[Rd] Color mapping for R : col2rgb()

[Martin Maechler]

>>>>> "BDR" == Prof Brian Ripley <ripley@stats.ox.ac.uk> writes:

    BDR> On Mon, 2 Apr 2001, Martin Maechler wrote:
    >> 
    >> 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},
    >> when
    >> r <- col2rgb(col)
    >> r is a list() of the same length as col, and  r[[j]] = c(red_j, green_j, blue_j)
    >> e.g.
    >> > 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.

    BDR> What do you get if you ask for one colour (the most common case?)?
    BDR> A one-element list seems silly.

agreed.  Also, a matrix *is* a simpler structure anyway.
I'd plan to rather go for an  
  3 x n 
matrix (even though  [n x 3] prints more nicely)
with  dimnames = list(c("red,"green","blue"), NULL)
I'd still return a (1-row) matrix even for length(col) == 1,
for (R) API reasons, but at least that can be used as if it was a 3-vector
in almost all situations.

Martin
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