradient, or background image, or positioned over an actual <img>, etc) I guess you calculate the *actual* colors as rendered by the browser. That does start to get very tricky for an automated tool to work out though, and is prone to false positives/negatives since the tools would generally not take into account images etc which might be present and skew the results. > However, if you use a tool with an eye-dropper style mechanism on the > resulting colours, that should be accurate. Unless I’m missing > something? Yes, using an eye-dropper to test the actual final render would work (though this can get complicated if the layout potentially changes depending on viewport/window size, and semi-transparent elements all of a sudden move over other elements...you end up with different backgrounds, resulting in different final rendered color). > The contrast adjuster function in the CSS-color-4 spec looks cool (if > it’s implemented?), it references WCAG2 1.4.3 so should line up. > > Perhaps we need to make G18 more specific about testing the result > rather than the colour definition? > > And perhaps we could create a test page that would pass if > transparency is not accounted for, but should fail, and check that > each of the tools listed in G18 accounts for the transparency? > (Perhaps the browser based ones don’t?) > > Cheers, > > -Alastair > -- Patrick H. Lauke www.splintered.co.uk | https://github.com/patrickhlauke http://flickr.com/photos/redux/ | http://redux.deviantart.com twitter: @patrick_h_lauke | skype: patrick_h_lauke
Received on Friday, 29 July 2016 08:54:50 UTC