[R] repeating colors in graph

[Barry Rowlingson]

Thomas Lumley wrote:

> See the help pages for color() and palette(), and perhaps the RColorBrewer
> package.

Splus lets you define colour palettes in its Gui, using a nice little 
notation where you specify something like "black 8 white 6 red" to get a 
17-colour palette with 8 colours between balck and white and then 6 
colours up to red (linearly interpolating RGB values). Here's a little R 
function to do something similar:

smoothColours <-
function(...){
###
### usage: smoothColours(colour,[n|colour],...)
### like smoothColours("white",10,"black") 12 colours, white to black
###
   args <- list(...)
   r <- g <- b <- NULL
   while(length(args)>0){
     if(!is.character(args[[1]])){
       stop("bad args")
     }

     if(length(args)>1){
       if(is.numeric(args[[2]])){
         ## do interpolate:
         from <- col2rgb(args[[1]])
         too <- col2rgb(args[[3]])
         ## generate args[[2]] colours between specified colours:
         n <- args[[2]]+2 # add 2 for start and finish

         ## chop off last one since it will be added on the next iteration:
         r <- c(r,seq(from[1,],too[1,],length=n))
         i <- length(r)
         r <- r[-i]
         g <- c(g,seq(from[2,],too[2,],length=n))
         g <- g[-i]
         b <- c(b,seq(from[3,],too[3,],length=n))
         b <- b[-i]
         ## cut colour and n from list and back we go
         args <- args[-(1:2)]

       }else{
         ## insert colour, chop off 1
         cc <- col2rgb(args[[1]])
         r <- c(r,cc[1,])
         g <- c(g,cc[2,])
         b <- c(b,cc[3,])
         args <- args[-1]
       }
     }else{
       ## insert colour, chop off 1
       cc <- col2rgb(args[[1]])
       r <- c(r,cc[1,])
       g <- c(g,cc[2,])
       b <- c(b,cc[3,])
       args <- args[-1]
     }
    }
   rgb(r,g,b,max=255)
  }

  You can then do something like:
 
image(matrix(runif(100),10,10),col=smoothColours("red",5,"green",6,"blue"))

  You can use as many "colour",n,"colour" things as you like, as long as 
it starts and ends with a colour name. You can even do:

  smoothColours("red","green",10,"white")

  And you can use "#abcdef" notation too.

  Of course, linearly interpolating in RGB might not be the right thing 
to do...

Baz