How to convert hex color code to hue rotation using scss mixin

Question

Please run below code snippet example, all images are same url and the original image is red. Then by using grayscale and finding hue-rotate degree from here I was able to change the color to green and blue.

Now I want to have a mixin in scss to have a hex color code as input parameter and calculate hue-rotation degree and return it. How can I have a mixin to caluclate it?

.colorRed,.colorBlue, .colorGreen {filter:grayscale(0);}
.colorRed img {filter:hue-rotate(0deg);}
.colorBlue img {filter:hue-rotate(215deg);}
.colorGreen img {filter:hue-rotate(100deg);}

<span class="original"><img src="https://i.imgur.com/ZbOHifU.png" width="100px";></span>
<span class="colorGreen"><img src="https://i.imgur.com/ZbOHifU.png" width="100px";></span>
<span class="colorBlue"><img src="https://i.imgur.com/ZbOHifU.png" width="100px";></span>
<span class="colorRed"><img src="https://i.imgur.com/ZbOHifU.png" width="100px";></span>

p.s

I DONT want to use svg solution as mentioned in here

I DONT want to use javascript solutions as well

Answer 1

It is difficult to do it using scss mixins, but using Javascript, we can get the filter based on the hex color.

This example shows how to generate filter values from hex color.

'use strict';

class Color {
  constructor(r, g, b) {
    this.set(r, g, b);
  }
  
  toString() {
    return `rgb(${Math.round(this.r)}, ${Math.round(this.g)}, ${Math.round(this.b)})`;
  }

  set(r, g, b) {
    this.r = this.clamp(r);
    this.g = this.clamp(g);
    this.b = this.clamp(b);
  }

  hueRotate(angle = 0) {
    angle = angle / 180 * Math.PI;
    const sin = Math.sin(angle);
    const cos = Math.cos(angle);

    this.multiply([
      0.213 + cos * 0.787 - sin * 0.213,
      0.715 - cos * 0.715 - sin * 0.715,
      0.072 - cos * 0.072 + sin * 0.928,
      0.213 - cos * 0.213 + sin * 0.143,
      0.715 + cos * 0.285 + sin * 0.140,
      0.072 - cos * 0.072 - sin * 0.283,
      0.213 - cos * 0.213 - sin * 0.787,
      0.715 - cos * 0.715 + sin * 0.715,
      0.072 + cos * 0.928 + sin * 0.072,
    ]);
  }

  grayscale(value = 1) {
    this.multiply([
      0.2126 + 0.7874 * (1 - value),
      0.7152 - 0.7152 * (1 - value),
      0.0722 - 0.0722 * (1 - value),
      0.2126 - 0.2126 * (1 - value),
      0.7152 + 0.2848 * (1 - value),
      0.0722 - 0.0722 * (1 - value),
      0.2126 - 0.2126 * (1 - value),
      0.7152 - 0.7152 * (1 - value),
      0.0722 + 0.9278 * (1 - value),
    ]);
  }

  sepia(value = 1) {
    this.multiply([
      0.393 + 0.607 * (1 - value),
      0.769 - 0.769 * (1 - value),
      0.189 - 0.189 * (1 - value),
      0.349 - 0.349 * (1 - value),
      0.686 + 0.314 * (1 - value),
      0.168 - 0.168 * (1 - value),
      0.272 - 0.272 * (1 - value),
      0.534 - 0.534 * (1 - value),
      0.131 + 0.869 * (1 - value),
    ]);
  }

  saturate(value = 1) {
    this.multiply([
      0.213 + 0.787 * value,
      0.715 - 0.715 * value,
      0.072 - 0.072 * value,
      0.213 - 0.213 * value,
      0.715 + 0.285 * value,
      0.072 - 0.072 * value,
      0.213 - 0.213 * value,
      0.715 - 0.715 * value,
      0.072 + 0.928 * value,
    ]);
  }

  multiply(matrix) {
    const newR = this.clamp(this.r * matrix[0] + this.g * matrix[1] + this.b * matrix[2]);
    const newG = this.clamp(this.r * matrix[3] + this.g * matrix[4] + this.b * matrix[5]);
    const newB = this.clamp(this.r * matrix[6] + this.g * matrix[7] + this.b * matrix[8]);
    this.r = newR;
    this.g = newG;
    this.b = newB;
  }

  brightness(value = 1) {
    this.linear(value);
  }
  contrast(value = 1) {
    this.linear(value, -(0.5 * value) + 0.5);
  }

  linear(slope = 1, intercept = 0) {
    this.r = this.clamp(this.r * slope + intercept * 255);
    this.g = this.clamp(this.g * slope + intercept * 255);
    this.b = this.clamp(this.b * slope + intercept * 255);
  }

  invert(value = 1) {
    this.r = this.clamp((value + this.r / 255 * (1 - 2 * value)) * 255);
    this.g = this.clamp((value + this.g / 255 * (1 - 2 * value)) * 255);
    this.b = this.clamp((value + this.b / 255 * (1 - 2 * value)) * 255);
  }

