[R] Palettes {grDevices} - wrong number of colors returned?

[Achim Zeileis]

Duncan,

thanks for your feedback. I just received your response after sending out 
mine. You came to the same conclusion. Should I prepare a patch and send 
it to you so that you can also have a look? Or view Bugzilla?

Best wishes,
Achim

On Thu, 23 Feb 2023, Duncan Murdoch wrote:

> On 23/02/2023 4:36 a.m., Sigbert Klinke wrote:
>> Hi,
>> 
>> I would have expected that I get always 3 colors as result which is not
>> true:
>> 
>> hcl.colors(3, alpha=c(0, 0.5, 1))     # 3 colors
>> 
>> rainbow(3, alpha=c(0, 0.5, 1))        # 3 colors
>> 
>> heat.colors(3, alpha=c(0, 0.5, 1))    # 3 colors
>> 
>> terrain.colors(3, alpha=c(0, 0.5, 1)) # 6 colors
>> 
>> cm.colors(3, alpha=c(0, 0.5, 1))      # 6 colors
>> 
>> topo.colors(3, alpha=c(0, 0.5, 1))    # 9 colors
>> 
>> R-Version and platform:
>> R version 4.2.2 Patched (2022-11-10 r83330) -- "Innocent and Trusting"
>> 
>> Copyright (C) 2022 The R Foundation for Statistical Computing
>> 
>> Platform: x86_64-pc-linux-gnu (64-bit)
>> 
>> Bug or feature?
>
> Looks like a bug to me, they should all be length 3.  The reason it happens 
> in terrain.colors (I didn't look at the others) is that it breaks the range 
> into two parts, and relies on the number of values of the parameters in each 
> part to get the length, but passes the full alpha vector in:
>
> terrain.colors <- function (n, alpha, rev = FALSE)
> {
>    if ((n <- as.integer(n[1L])) > 0) {
> 	k <- n%/%2
> 	h <- c(4/12, 2/12, 0/12)
> 	s <- c(1, 1, 0)
> 	v <- c(0.65, 0.9, 0.95)
> 	cols <- c(hsv(h = seq.int(h[1L], h[2L], length.out = k),
>                      s = seq.int(s[1L], s[2L], length.out = k),
>                      v = seq.int(v[1L], v[2L], length.out = k), alpha = 
> alpha),
>                  hsv(h = seq.int(h[2L], h[3L], length.out = n - k + 1)[-1L],
>                      s = seq.int(s[2L], s[3L], length.out = n - k + 1)[-1L],
>                      v = seq.int(v[2L], v[3L], length.out = n - k + 1)[-1L],
>                      alpha = alpha))
>        if(rev) rev(cols) else cols
>    } else character()
> }
>
>
> A bug fix would be to recycle alpha to the right length and only pass in 
> portions of it, e.g.
>
> terrain.colors <- function (n, alpha, rev = FALSE)
> {
>    if ((n <- as.integer(n[1L])) > 0) {
>        alpha <- rep_len(alpha, length.out = n)
> 	k <- n%/%2
> 	h <- c(4/12, 2/12, 0/12)
> 	s <- c(1, 1, 0)
> 	v <- c(0.65, 0.9, 0.95)
> 	cols <- c(hsv(h = seq.int(h[1L], h[2L], length.out = k),
>                      s = seq.int(s[1L], s[2L], length.out = k),
>                      v = seq.int(v[1L], v[2L], length.out = k), alpha = 
> alpha[seq_len(k)]),
>                  hsv(h = seq.int(h[2L], h[3L], length.out = n - k + 1)[-1L],
>                      s = seq.int(s[2L], s[3L], length.out = n - k + 1)[-1L],
>                      v = seq.int(v[2L], v[3L], length.out = n - k + 1)[-1L],
>                      alpha = alpha[seq_len(n - k) + k]))
>        if(rev) rev(cols) else cols
>    } else character()
> }
>
> I'd guess the same sort of approach would fix cm.colors and topo.colors.
>
> Duncan Murdoch
>
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