Promote mostly black pixels to K only in a CMYK TIFF

promote-to-black.js

function promoteTiffBufferToBlack(tiffBuffer) {
  const ifds = UTIF.decode(tiffBuffer);

  // tiff tag descriptions: https://www.loc.gov/preservation/digital/formats/content/tiff_tags.shtml
  // tiff image file format: http://www.fileformat.info/format/tiff/corion.htm

  const bitsPerSample = ifds[0].t258;
  const stripOffsets = ifds[0].t273[0];
  const stripByteCounts = ifds[0].t279[0];

  // loop over the image in tiffBuffer to promote mostly black pixels to K only
  for (
    let i = stripOffsets; // start at the offset
    i < stripOffsets + stripByteCounts; // keep going until we reach the end of the strip
    i = i + bitsPerSample.length // seek by the amount of bitsPerSample sizes (this is 4 or 5, depending on the number of channels in the source image.)
  ) {
    const cmyk = [
      tiffBuffer[i], // C
      tiffBuffer[i + 1], // M
      tiffBuffer[i + 2], // Y
      tiffBuffer[i + 3] // K
    ];

    // if K (black) is the dominant channel, we set the others to 0
    if (Math.max(...cmyk) === cmyk[3]) {
      tiffBuffer[i] = 0; // C
      tiffBuffer[i + 1] = 0; // M
      tiffBuffer[i + 2] = 0; // Y

      // add 5% additional black to the K channel in case we stripped the others
      // but only if it is already pretty black
      if (tiffBuffer[i + 3] > 150) {
        tiffBuffer[i + 3] = Math.min(tiffBuffer[i + 3] + 255 / 20, 255);
      }
    }
  }

  return tiffBuffer;
}