Marker symbol colour

[skeenp]

As per the pull request which isn't getting too much love, it would be good to have the spec allow users to deal with colouring the marker symbol. Something like the following for consideration.

// OPTIONAL: default "555555"
// marker's text or symbol color
//
// value must follow COLOR RULES
"marker-symbol-color": "#555555",

[adamwight]

We looked at this problem briefly and it seems reasonable to simply pick white or black depending on whether the background color crosses a relative luminance of 0.5 0.179, following this WCAG formula:

L = 0.2126 * R + 0.7152 * G + 0.0722 * B

The threshold 0.179 was chosen by solving for a luminance where the contrast

(L1 + 0.05) / (L2 + 0.05)

to black is equal to the contrast to white:

(L + 0.05) / (0 + 0.05) = (1 + 0.05) / (L + 0.05)