Generate images

Let me first generate some data in the following format: Each row in a tibble has the title of the image and the x and y data to be plotted. I am deliberately using base R plotting – I find this much more convenient than transforming my data into a format that {ggplot2} can handle.

tbl <- tibble::tibble(
    x = list(runif(10), runif(8)),
    y = list(runif(10), runif(8)),
    title = c("a", "b")
)

I then define a function for plotting a row of data:

my_plot <- function(x, y, title) {
    plot(x = x, y = y, main = title)
}

I also need a function for saving the plots. This function is where there are potential speed-ups. My first version saves each plot and closes the connection:

save_plots <- function(x, y, title, save_dir) {
    png(filename = fs::path(save_dir, title, ext = "png"))
    my_plot(x, y, title)
    dev.off()
}

The plots can now be generated by sweeping through the rows of tbl:

save_dir <- fs::path_temp("my_images")
fs::dir_create(save_dir)
purrr::pwalk(tbl, save_plots, save_dir = save_dir)

When tbl is large this is very time consuming.

Faster image generation

The png function has a trick: We can specify a “family” of filenames (just as the default filename argument of png) and only call dev.off once after all images are saved.

png(filename = fs::path(save_dir, "image%03d.png"))
purrr::pwalk(tbl, my_plot)
dev.off()

The sprintf format “%03d” means that each image is numbered sequentially with 3 digits that are left-padded with zeros if needed (this ensures that the lexicographical ordering aligns with the index ordering). We can see this in the generated images

list.files(save_dir)

## [1] "a.png"        "b.png"        "image001.png" "image002.png"