if [[ ! -f ${tmp_png_file} ]] ; then
    echo "pdftoppm does not work."
    exit 1
  fi
  convert -thumbnail ${image_size} ${tmp_png_file} "${output_file}"
  rm -f ${tmp_png_prefix}*png
elif [[ x"${cmd_type}" = "xps" ]] ; then
  check_command convert
  check_command pdftoppm
  check_command ps2pdf
  target_file=$(echo "$url" | sed -e "s#^file:/*#/#g")
  tmp_pdf_file=/tmp/thumbnail.pdf.$$
  ps2pdf "${target_file}" ${tmp_pdf_file}