mono.js

/**
 * ImageMagick MONO format encoder.
 *
 * https://www.imagemagick.org/script/formats.php
 * Bi-level bitmap in least-significant-byte first order
 *
 * Turning 8 bytes to 1 byte (least significant bit first):
 * 00 ff ff ff ff ff ff ff => 01111111 => 111111110
 * @param greyscaleBuffer
 * @returns {Buffer}
 */
function asMono(greyscaleBuffer) {
  const bitsInBytes = 8;
  const bytes = greyscaleBuffer.length;
  const resultSize = bytes / bitsInBytes;
  const result = new Buffer(resultSize);

  for (let i = 0; i < resultSize; i++) {
    let byte = 0x00;
    const start = i * bitsInBytes;
    const end = start + bitsInBytes - 1;

    // 8 bytes to a byte, reversed
    for (let j = end; j >= start; j--) {
      // greyscale to 0 or 1
      const bit = greyscaleBuffer[j] >= 128 ? 0x1 : 0x0;
      byte = (byte << 1) | bit;
    }
    result[i] = byte;
  }
  return result;
}