[R] measuring distances between colours?

[John Fox]

Dear r-helpers,

I'm interested in locating the named colour that's "closest" to an arbitrary RGB colour. The best that I've been able to come up is the following, which uses HSV colours for the comparison:

r2c <- function(){
    hexnumerals <- 0:15
    names(hexnumerals) <- c(0:9, LETTERS[1:6])
    hex2decimal <- function(hexnums){
        hexnums <- strsplit(hexnums, "")
        decimals <- matrix(0, 3, length(hexnums))
        decimals[1, ] <- sapply(hexnums, function(x)               
                                 sum(hexnumerals[x[1:2]] * c(16, 1)))
        decimals[2, ] <- sapply(hexnums, function(x) 
                                 sum(hexnumerals[x[3:4]] * c(16, 1)))
        decimals[3, ] <- sapply(hexnums, function(x) 
                                 sum(hexnumerals[x[5:6]] * c(16, 1)))
        decimals
    }
    colors <- colors()
    hsv <- rgb2hsv(col2rgb(colors))
    function(cols){
        cols <- sub("^#", "", toupper(cols))
        dec.cols <- rgb2hsv(hex2decimal(cols))
        colors[apply(dec.cols, 2, function(dec.col) 
            which.min(colSums((hsv - dec.col)^2)))]
    }
}

rgb2col <- r2c()

I've programmed this with a closure so that hsv gets computed only once.

Examples:

> rgb2col(c("AA0000", "002200", "000099", "333300", "BB00BB", "#005555"))
[1] "darkred"   "darkgreen" "blue4"     "darkgreen" "magenta3"  "darkgreen"
> rgb2col(c("AAAA00", "#00AAAA"))
[1] "darkgoldenrod" "cyan4"      

Some of these colour matches, e.g., "#005555" -> "darkgreen" seem poor to me. Even if the approach is sound, I'd like to be able to detect that there is no sufficiently close match in the vector of named colours. That is, can I establish a maximum acceptable distance in the HSV (or some other) colour space?

I vaguely recall a paper or discussion concerning colour representation in R but can't locate it.

Any suggestions would be appreciated.

John

------------------------------------------------
John Fox
Sen. William McMaster Prof. of Social Statistics
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox/