A Special Graphics Device in R: the Null Device

Update on 2024-07-04

Since Mehrad discovered and mentioned this post on Mastodon, I want to point out that the null device has been officially introduced into R since v2.15.0 (2012-03-30), so you can set:

options(device = function(...) {
  pdf(NULL, ...)
})

The R_GD_nullDevice was said to be removed in the future, so you’d better not use it any more.

It is well-known that R has several graphics devices – either the screen devices (X11(), windows(), …) or the off-screen devices (pdf(), png(), …). We can query the default graphics device in options():

getOption('device')

In a non-interactive session, the default device is pdf(). This is why Sweave has to create a file named Rplots.pdf no matter if you want it or not when you run Sweave on an Rnw file which has code chunks creating plots. Such a behaviour is annoying to me – the PDF file is not only unnecessary, but also time-consuming (creating this PDF file is completely a waste of time). Is there a way to set a “null” device? (like the /dev/null for *nix users) The answer is yes, but not so obvious. I have not found the device below documented anywhere:

options(device = function(...) {
    .Call("R_GD_nullDevice", PACKAGE = "grDevices")
})

This device can speed up Sweave a lot when there are many plots to draw. Here is a comparison:

x = rnorm(1000)
system.time({
    .Call("R_GD_nullDevice", PACKAGE = "grDevices")
    replicate(500, plot(x, pch = 1:21))
    dev.off()
})
#   user  system elapsed
#   1.51    0.02    1.53
system.time({
    pdf(file.path(tempdir(), "Rplots.pdf"))
    replicate(500, plot(x, pch = 1:21))
    dev.off()
})
#   user  system elapsed
#  47.81    0.20   48.10

One thing I don’t understand in Sweave is that it evaluates the code chunk twice if its Sweave options contain fig=TRUE. I think this might be a waste of time as well, and this is why I like pgfSweave, which has both the mechanism of caching R objects (using cacheSweave) and a smart way to cache graphics (using pgf).

WARNING: this null device may not work with plots that contain (math) expressions! (take a look at demo(plotmath) in case you do not know what are expressions in R graphics)