Dark frame subtraction script for removing digital camera sensor noise from long-exposure photos

darksub.js

#!/usr/bin/env node

/*
  darksub.js

  Implements the dark frame subtraction pipeline using ImageMagick described here:
  https://www.imagemagick.org/discourse-server/viewtopic.php?p=62840&sid=c6a5c35cc2805a51e37ae5f18895b609#p62840

  It's very slow.

  This is really just a batch script; you'll need to have ImageMagick's
  command-line utility `convert` installed (try `apt install imagemagick`
  or `brew install imagemagick`).

  Usage: $ darksub.js <darkframe.jpg> <pic1.jpg...picN.jpg>

  The first argument should be the dark frame to be subtracted.
  The remaining arguments are the photos from which to remove the dark frame noise.

  Files are written to the directory named in OUTPUT_DIR.
*/

const fs = require('fs');
const path = require('path');
const execSync = require('child_process').execSync;

const BLUR_RADIUS = 20;
const OUTPUT_DIR = 'darksub-out';

if (process.argv.length < 4) {
  console.log(`Usage: ${path.basename(__filename)} <darkframe.jpg> <target0.jpg...targetN.jpg>`);
  process.exit(1);
}

files = Array.from(process.argv.slice(2));
darkFrame = files.shift();

if (!fs.existsSync(OUTPUT_DIR))
  fs.mkdirSync(OUTPUT_DIR);

for (file of files) {
  // Don't de-noise the dark frame if it's among the targets.
  if (file === darkFrame) continue;

  outfile = path.join(OUTPUT_DIR, file);
  darkProduct = path.join(OUTPUT_DIR, 'darkProduct.png');
  oneMinusDarkProduct = path.join(OUTPUT_DIR, 'oneMinusDarkProduct.png');
  corrections = path.join(OUTPUT_DIR, 'corrections.png');
  blurFile = path.join(OUTPUT_DIR, 'blurFile.png');

  process.stdout.write(`Processing ${file}....`);

  // the product of the dark frame and the starting image is now in startbydark
  execSync(`convert -compose multiply -composite ${darkFrame} ${file} ${darkProduct}`);

  // I multiply by 2.54, as the math has some weird 255=100 thing, I'm sure it's documented, but...
  execSync(`convert -evaluate Multiply 2.54 ${darkProduct} ${darkProduct}`);

  //start - dark
  // oneMinusDarkProduct = start - start*dark = start (1-dark)
  execSync(`convert -compose minus -composite ${darkProduct} ${file} ${oneMinusDarkProduct}`);

  //blurs
  // so I can easily get the local color
  execSync(`convert -gaussian-blur 20x${BLUR_RADIUS} ${file} ${blurFile}`);

  //local * dark
  // corrections = dark * local_color
  execSync(`convert -compose multiply -composite ${darkFrame} ${blurFile} ${corrections}`);

  // fixing the math again, yes I realize this should be 2.55 and not 2.54
  execSync(`convert -evaluate Multiply 2.54 ${corrections} ${corrections}`);

  // final = start - start*dark + dark*local_color = (1-dark) * start + dark * local
  execSync(`convert -compose plus -composite ${oneMinusDarkProduct} ${corrections} ${outfile}`);

  [
    darkProduct,
    oneMinusDarkProduct,
    blurFile,
    corrections,
  ].forEach(fs.unlinkSync);

  process.stdout.write('done\n');
}