# Re: [R] Selecting complementary colours

From: Charles C. Berry <cberry_at_tajo.ucsd.edu>
Date: Mon, 21 May 2007 17:49:21 -0700

On Mon, 21 May 2007, John Fox wrote:

> Dear Deepayan,
>
> I actually thought of the equivalent of this approach, but it doesn't do
> quite what I want.
>
> In retrospect, I didn't specify the problem clearly: What I want to be able
> to do is to place text on a background of arbitrary (but known RGB) colour
> so that the text is legible. I guess that this is better described as a
> "contrasting" than a "complementary" colour.

John,

There may be no unique solution. (For gray, for example.)

I am not sure (in terms of color theory) that maximizing in rgb space really is the right thing to do, but perhaps this will help you:

> cval <- function(x,y) -sum((x-y)^2)
> contrasting <- function(x) optim(runif(3,0,255),cval,lower=0,upper=255,method="L-BFGS-B",y=x)\$par
> do.call(rgb,as.list(contrasting(col2rgb("gray"))/255))
[1] "#000000"
> do.call(rgb,as.list(contrasting(col2rgb("gray"))/255))
[1] "#0000FF"
> do.call(rgb,as.list(contrasting(col2rgb("pink"))/255))
[1] "#00FF00"

Regards,

Chuck

>

>
> Your solution, for example breaks down for grays:
>
>> mycol <- "#888888"
>> do.call(rgb, as.list(1 - col2rgb(mycol) / 255))
> [1] "#777777"
>
> Thank you for the suggestion.
>
> John
>
> --------------------------------
> John Fox, Professor
> Department of Sociology
> McMaster University
> Hamilton, Ontario
> 905-525-9140x23604
> http://socserv.mcmaster.ca/jfox
> --------------------------------
>
>> -----Original Message-----
>> From: r-help-bounces_at_stat.math.ethz.ch
>> [mailto:r-help-bounces_at_stat.math.ethz.ch] On Behalf Of Deepayan Sarkar
>> Sent: Monday, May 21, 2007 6:45 PM
>> To: John Fox
>> Cc: r-help_at_stat.math.ethz.ch
>> Subject: Re: [R] Selecting complementary colours
>>
>> On 5/21/07, John Fox <jfox_at_mcmaster.ca> wrote:
>>> Dear r-helpers,
>>>
>>> I wonder whether, given the "#rrggbb" representation of a colour,
>>> there is a simple way to select the complementary colour,
>> also expressed as a "#rrggbb"
>>> string.
>>>
>>> Any suggestions would be appreciated.
>>
>> You want rgb2col. The following should work for any standard color
>> specification:
>>
>>> mycol = "royalblue"
>>> do.call(rgb, as.list(1 - col2rgb(mycol) / 255))
>> [1] "#BE961E"
>>
>> -Deepayan
>>
>> ______________________________________________
>> R-help_at_stat.math.ethz.ch mailing list
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>>
>
> ______________________________________________
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> and provide commented, minimal, self-contained, reproducible code.
>

```Charles C. Berry                        (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cberry_at_tajo.ucsd.edu	         UC San Diego
http://biostat.ucsd.edu/~cberry/         La Jolla, San Diego 92093-0901

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