Contrast between label and background: determine if color is light or dark

Question

I have a barplot with labels in white. Sometimes the color of background is too light and the white label becomes illegible. I'm looking for a function that takes a color value and returns whether the color is dark or light. Then I can set the label color to white or black accordingly to obtain the best contrast against the background.

Answer 1

Here's a strategy to implement picking a text color of black vs white based on the intensity scale in the (second) link provided by @MrFlick.

The blog cited a W3C publication: a standard formula for calculating the perceived brightness of a color that used an algorithm for RGB encoded colors:

 ((Red value X 299) + (Green value X 587) + (Blue value X 114)) / 1000

The col2rgb function delivers a 3-row matrix which I multiply by the factors offered in that webpage. I used an example of "red" as a background color and the chosen text would then be "white"

 c( "black", "white")[  1+(sum( col2rgb("red") *c(299, 587,114))/1000 < 123) ]
[1] "white"

Implemented as a function:

isDark <- function(colr) { (sum( col2rgb(colr) * c(299, 587,114))/1000 < 123) }
isDark("red")
[1] TRUE