What is the [0,1] equivalent gray value of a given hex color

to.greyvalue.R

# I use this frequently enough .... helps
# with determining text value on top of given background color

to.greyvalue <- function(hexcolor, method = c("luminosity","average","lightness")[1]){
	# https://www.johndcook.com/blog/2009/08/24/algorithms-convert-color-grayscale/
	stopifnot(length(hexcolor) == 1)
	rgbspec <- as.vector(col2rgb(hexcolor)) / 255
	if (method == "luminosity"){
		grayval <- sum(c(0.21, 0.72, 0.07) *  rgbspec)
	}
	if (method == "average"){
		grayval <- mean(rgbspec)
	}
	if (method == "lightness"){
		grayval <- (max(rgbspec) + min(rgbspec)) / 2
	}
	grayval
}