(This article was first published on Dan Kelley Blog/R, and kindly contributed to Rbloggers)
Introduction
Over the past month or so I have been trying different ways of handling GMTstyle colormaps in Oce. I think my present solution is on the right track, but I am posting here to get more eyes on the problem.
Note that the function called Colormap()
here will be called colormap()
if I decided to incorporate it into Oce. That will mean that it replaces an old function of the same name. Also, as part of the change, the old function colorize()
will disappear. (To call them “old” is a stretch. They are about a month old and were marked as “Alpha” code from the start.)
Procedure
The following code is direct from the help for Colormap()
; all I’ve done is to put the example code into Rmarkdown to make for easier comparision with the resultant graphs.

library(oce)

## Loading required package: methods
## Loading required package: mapproj
## Loading required package: maps
## Loading required package: ncdf4
## Loading required package: tiff

## Example 1. color scheme for points on xy plot
x < seq(0, 1, length.out = 40)
y < sin(2 * pi * x)
par(mar = c(3, 3, 1, 1))
mar < par("mar") # prevent margin creep by drawPalette()
## First, default breaks
c < Colormap(y)
drawPalette(c$zlim, col = c$col, breaks = c$breaks)
plot(x, y, bg = c$zcol, pch = 21, cex = 1)
grid()


par(mar = mar)
## Second, 100 breaks, yielding a smoother palette
c < Colormap(y, breaks = 100)
drawPalette(c$zlim, col = c$col, breaks = c$breaks)
plot(x, y, bg = c$zcol, pch = 21, cex = 1)
grid()


par(mar = mar)
## Example 2. topographic image with a standard color scheme
par(mfrow = c(1, 1))
data(topoWorld)
cm < Colormap(name = "gmt_globe")
imagep(topoWorld, breaks = cm$breaks, col = cm$col)


## Example 3. topographic image with modified colors
cm < Colormap(name = "gmt_globe")
deep < cm$x0 < 4000
cm$col0[deep] < "black"
cm$col1[deep] < "black"
cm < Colormap(x0 = cm$x0, x1 = cm$x1, col0 = cm$col0, col1 = cm$col1)
imagep(topoWorld, breaks = cm$breaks, col = cm$col)


## Example 4. image of world topography with water colorized smoothly from
## violet at 8km depth to blue at 4km depth, then blending in 0.5km
## increments to white at the coast, with tan for land.
cm < Colormap(x0 = c(8000, 4000, 0, 100), x1 = c(8000, 4000, 0, 100), col0 = c("violet",
"blue", "white", "tan"), col1 = c("violet", "blue", "white", "tan"), n = c(100,
8, 1))
lon < topoWorld[["longitude"]]
lat < topoWorld[["latitude"]]
z < topoWorld[["z"]]
imagep(lon, lat, z, breaks = cm$breaks, col = cm$col)
contour(lon, lat, z, levels = 0, add = TRUE)

Resources
 Source code: 20140430colormap.R
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