make_thumbnails.R

#' @title make_thumbnails
#' @description Convert Pages to Image Thumbnails
#' @param file A character string specifying the path or URL to a PDF file.
#' @param outdir An optional character string specifying a directory into which
#' to split the resulting files. If \code{NULL}, the \code{outdir} is
#' \code{tempdir()}. If \code{file} is a URL, both file and thumbnails are
#' stored in the R session's temporary directory.
#' @param pages An optional integer vector specifying pages to extract from.
#' @param format A character string specifying an image file format.
#' @param resolution A numeric value specifying the image resolution in DPI.
#' @param password Optionally, a character string containing a user password
#' to access a secured PDF.
#' @param copy Specifies whether the original local file(s) should be copied to
#' \code{tempdir()} before processing. \code{FALSE} by default. The argument is
#' ignored if \code{file} is URL.
#' @details This function save each (specified) page of a document as an image
#' with 720 dpi resolution. Images are saved in the same directory as the
#' original file, with file names specified by the original file name,
#' a page number, and the corresponding file format extension.
#' @note This may generate Java \dQuote{INFO} messages in the console,
#' which can be safely ignored.
#' @return A character vector of file paths.
#' @references \href{https://tabula.technology/}{Tabula}
#' @author Thomas J. Leeper <thosjleeper@gmail.com>
#' @examples
#' # simple demo file
#' f <- system.file("examples", "mtcars.pdf", package = "tabulapdf")
#'
#' make_thumbnails(f)
#' @importFrom tools file_path_sans_ext
#' @importFrom rJava J new .jfloat
#' @seealso \code{\link{extract_tables}}, \code{\link{extract_text}},
#' \code{\link{make_thumbnails}}
#' @export
make_thumbnails <- function(file,
                            outdir = NULL,
                            pages = NULL,
                            format = c("png", "jpeg", "bmp", "gif"),
                            resolution = 72,
                            password = NULL,
                            copy = FALSE) {
  file <- localize_file(file, copy = copy)
  pdfDocument <- load_doc(file, password = password, copy = copy)
  on.exit(pdfDocument$close())

  if (!is.null(pages)) {
    pages <- as.integer(pages)
  } else {
    pages <- 1L:(get_n_pages(doc = pdfDocument))
  }

  format <- match.arg(format)
  fileseq <- formatC(pages, width = max(nchar(pages)), flag = 0)
  if (is.null(outdir)) {
    outdir <- tempdir()
  }
  filename <- paste0(file_path_sans_ext(basename(file)), fileseq, ".", format)
  outfile <- normalizePath(file.path(outdir, filename), mustWork = FALSE)

  for (i in seq_along(pages)) {
    pageIndex <- pages[i] - 1L
    PDFRenderer <- new(J("org.apache.pdfbox.rendering.PDFRenderer"),
      document = pdfDocument
    )
    BufferedImage <- PDFRenderer$renderImageWithDPI(pageIndex,
      scale = .jfloat(resolution)
    )
    JavaFile <- new(J("java.io.File"), pathname = outfile[i])
    J("javax.imageio.ImageIO")$write(
      BufferedImage,
      format,
      JavaFile
    )
  }
  ifelse(file.exists(outfile), outfile, NA_character_)
}