How to merge images into one file in a defined order

Question

I have around 100 images(png). Instead of doing that manually, I want to put them beside each other (12 images per line) in one single pdf in a defined order (based on file names).

Does anybody have any suggestion?

I tried according to what Thomas told me below, it paste them beside with a black margin, how can I remove that?

setwd(workingDir);
files <- list.files(path=".", pattern="*.png", all.files=T, full.names=T)
filelist <- lapply(files, readPNG)
names(filelist) <- paste0(basename((files)))
list2env(filelist, envir=.GlobalEnv)


par(mar=rep(0,4))
layout(matrix(1:length(names(filelist)), ncol=15, byrow=TRUE))

for(i in 1:length(names(filelist))) {
  img <- readPNG(names(filelist[i]))
  plot(NA,xlim=0:1,ylim=0:1,xaxt="n",yaxt="n")
  rasterImage(img,0,0,1,1)
}


dev.print(pdf, "output.pdf") 

Answer 1

Note that the solution outlined by Thomas introduces some whitespace into the multipane image not present in the source image. Adding the arguments xaxs = 'i' and yaxs='i' into plot() will remove it.

library("png") # for reading in PNGs

# example image
img <- readPNG(system.file("img", "Rlogo.png", package="png"))

# setup plot
dev.off()
par(mai=rep(0,4)) # no margins

# layout the plots into a matrix w/ 12 columns, by row
layout(matrix(1:120, ncol=12, byrow=TRUE))

# do the plotting
for(i in 1:120) {
  plot(NA,xlim=0:1,ylim=0:1,bty="n",axes=0,xaxs = 'i',yaxs='i')
  rasterImage(img,0,0,1,1)
}

# write to PDF
dev.print(pdf, "output.pdf")