Hi John
This has been an interesting discussion.
Though you have a solution for your needs, you might be interested in
this javascript implementation that allows you to visually compare color
distances in various color spaces
http://stevehanov.ca/blog/index.php?id=116
And, all the theory of color distance is described in
http://en.wikipedia.org/wiki/Color_difference
PS: This is a very handy function. When I last tried
aplpack::bagplot(), it was annoying that the colors could *only*
be specified in hex.
-Michael
On 5/30/2013 5:14 PM, John Fox wrote:
Dear all,
My thanks to everyone who addressed my question. I've incorporated Eik
Vettorazzi's suggestion for improved conversion of hexadecimal RGB colours
to decimal numbers, and Martin Maechler's hint to look at demo(colors). I've
loosened the default definition of "close enough" from the latter, since the
following seems to work well for my purposes.
r2c <- function(){
hex2dec <- function(hexnums) {
# suggestion of Eik Vettorazzi
sapply(strtoi(hexnums, 16L), function(x) x %/% 256^(2:0) %% 256)
}
findMatch <- function(dec.col) {
sq.dist <- colSums((hsv - dec.col)^2)
rbind(which.min(sq.dist), min(sq.dist))
}
colors <- colors()
hsv <- rgb2hsv(col2rgb(colors))
function(cols, near=0.25){
cols <- sub("^#", "", toupper(cols))
dec.cols <- rgb2hsv(hex2dec(cols))
which.col <- apply(dec.cols, 2, findMatch)
matches <- colors[which.col[1, ]]
unmatched <- which.col[2, ] > near^2
matches[unmatched] <- paste("#", cols[unmatched], sep="")
matches
}
}
rgb2col <- r2c()
For example,
rgb2col(c("010101", "EEEEEE", "AA0000", "00AA00", "0000AA", "AAAA00",
"AA00AA", "00AAAA"))
[1] "black" "gray93" "darkred" "green4"
[5] "blue4" "darkgoldenrod" "darkmagenta" "cyan4"
rgb2col(c("010101", "090909", "090000", "000900", "000009", "090900",
"090009", "000909"))
[1] "black" "gray3" "#090000" "#000900" "#000009" "#090900"
[7] "#090009" "#000909"
Thanks again,
John
--
Michael Friendly Email: friendly AT yorku DOT ca
Professor, Psychology Dept. & Chair, Quantitative Methods
York University Voice: 416 736-2100 x66249 Fax: 416 736-5814
4700 Keele Street Web: http://www.datavis.ca
Toronto, ONT M3J 1P3 CANADA
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