On 05/30/2013 10:13 PM, John Fox wrote:
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
Hi John,
Ben Bolker contributed a function (color.id) to the plotrix package that
does something like this. Although it uses RGB colorspace, it might be
useful:
color.id<-function (col) {
c2 <- col2rgb(col)
coltab <- col2rgb(colors())
cdist <- apply(coltab, 2, function(z) sum((z - c2)^2))
colors()[which(cdist == min(cdist))]
}
Jim
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