R/pdf_to_png.R

In skgrange/systemr: Tools to Return Structured System Calls

#' Function to convert a pdf file to a png file, usually used for single page 
#' pdf images. 
#' 
#' @author Stuart K. Grange
#' 
#' @param file Vector of file names. 
#' 
#' @param file_output Optional vector of output file names. 
#' 
#' @param resolution Resolution of png file in DPI. 
#' 
#' @param verbose Should the function give messages?
#' 
#' @return Invisible. 
#' 
#' @export
pdf_to_png <- function(file, file_output = NA, resolution = 320, verbose = FALSE) {
  
  # Check if system programme is installed
  if (!pdftoppm_install_check()) {
    stop("`pdftoppm` system programme is not installed...", call. = FALSE)
  }
  
  # Do
  purrr::walk2(
    file, 
    file_output, 
    pdf_to_png_worker, 
    resolution = resolution, 
    verbose = verbose
  )
  
}
  

pdf_to_png_worker <- function(file, file_output, resolution, verbose) {
  
  # Ensure path is expanded 
  file <- fs::path_expand(file)
  
  # Make file name if not given
  if (is.na(file_output))  {
    file_output <- file %>% 
      fs::path() %>% 
      fs::path_ext_remove() %>% 
      stringr::str_c(".png")
  } else {
    file_output <- fs::path_expand(file_output)
  }
  
  # Use redirect here
  command <- stringr::str_c(
    "pdftoppm -rx ", 
    resolution, " -ry ", resolution, " -png ",
    file,
    " > ", 
    file_output
  )
  
  # Print
  if (verbose) message(threadr::date_message(), "`", file, "`...")
  
  # Do
  system(command)
  
  return(invisible(file))
  
}


pdftoppm_install_check <- function() {
  
  # System call
  suppressWarnings(
    x <- system("which pdftoppm", intern = TRUE)
  )
  
  # Test
  if (length(x) == 0) {
    x <- FALSE
  } else if (grepl("pdftoppm", x, ignore.case = TRUE)) {
    x <- TRUE
  } else {
    x <- FALSE
  }
  
  return(x)
  
}