D11262: KDE logout screen background color fix

[Christoph Feck]

cfeck added a comment.


  In D11262#235414 <https://phabricator.kde.org/D11262#235414>, @Pitel wrote:
  
  > 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...
  
  
  You can adjust the coefficients. Please check color (0, 0, 128), which is - according to the committed way - considered a light color.
  
  Maybe try something like (r+2*g+b) > 0.8.

REPOSITORY
  R120 Plasma Workspace

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

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