[R] measuring distances between colours?

[Ken Knoblauch]

Hi John,

Out of curiosity and if it is not much trouble, I would be curious if Luv worked any better than Lab. I think that Luv is supposed to be preferred for monitors and Lab for surfaces but they are generally pretty similar. 

Best,

Ken

Sent from my iPhone

___
Ken Knoblauch
Inserm U846
Stem-Cell and Brain Research Institute
18 av du Doyen Lépine
69500 Bron France
Tel : 04 72 91 34 77
Fax :  04 72 91 34 61
portable : 06 84 10 64 10
http://www.sbri.fr/members/kenneth-knoblauch.html

On 1 juin 2013, at 17:31, "John Fox" <jfox at mcmaster.ca> wrote:

> Hi Michael,
> 
> Thanks for the Wikipedia tip -- I'd looked there but didn't find this
> article. The article explains that the Lab colour space was formulated to
> provide uniform perceptual differences between colours, with a JND of
> approximately of 2.3. Ken Knoblauch made a similar point. The article goes
> on to describe relatively complicated adjustments meant to improve the LAV
> distance metric, which are probably overkill for my application.
> 
> I've programmed Lab colour matching as follows, using Euclidean distances
> and adapting Kevin Wright's modification of my original code. I used
> convertColor(), which Martin Maechler pointed out to me.
> 
> ----------- snip --------------
> 
> r2c <- function(){
>    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(cols.near[2, ] < near^2, cols.near[1, ], cols.hex)
>    }
> }
> 
> rgb2col <- r2c()
> 
> ----------- snip --------------
> 
> A bit of experimentation suggests that this works better than using (as I
> did previously) direct RGB distances, matching more colours to names and
> providing (to my eye, with my monitor) perceptually closer matches, though
> sometimes with (again to my eye) perceptible differences. Here's an
> illustration, adapting one of Kevin's examples:
> 
> ----------- snip --------------
> 
> cols <- c("#010101", "#EEEEEE", "#AA0000", "#00AA00", "#0000AA", "#AAAA00",
> "#AA00AA", "#00AAAA")
> (nms <- rgb2col(cols))
> pie(rep(1, 2*length(cols)), labels=c(cols, nms), col=c(cols, nms))
> 
> ----------- snip --------------
> 
> Thanks again to everyone who responded to my original, naïve, question.
> 
> Best,
> John
> 
>> -----Original Message-----
>> From: Michael Friendly [mailto:friendly at yorku.ca]
>> Sent: Friday, May 31, 2013 10:24 AM
>> To: John Fox
>> Cc: r-help at r-project.org; 'Martin Maechler'
>> Subject: Re: measuring distances between colours?
>> 
>> 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
>