R: Bitmap output in PDF - r

Question

A lot of the time, I find it very useful to output graphics with pdf() as it allows me to scroll through pages and observe subtle differences (e.g. the page numbers may correspond to a particular parameter in a simulation).
Sometimes if the plot is quite packed with information, the fact that the PDF is a vector graphic means that it takes a long time to load in a PDF reader and is useless for scrolling through pages. I could plot with png(), but this would result in many image files.
My ideal solution would be to have a device that will plot a bitmap graphic (e.g. PNG) to a PDF.
I have read that cairo_pdf() outputs to a bitmap sometimes? Or I could write something that outputs to PNG, then combines these all together into a PDF?
Any other thoughts? Or does anyone have a solution for this already?

Answer 1

UPDATE: have now added method based on readPNG() as suggested in comments above. It's a bit slower (3s vs 9s) and seems to result in slightly larger file sizes than ImageMagick. rasterImage() interpolation makes no difference to filesize or timing, but alters the appearance slightly. If it's FALSE, then it looks the same as ImageMagick
I have just come up with the following solution using ImageMagick. It's not perfect, but it seems to work well so far.
png2pdf <- function(name=NULL,removepngs=TRUE,method="imagemagick",pnginterpolate=FALSE){
# Run the png() function with a filename of the form name%03d.png
# Then the actual plotting functions, e.g. plot(), lines() etc.
# Then dev.off()
# Then run png2pdf() and specify the name= argument if other pngs exist in the directory
# Need to incorporate a way of dealing with non-square plots
if(is.null(name)){
names <- list.files(pattern="[.]png")
name <- unique(sub("[0-9][0-9][0-9][.]png","",names))
if(length(name)!=1) stop("png2pdf() error: Check filenames")
}else{
names <- list.files(pattern=paste0(name,"[0-9][0-9][0-9][.]png"))
}
# Can change this to "convert" if it is correctly in the system path
if(method=="imagemagick"){
cmd <- c('C:\\Program Files\\ImageMagick-6.9.0-Q16\\convert.exe',names,paste0(name,".pdf"))
system2(cmd[1],cmd[-1])
}else if(method=="readPNG"){
library(png)
pdf(paste0(name,".pdf"))
par(mar=rep(0,4))
for(i in 1:length(names)){
plot(c(0,1),c(0,1),type="n")
rasterImage(readPNG(names[i]),0,0,1,1,interpolate=pnginterpolate)
}
dev.off()
}
if(removepngs) file.remove(names)
}