fv/lib/filters/darken_ink.js

FormVision is a node.js library for extracting data from scanned forms

// Generated by CoffeeScript 2.3.1
var dv;

dv = require('dv');

// Darken very light ink to enhance readability.
module.exports = function(image) {
  var curve, darkValueMask, deltaV, desaturatedMask, h, hsv, i, j, k, len, protectionMask, rect, ref, s, v, vOrig, x;
  // Transform to HSV.
  hsv = image.toColor().toHSV();
  // Select desatured (grayish) and dark pixels.
  desaturatedMask = hsv.inRange(0, 0, 0, 239, 0.1 * 255, 255);
  darkValueMask = hsv.inRange(0, 0, 0, 239, 255, 0.5 * 255).invert();
  // Split HSV channels.
  h = hsv.toGray(1, 0, 0);
  s = hsv.toGray(0, 1, 0);
  v = hsv.toGray(0, 0, 1);
  vOrig = new dv.Image(v);
  // Compute "fuzzy value protection mask" and apply linear spline.
  curve = (function() {
    var results = [];
    for (var i = 0; i <= 255; i++){ results.push(i); }
    return results;
  }).apply(this);
  for (x = i = 0; i <= 199; x = ++i) {
    curve[x] = 0;
  }
  for (x = j = 0; j <= 54; x = ++j) {
    curve[x + 200] = x / 54 * 220;
  }
  protectionMask = desaturatedMask.and(darkValueMask.erode(5, 5));
  v.applyCurve(curve, protectionMask);
  // Extract changes to value channel.
  deltaV = vOrig.subtract(v);
  // Filter undesired changes (blobs and noise).
  deltaV = deltaV.invert();
  ref = deltaV.threshold(254).dilate(5, 5).connectedComponents(8);
  for (k = 0, len = ref.length; k < len; k++) {
    rect = ref[k];
    if (rect.height > 100 || rect.width < 5 || rect.height < 5) {
      deltaV.clearBox(rect);
    }
  }
  deltaV = deltaV.invert();
  // Apply filtered changes to unmodified value channel.
  v = vOrig.subtract(deltaV);
  // Merge HSV channels.
  return new dv.Image(h, s, v).toRGB();
};