fast way to read in png, add grid & coords, and output - r

Question

I have a lot of png files of a floor plan (mapping) layout that I want to:
read into R
Add grid lines
Add coordinates per cell in grid
Output
There are 1000 of these files so I'm looking for a speedy method. What would be a fast way to accomplish this task. These don't need to be publication quality as I'm looking for certain behavior clusters within cells and want recording the coordinated for these events for each of the 100 frames (pngs).
Here is a MWE that produces 10 png files:
x <- y <- seq(-4*pi, 4*pi, len = 27)
r <- sqrt(outer(x^2, y^2, "+"))
dir.create("delete_me")
wd <- getwd()
setwd("delete_me")
lapply(1:10, function(x){
png(sprintf("file_%s.png", x))
image(z = z <- cos(r^2)*exp(-r/x))
dev.off()
})
setwd(wd)
The final output will look like this for each png (with all the coords filled in).
I assume grid will be the way to create the gridlines quickly but am not sure about reading the png in quickly or plotting the coordinates (assume we'll use a 10 x 10 grid on each png).

Answer 1

How about using ggplot() and annotation_custom() to plot the image across the entire plot area, then manually overplot the grid lines.
(In the image, I trimmed the excess whitespace and axis from the png file in advance)
# pre-req libraries
require(ggplot2)
require(grid) # rasterGrob function
require(png) # to read the PNG file
width<-10
height<-10
# generate the points and labels for the grid
points<-data.frame(expand.grid(w=1:width,h=1:height))
points$labs<-paste0("(",points$w,",",points$h,")")
points$x<-points$w-0.5 # center
points$y<-points$h-0.5
# make the gridline co-ordinates
gridx<-data.frame(x=0:width,xend=0:width,y=rep(0,width+1),yend=rep(height,width+1))
gridy<-data.frame(x=rep(0,height+1),xend=rep(width,height+1),y=0:height,yend=0:height)
grids<-rbind(gridx,gridy)
# function to plot using ggplot with annotation_custom for the image
plotgrid<-function(file){
g<-ggplot(points)+theme_bw()+
annotation_custom(rasterGrob(readPNG(file),0,0,1,1,just=c("left","bottom")),0,width,0,height)+
geom_text(aes(x=x,y=y,label=labs))+
geom_segment(aes(x=x,xend=xend,y=y,yend=yend),data=grids) +
coord_cartesian(c(0,width),c(0,height))
return(g)
}
# run the function for each file in the folder
setwd("delete_me")
lapply(list.files(),function(x)plotgrid(x))
setwd(wd)