D11262: KDE logout screen background color fix

[Radek Hušek]

Pitel added a comment.


  I finally got to do some testing and it is not looking good for luminance formula `0.2126*r + 0.7152*g + 0.0722*b`.
  
  Screenshot with background color `#5500FF` (blueish, button background on picture), you can see that black text is readable on it, but it has luminance only 0.14 so in overlay it is replaced by black.
  F5769603: Screenshot_20180327_174715.png <https://phabricator.kde.org/F5769603>
  
  So I would rather stay conservative, and used formula `max(r, g, b)`. I am still not sure whether the threshold should be `0.5` or a bit lower...

REPOSITORY
  R120 Plasma Workspace

REVISION DETAIL
  https://phabricator.kde.org/D11262

To: Pitel, mart, #plasma
Cc: fredrik, ngraham, abetts, broulik, mvourlakos, plasma-devel, ragreen, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, sebas, apol, mart
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