2d color gradient plot in R

Question

I want to produce a 2d color gradient rectangle like the ones in the picture below on the right hand side. How can I do this in R? Using colorRamp or RColorBrewer or other functions/packages I can produce nice 1D dolor ramps. But how do I do this for 2D including several colors in the corners, like e.g. the upper right rectangle?

What I want to get is e.g. the following two gradient types:

BTY: I completely forgot to mention that I found the above chart here (produced by Luca Fenu).

Answer 1

Try this:

 m = tcrossprod(sin(seq(0,pi,length=1e2)), cos(seq(0, 3*pi, length=1e2)))
 cols = matrix(hcl(h=scales::rescale(m, c(0, 360))), nrow(m))
 grid::grid.raster(cols)

You'll need to find which function describes the colour gradient that you want (I used sine waves for illustration).

Edit: linear interpolation between 4 corners

library(grid)
library(scales)

m = tcrossprod(seq(1,2,length=1e2), seq(2, 3, length=1e2))
pal <- gradient_n_pal(c("red","green","yellow","blue"), values = c(2, 3, 4, 6), space = "Lab")
cols = matrix(pal(m), nrow(m))
grid.raster(cols)

Edit 2: When the function is not separable, use outer,

fun_xy <- function(x, y){

  abs(y-x) * abs(y+x)

}

z <- outer(seq(-1,1,length=100), seq(-1,1,length=100), FUN = fun_xy)

cols = matrix(hcl(h=scales::rescale(z, c(0, 200))), nrow(z))
grid::grid.raster(cols)

You can also do the colour mixing directly inside the function instead of mapping values to a colour scale afterwards,

fun_xy <- function(x, y){

  R <- (x+1)/2
  G <- (1-x)/2
  B <- (y+1)/2
  A <- 1- 0.5*exp(-(x^2+y^2)/0.2)

  rgb(R, G, B, A)

}

z <- outer(seq(-1,1,length=100), seq(-1,1,length=100), FUN = fun_xy)

library(grid)
grid.newpage()
grid::grid.raster(z)