Detect if area under panel is complex or even-toned [$50]

[Cassidy James Blaede]

For determining the opacity of the panel, Gala should also check if the area under the panel is evenly-toned or more complex.

For example, if a wallpaper under the panel has many small bright spots but is overall dark, currently the panel is set to transparent. This can be problematic when trying to see the indicator icons, clock, or "Applications" text when a bright spot is underneath. In that case, we should still have some opacity to the panel to counteract the bright spots.

[Cassidy James Blaede (cassidyjames)]

For a similar concept applied in Android, see "Color Variants" at https://www.google.com/design/spec/layout/structure.html#structure-system-bars

[Cassidy James Blaede (cassidyjames)]

Here is an example of when Gala gets it wrong and would be improved by detecting the even-ness of the area under the panel. Check the contrast in the top-right.

[Danielle Foré (danrabbit)]

https://www.bountysource.com/issues/24104459-detect-if-area-under-panel-is-complex-or-even-toned

[Erasmo Marín (erasmo-marin)]

Basically, what we need to detect is any case where there is an big horizontal variance of the luminosity like the one in the screenshot. Right now, Gala calculates the color variance, so, maybe that's enough and the fix should be done in Wingpanel instead of Gala.

Relevant code is here:
http://bazaar.launchpad.net/~elementary-pantheon/wingpanel/trunk/view/head:/src/Services/BackgroundManager.vala#L82