Question

I’m using JRI to generate ggplot2 plots from Java. Currently I have to write plots to disk. How do I do this without going through files, i.e. just rendering the plots in memory?

I tried using the Cairo package to plot to a textConnection, but that doesn’t work without the “R Connections Patch,” which after some Googling turns out to be ancient history.

Answer 1

Mostly from https://stat.ethz.ch/pipermail/r-devel/2010-August/058253.html.

library(Cairo)
library(png)
library(ggplot2)

Cairo(file='/dev/null')

qplot(rnorm(5000)) # your plot

# hidden stuff in Cairo
i = Cairo:::.image(dev.cur())
r = Cairo:::.ptr.to.raw(i$ref, 0, i$width * i$height * 4)
dim(r) = c(4, i$width, i$height) # RGBA planes
# have to swap the red & blue components for some reason
r[c(1,3),,] = r[c(3,1),,]
# now use the png library
p = writePNG(r, raw()) # raw PNG bytes

[Update: JRI can handle raws, you just need to use the REngine abstractions and not the JRI ones.]