Create image covers for PDFs

create_covers.R

#' Create cover images for talks from a folder of PDFs
#' 
#' Requires imagemagick to be installed and in the path for the convert command
#' @param pdf_dir Directory containing PDF files of papers
#' @param img_dir Directory to output all the image files
#' @param crop_height How big to make the cropped images, relative to full page
#' @param fade_height How much of the image to fade to transparent at the bottom
#' @param density resolution for PNG images
#'
create_covers = function(pdf_dir, img_dir, crop_height = .5, fade_height = .25, density = 300){
  if(!dir.exists(file.path(img_dir,'covers')))
    dir.create(file.path(img_dir,'covers'))
  dir(pdf_dir, full.names = TRUE, pattern = '*.pdf') %>%
    purrr::walk(function(fn){
      basename(fn) %>%
        tools::file_path_sans_ext() -> fn0
      img = magick::image_read_pdf(fn, pages = 1, density = density)[1] 
      # Full cover
      magick::image_write(
        image = img,
        path = glue::glue('{img_dir}/covers/{fn0}_cover.png'),
        format = "png", density = density
      )
      # Cropped
      img_height = magick::image_info(img)$height
      magick::image_crop(img, glue::glue('x{img_height*crop_height}')) %>%
        image_write(
          path = glue::glue('{img_dir}/covers/{fn0}_cover_cropped.png'),
          format = "png", density = density
        )
    })
  
  ## Add fade effect to all cropped images
  ## I like to display these with css:
  ## .img-cover {
  ##   width:90%;
  ##   border-right: solid 1px black;
  ##   border-left: solid 1px black;
  ##   border-top: solid 1px black;
  ##  }
  ## which will add black borders to the right, left, 
  ## and bottom.
  glue::glue('{img_dir}/covers') %>%
    dir(pattern = '*_cover_cropped.png', full.names = TRUE) %>% 
    purrr::walk(function(fn){
      magick::image_read(fn) %>%
        magick::image_info() %>%
        `$`('height') -> img_height
      fn0 = tools::file_path_sans_ext(fn)
      glue::glue('convert {fn0}.png -alpha set -background none -channel A -sparse-color barycentric "0,%[h] none 0,{img_height*(1-fade_height)} white" +channel {fn0}_fade.png') %>%
        system()
    })
}

## Example:
# create_covers('readings', 'img')