multimath/lib/unsharp_mask/unsharp_mask.js

Core to create fast image math in WebAssembly and JS.

// Unsharp mask filter
//
// http://stackoverflow.com/a/23322820/1031804
// USM(O) = O + (2 * (Amount / 100) * (O - GB))
// GB - gaussian blur.
//
// Image is converted from RGB to HSL, unsharp mask is applied to the
// lightness channel and then image is converted back to RGB.
//
'use strict';


var glur_mono16 = require('glur/mono16');
var hsl_l16     = require('./hsl_l16');


module.exports = function unsharp(img, width, height, amount, radius, threshold) {
  var r, g, b;
  var h, s, l;
  var min, max;
  var m1, m2, hShifted;
  var diff, iTimes4;

  if (amount === 0 || radius < 0.5) {
    return;
  }
  if (radius > 2.0) {
    radius = 2.0;
  }

  var lightness = hsl_l16(img, width, height);

  var blured = new Uint16Array(lightness); // copy, because blur modify src

  glur_mono16(blured, width, height, radius);

  var amountFp = (amount / 100 * 0x1000 + 0.5)|0;
  var thresholdFp = (threshold * 257)|0;

  var size = width * height;

  /* eslint-disable indent */
  for (var i = 0; i < size; i++) {
    diff = 2 * (lightness[i] - blured[i]);

    if (Math.abs(diff) >= thresholdFp) {
      iTimes4 = i * 4;
      r = img[iTimes4];
      g = img[iTimes4 + 1];
      b = img[iTimes4 + 2];

      // convert RGB to HSL
      // take RGB, 8-bit unsigned integer per each channel
      // save HSL, H and L are 16-bit unsigned integers, S is 12-bit unsigned integer
      // math is taken from here: http://www.easyrgb.com/index.php?X=MATH&H=18
      // and adopted to be integer (fixed point in fact) for sake of performance
      max = (r >= g && r >= b) ? r : (g >= r && g >= b) ? g : b; // min and max are in [0..0xff]
      min = (r <= g && r <= b) ? r : (g <= r && g <= b) ? g : b;
      l = (max + min) * 257 >> 1; // l is in [0..0xffff] that is caused by multiplication by 257

      if (min === max) {
        h = s = 0;
      } else {
        s = (l <= 0x7fff) ?
          (((max - min) * 0xfff) / (max + min))|0 :
          (((max - min) * 0xfff) / (2 * 0xff - max - min))|0; // s is in [0..0xfff]
        // h could be less 0, it will be fixed in backward conversion to RGB, |h| <= 0xffff / 6
        h = (r === max) ? (((g - b) * 0xffff) / (6 * (max - min)))|0
          : (g === max) ? 0x5555 + ((((b - r) * 0xffff) / (6 * (max - min)))|0) // 0x5555 == 0xffff / 3
          : 0xaaaa + ((((r - g) * 0xffff) / (6 * (max - min)))|0); // 0xaaaa == 0xffff * 2 / 3
      }

      // add unsharp mask mask to the lightness channel
      l += (amountFp * diff + 0x800) >> 12;
      if (l > 0xffff) {
        l = 0xffff;
      } else if (l < 0) {
        l = 0;
      }

      // convert HSL back to RGB
      // for information about math look above
      if (s === 0) {
        r = g = b = l >> 8;
      } else {
        m2 = (l <= 0x7fff) ? (l * (0x1000 + s) + 0x800) >> 12 :
          l  + (((0xffff - l) * s + 0x800) >>  12);
        m1 = 2 * l - m2 >> 8;
        m2 >>= 8;
        // save result to RGB channels
        // R channel
        hShifted = (h + 0x5555) & 0xffff; // 0x5555 == 0xffff / 3
        r = (hShifted >= 0xaaaa) ? m1 // 0xaaaa == 0xffff * 2 / 3
          : (hShifted >= 0x7fff) ?  m1 + ((m2 - m1) * 6 * (0xaaaa - hShifted) + 0x8000 >> 16)
          : (hShifted >= 0x2aaa) ? m2 // 0x2aaa == 0xffff / 6
          : m1 + ((m2 - m1) * 6 * hShifted + 0x8000 >> 16);
        // G channel
        hShifted = h & 0xffff;
        g = (hShifted >= 0xaaaa) ? m1 // 0xaaaa == 0xffff * 2 / 3
          : (hShifted >= 0x7fff) ?  m1 + ((m2 - m1) * 6 * (0xaaaa - hShifted) + 0x8000 >> 16)
          : (hShifted >= 0x2aaa) ? m2 // 0x2aaa == 0xffff / 6
          : m1 + ((m2 - m1) * 6 * hShifted + 0x8000 >> 16);
        // B channel
        hShifted = (h - 0x5555) & 0xffff;
        b = (hShifted >= 0xaaaa) ? m1 // 0xaaaa == 0xffff * 2 / 3
          : (hShifted >= 0x7fff) ?  m1 + ((m2 - m1) * 6 * (0xaaaa - hShifted) + 0x8000 >> 16)
          : (hShifted >= 0x2aaa) ? m2 // 0x2aaa == 0xffff / 6
          : m1 + ((m2 - m1) * 6 * hShifted + 0x8000 >> 16);
      }

      img[iTimes4] = r;
      img[iTimes4 + 1] = g;
      img[iTimes4 + 2] = b;
    }
  }
};