[R] measuring distances between colours?
John Fox
jfox at mcmaster.ca
Sat Jun 1 18:30:05 CEST 2013
Hi Michael,
This has become a bit of a comedy of errors.
The bug is in Kevin Wright's code, which I adapted, and you too in your
version, which uses local() rather than function() to produce the closure.
The matrix which.col contains character data, as a consequence of binding
the minimum squared distances to colour names, and thus the comparison
cols.near[2,] < near^2 doesn't work properly when, ironically, the distance
is small enough so that it's rendered in scientific notation.
Converting to numeric appears to work:
> rgb2col2 <- local({
+ all.names <- colors()
+ all.hsv <- rgb2hsv(col2rgb(all.names))
+ find.near <- function(x.hsv) {
+ # return the nearest R color name and distance
+ sq.dist <- colSums((all.hsv - x.hsv)^2)
+ rbind(all.names[which.min(sq.dist)], min(sq.dist))
+ }
+ function(cols.hex, near=.25){
+ cols.hsv <- rgb2hsv(col2rgb(cols.hex))
+ cols.near <- apply(cols.hsv, 2, find.near)
+ ifelse(as.numeric(cols.near[2,]) <= near^2, cols.near[1,],
cols.hex)
+ }
+ })
> rgb2col2(c("#010101", "#EEEEEE", "#AA0000", "#00AA00", "#0000AA",
+ "#AAAA00", "#AA00AA", "#00AAAA"))
[1] "black" "gray93" "darkred" "green4" "blue4"
"darkgoldenrod"
[7] "darkmagenta" "cyan4"
The same bug is in the code that I just posted using Lab colours, so (for
posterity) here's a fixed version of that, using local():
> rgb2col <- local({
+ all.names <- colors()
+ all.lab <- t(convertColor(t(col2rgb(all.names)), from = "sRGB",
+ to = "Lab", scale.in = 255))
+ find.near <- function(x.lab) {
+ sq.dist <- colSums((all.lab - x.lab)^2)
+ rbind(all.names[which.min(sq.dist)], min(sq.dist))
+ }
+ function(cols.hex, near = 2.3) {
+ cols.lab <- t(convertColor(t(col2rgb(cols.hex)), from = "sRGB",
+ to = "Lab", scale.in = 255))
+ cols.near <- apply(cols.lab, 2, find.near)
+ ifelse(as.numeric(cols.near[2, ]) < near^2, cols.near[1, ],
toupper(cols.hex))
+ }
+ })
> rgb2col(c("#010101", "#EEEEEE", "#AA0000", "#00AA00", "#0000AA",
"#AAAA00", "#AA00AA", "#00AAAA"))
[1] "black" "gray93" "#AA0000" "#00AA00" "#0000AA" "#AAAA00"
[7] "#AA00AA" "#00AAAA"
> rgb2col(c("#010101", "#EEEEEE", "#AA0000", "#00AA00", "#0000AA",
"#AAAA00", "#AA00AA", "#00AAAA"), near=15)
[1] "black" "gray93" "firebrick3" "limegreen"
[5] "blue4" "#AAAA00" "darkmagenta" "lightseagreen"
So with Lab colours, setting near to the JND of 2.3 leaves many of these
colours unmatched. I experimented a bit, and using 15 (as above) produces
matches that appear reasonably "close" to me.
I used squared distances to avoid taking the square-roots of all the
distances. Since the criterion for "near" colours, which is on the distance
scale, is squared to make the comparison, this shouldn't be problematic.
I hope that finally this will be a satisfactory solution.
Best,
John
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Michael Friendly
> Sent: Saturday, June 01, 2013 11:33 AM
> To: John Fox
> Cc: 'r-help'; 'Martin Maechler'
> Subject: Re: [R] measuring distances between colours?
>
> Just a quick note: The following two versions of your function don't
> give the same results. I'm not sure why, and also not sure why the
> criterion for 'near' should be expressed in squared distance.
>
> # version 1
> rgb2col <- local({
> 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
> }
> })
>
> # version 2
> rgb2col2 <- local({
> all.names <- colors()
> all.hsv <- rgb2hsv(col2rgb(all.names))
> find.near <- function(x.hsv) {
> # return the nearest R color name and distance
> sq.dist <- colSums((all.hsv - x.hsv)^2)
> rbind(all.names[which.min(sq.dist)], min(sq.dist))
> }
> function(cols.hex, near=.25){
> cols.hsv <- rgb2hsv(col2rgb(cols.hex))
> cols.near <- apply(cols.hsv, 2, find.near)
> ifelse(cols.near[2,] < near^2, cols.near[1,], cols.hex)
> }
> })
>
> # tests
> > rgb2col(c("#010101", "#EEEEEE", "#AA0000", "#00AA00", "#0000AA",
> "#AAAA00", "#AA00AA", "#00AAAA"))
> [1] "black" "gray93" "darkred" "green4"
> [5] "blue4" "darkgoldenrod" "darkmagenta" "cyan4"
> > rgb2col2(c("#010101", "#EEEEEE", "#AA0000", "#00AA00", "#0000AA",
> "#AAAA00", "#AA00AA", "#00AAAA"))
> [1] "#010101" "#EEEEEE" "darkred" "green4"
> [5] "blue4" "darkgoldenrod" "darkmagenta" "cyan4"
> >
>
>
> On 5/31/2013 7:42 PM, John Fox wrote:
> > Dear Kevin,
> >
> > I generally prefer your solution. I didn't realize that col2rgb()
> worked
> > with hex-colour input (as opposed to named colours), so my code
> converting
> > hex numbers to decimal is unnecessary; and using ifelse() is clearer
> than
> > replacing the non-matches.
