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Colour to RGB value?

2 messages · Duncan Murdoch, Ben Bolker

Duncan Murdoch
# 
There are a lot of ways to specify colours in R plot functions (number
from the palette, by name, etc.).  I'd like to pass a colour from an R
function to an external function, and I'd like it to have the same
flexibility.  To avoid having to interpret all possible colour
specifications myself, I need a function to convert a general colour
specification into a standard form (e.g. r,g,b values).  

Is there such a thing somewhere? 

Duncan Murdoch

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Ben Bolker
# 
Yes, with one Unix-specific hitch: the colorname-to-code database is
hard-coded in src/main/graphics.c and not (as far as I know) accessible
from R code.  However, on Unix you can just look at /usr/lib/X11/rgb.txt
(or some such file) to get the same information.

  Except for the unwanted line break in the first line (there should be a
space after the ")" and before the "\\" at the beginning of the next line)
these are my hacks for converting a name, internal color specification
(hex string beginning with "#"), or palette number to an RGB.  Have fun.

system("tr A-Z a-z </usr/lib/X11/rgb.txt | sed -e 's/\\([a-zA-Z]\\)
\\([a-zA-Z]\\)/\\1\\2/g' | uniq >colors.tmp")
color.list <- read.table("colors.tmp",skip=1,as.is=TRUE)
names(color.list) <- c("red","green","blue","name")

colorname.to.rgb <- function(name) {
  color.list[pmatch(name,color.list[,"name"]),1:3]
}

color.to.rgb <- function(color) {
#  given color as hexadecimal, return RGB values
  hexvec <- c(0:9,"A","B","C","D","E","F")
  rgb <- numeric(3)
  names(rgb) <- c("red","blue","green")
  for (i in (0:2)) {
    h1 <- which(hexvec==substr(color,i*2+2,i*2+2))-1
    h2 <- which(hexvec==substr(color,i*2+3,i*2+3))-1
    rgb[i+1] <- h1*16+h2
  }
  rgb
}
}

mma.color <- function(col) {
  if (is.character(col))
    if (substr(col,1,1)=="#")
  rgbvec <- color.to.rgb(col)
    else
      rgbvec <- colorname.to.rgb(col)
  else
    if (is.numeric(col)) {
      pal <- palette()
      if (col>length(pal))
        col <- col %% length(pal)
      rgbvec <- colorname.to.rgb(pal[col])
    }
  paste("RGBColor[",paste(rgbvec/255,collapse=","),"]",sep="")
}
On Fri, 15 Dec 2000, Duncan Murdoch wrote: