A Special Graphics Device in R: the Null Device

December 22, 2010
By

(This article was first published on Yihui Xie, and kindly contributed to R-bloggers)

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)

To leave a comment for the author, please follow the link and comment on his blog: Yihui Xie.

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