D11262: KDE logout screen background color fix

[Radek Hušek]

Pitel added a comment.


  I see that relative luminance formula is more correct but I would like to point out that we do not need here to calculate precise luminance. The point of the `?:` statement is to collapse colors close to back to real black because gray overlay with 50 % opacity does not look good. So the current formula looks good enough for me and I would even think about moving the threshold down a bit to 0.4 or 0.3. (Generally not collapsing to black is the save way, it may not look so good but the text will be still easy to read.)
  
  For reference a screenshot with gray background (similar to default breeze) without collapsing it to black: F5752690: Screenshot_20180313_175108.png <https://phabricator.kde.org/F5752690>

REPOSITORY
  R120 Plasma Workspace

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

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