type Option = {
  extension: 'ppm' | 'pbm' | 'pgm' | 'png' | 'jpeg' | 'tiff';
  verbose?: boolean;
};
export async function convertPDFtoImage(pdfFileName: string, outputName: string, {extension, verbose}: Option = {extension: 'png'}) {
  if (verbose) console.log(`Converting ${pdfFileName} to ${extension} images...`);
  
  let flag: string | undefined;
  switch (extension) {
    case 'ppm':
      flag = undefined;
      break;
    case 'pbm':
      flag = 'mono';
      break;
    case 'pgm':
      flag = 'gray';
      break;
    default:
      flag = extension;
      break;
  }
     
  await exec(`pdftoppm${flag ? ` -${flag}` : ''} ${pdfFileName} ${outputName} -r ${dpi}`);
  if (verbose) console.log(`Finish.`);
}