[Rd] Color mapping for R : col2rgb()

Martin Maechler Martin Maechler <maechler@stat.math.ethz.ch>
Mon, 2 Apr 2001 12:20:10 +0200

This has been an old topic, brought up last year by 
Jens Oehlschlaegel and even before by Ben Bolker and maybe others.

Over the weekend I've finally taken some time, 
and have implemented one new function on the R level,
basically exposing the underlying C function str2col() [main/graphics.c]

col2rgb() accepts a vector of all three kind of R colors :

1:n     =  indices into palette()
name    =  an element of colors()
#rrggbb =  character with the rgb hex code.

Per color, it returns an integer(3) vector   c(red,green,blue)
with elements from 0:255.

Currently {in my implementation},
  r <- col2rgb(col) 
r is a list() of the same length as col, and  r[[j]] = c(red_j, green_j, blue_j)
    > str(col2rgb(paste("gold", 1:4, sep="")))
    List of 4
     $ : int [1:3] 255 215 0
     $ : int [1:3] 238 201 0
     $ : int [1:3] 205 173 0
     $ : int [1:3] 139 117 0

This has been the easiest way of a vectorizing implementation.
Are the strong reasons why, e.g. a matrix should be returned instead ?
The matrix can `easily' be built by, e.g.

  matrix(c(col2rgb(...) , recursive = TRUE), nrow = 3))

If it is felt that one always wanted this matrix (and dimnames, too),
it would maybe make sense to compute it in C.



Martin Maechler <maechler@stat.math.ethz.ch>	http://stat.ethz.ch/~maechler/
Seminar fuer Statistik, ETH-Zentrum  LEO D10	Leonhardstr. 27
ETH (Federal Inst. Technology)	8092 Zurich	SWITZERLAND
phone: x-41-1-632-3408		fax: ...-1228			<><
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