> >
> > I'm not so sure about avoiding the closure, since for converting
> small
> > numbers of colours, your function will spend most of its time
> constructing
> > the local function find.near() and building all.hsv. Here's an
> example,
> > using your rgb2col() and a comparable function employing a closure,
> with one
> > of your examples executed 100 times:
> >
> >> r2c <- function(){
> > + all.names <- colors()
> > + all.hsv <- rgb2hsv(col2rgb(all.names))
> > + find.near <- function(x.hsv) {
> > + # return the nearest R color name and distance
> > + sq.dist <- colSums((all.hsv - x.hsv)^2)
> > + rbind(all.names[which.min(sq.dist)], min(sq.dist))
> > + }
> > + function(cols.hex, near=.25){
> > + cols.hsv <- rgb2hsv(col2rgb(cols.hex))
> > + cols.near <- apply(cols.hsv, 2, find.near)
> > + ifelse(cols.near[2,] < near^2, cols.near[1,], cols.hex)
> > + }
> > + }
> >
> >> mycols <- c("#010101", "#EEEEEE", "#AA0000", "#00AA00", "#0000AA",
> > + "#AAAA00", "#AA00AA", "#00AAAA")
> >
> >> system.time(for (i in 1:100) oldnew <- c(mycols, rgb2col(mycols,
> > near=.25)))
> > user system elapsed
> > 1.97 0.00 1.97
> >
> >> system.time({rgb2col2 <- r2c()
> > + for (i in 1:100) oldnew2 <- c(mycols, rgb2col2(mycols,
> near=.25))
> > + })
> > user system elapsed
> > 0.08 0.00 0.08
> >
> >> rbind(oldnew, oldnew2)
> > [,1] [,2] [,3] [,4] [,5] [,6]
> > oldnew "#010101" "#EEEEEE" "#AA0000" "#00AA00" "#0000AA" "#AAAA00"
> > oldnew2 "#010101" "#EEEEEE" "#AA0000" "#00AA00" "#0000AA" "#AAAA00"
> > [,7] [,8] [,9] [,10] [,11] [,12]
> > oldnew "#AA00AA" "#00AAAA" "#010101" "#EEEEEE" "darkred" "green4"
> > oldnew2 "#AA00AA" "#00AAAA" "#010101" "#EEEEEE" "darkred" "green4"
> > [,13] [,14] [,15] [,16]
> > oldnew "blue4" "darkgoldenrod" "darkmagenta" "cyan4"
> > oldnew2 "blue4" "darkgoldenrod" "darkmagenta" "cyan4"
> >
> > Does this really make a difference? Frankly, it wouldn't for my
> application
> > (for colour selection in the Rcmdr) where a user is likely to perform
> at
> > most one or two conversions of a small number of colours in a
> session. The
> > time advantage of the second approach will depend upon the number of
> times
> > the function is invoked and the number of colours converted each
> time.
> >
> > Best,
> > John
> >
> >> -----Original Message-----
> >> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> >> project.org] On Behalf Of Kevin Wright
> >> Sent: Friday, May 31, 2013 3:39 PM
> >> To: Martin Maechler
> >> Cc: r-help; John Fox
> >> Subject: Re: [R] measuring distances between colours?
> >>
> >> Thanks for the discussion. I've also wanted to be able to find
> nearest
> >> colors. I took the code and comments in this thread and simplified
> the
> >> function even further. (Personally, I think using closures results
> in
> >> Rube-Goldberg code. YMMV.) The first example below is what I use
> for
> >> 'group' colors in lattice.
> >>
> >> Kevin Wright
> >>
> >> rgb2col <- function(cols.hex, near=.25){
> >> # Given a vector of hex colors, find the nearest 'named' R colors
> >> # If no color closer than 'near' is found, return the hex color
> >> # Authors: John Fox, Martin Maechler, Kevin Wright
> >> # From r-help discussion 5.30.13
> >>
> >> find.near <- function(x.hsv) {
> >> # return the nearest R color name and distance
> >> sq.dist <- colSums((all.hsv - x.hsv)^2)
> >> rbind(all.names[which.min(sq.dist)], min(sq.dist))
> >> }
> >> all.names <- colors()
> >> all.hsv <- rgb2hsv(col2rgb(all.names))
> >> cols.hsv <- rgb2hsv(col2rgb(cols.hex))
> >> cols.near <- apply(cols.hsv, 2, find.near)
> >> ifelse(cols.near[2,] < near^2, cols.near[1,], cols.hex)
> >> }
> >>
> >> mycols <- c("royalblue", "red", "#009900", "dark orange", "#999999",
> >> "#a6761d", "#aa00da")
> >> mycols <- c("#010101", "#EEEEEE", "#AA0000", "#00AA00", "#0000AA",
> >> "#AAAA00", "#AA00AA", "#00AAAA")
> >> mycols <- c("#010101", "#090909", "#090000", "#000900", "#000009",
> >> "#090900", "#090009", "#000909")
> >> oldnew <- c(mycols, rgb2col(mycols, near=.25)) # Also try near=10
> >> pie(rep(1,2*length(mycols)), labels=oldnew, col=oldnew)
> >>
> >> [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide http://www.R-project.org/posting-
> >> guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >
>
>
> --
> 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
>
> ______________________________________________
> R-help at r-project.org mailing list
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> PLEASE do read the posting guide http://www.R-project.org/posting-
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> and provide commented, minimal, self-contained, reproducible code.
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