  hsl() {
    // Code taken from https://stackoverflow.com/a/9493060/2688027, licensed under CC BY-SA.
    const r = this.r / 255;
    const g = this.g / 255;
    const b = this.b / 255;
    const max = Math.max(r, g, b);
    const min = Math.min(r, g, b);
    let h, s, l = (max + min) / 2;

    if (max === min) {
      h = s = 0;
    } else {
      const d = max - min;
      s = l > 0.5 ? d / (2 - max - min) : d / (max + min);
      switch (max) {
        case r:
          h = (g - b) / d + (g < b ? 6 : 0);
          break;

        case g:
          h = (b - r) / d + 2;
          break;

        case b:
          h = (r - g) / d + 4;
          break;
      }
      h /= 6;
    }

    return {
      h: h * 100,
      s: s * 100,
      l: l * 100,
    };
  }

  clamp(value) {
    if (value > 255) {
      value = 255;
    } else if (value < 0) {
      value = 0;
    }
    return value;
  }
}

class Solver {
  constructor(target, baseColor) {
    this.target = target;
    this.targetHSL = target.hsl();
    this.reusedColor = new Color(0, 0, 0);
  }

  solve() {
    const result = this.solveNarrow(this.solveWide());
    return {
      values: result.values,
      loss: result.loss,
      filter: this.css(result.values),
    };
  }

  solveWide() {
    const A = 5;
    const c = 15;
    const a = [60, 180, 18000, 600, 1.2, 1.2];

    let best = { loss: Infinity };
    for (let i = 0; best.loss > 25 && i < 3; i++) {
      const initial = [50, 20, 3750, 50, 100, 100];
      const result = this.spsa(A, a, c, initial, 1000);
      if (result.loss < best.loss) {
        best = result;
      }
    }
    return best;
  }

  solveNarrow(wide) {
    const A = wide.loss;
    const c = 2;
    const A1 = A + 1;
    const a = [0.25 * A1, 0.25 * A1, A1, 0.25 * A1, 0.2 * A1, 0.2 * A1];
    return this.spsa(A, a, c, wide.values, 500);
  }

  spsa(A, a, c, values, iters) {
    const alpha = 1;
    const gamma = 0.16666666666666666;

    let best = null;
    let bestLoss = Infinity;
    const deltas = new Array(6);
    const highArgs = new Array(6);
    const lowArgs = new Array(6);

    for (let k = 0; k < iters; k++) {
      const ck = c / Math.pow(k + 1, gamma);
      for (let i = 0; i < 6; i++) {
        deltas[i] = Math.random() > 0.5 ? 1 : -1;
        highArgs[i] = values[i] + ck * deltas[i];
        lowArgs[i] = values[i] - ck * deltas[i];
      }

      const lossDiff = this.loss(highArgs) - this.loss(lowArgs);
      for (let i = 0; i < 6; i++) {
        const g = lossDiff / (2 * ck) * deltas[i];
        const ak = a[i] / Math.pow(A + k + 1, alpha);
        values[i] = fix(values[i] - ak * g, i);
      }

      const loss = this.loss(values);
      if (loss < bestLoss) {
        best = values.slice(0);
        bestLoss = loss;
      }
    }
    return { values: best, loss: bestLoss };

    function fix(value, idx) {
      let max = 100;
      if (idx === 2 /* saturate */) {
        max = 7500;
      } else if (idx === 4 /* brightness */ || idx === 5 /* contrast */) {
        max = 200;
      }

      if (idx === 3 /* hue-rotate */) {
        if (value > max) {
          value %= max;
        } else if (value < 0) {
          value = max + value % max;
        }
      } else if (value < 0) {
        value = 0;
      } else if (value > max) {
        value = max;
      }
      return value;
    }
  }

  loss(filters) {
    // Argument is array of percentages.
    const color = this.reusedColor;
    color.set(0, 0, 0);

    color.invert(filters[0] / 100);
    color.sepia(filters[1] / 100);
    color.saturate(filters[2] / 100);
    color.hueRotate(filters[3] * 3.6);
    color.brightness(filters[4] / 100);
    color.contrast(filters[5] / 100);

    const colorHSL = color.hsl();
    return (
      Math.abs(color.r - this.target.r) +
      Math.abs(color.g - this.target.g) +
      Math.abs(color.b - this.target.b) +
      Math.abs(colorHSL.h - this.targetHSL.h) +
      Math.abs(colorHSL.s - this.targetHSL.s) +
      Math.abs(colorHSL.l - this.targetHSL.l)
    );
  }

  css(filters) {
    function fmt(idx, multiplier = 1) {
      return Math.round(filters[idx] * multiplier);
    }
    return `filter: invert(${fmt(0)}%) sepia(${fmt(1)}%) saturate(${fmt(2)}%) hue-rotate(${fmt(3, 3.6)}deg) brightness(${fmt(4)}%) contrast(${fmt(5)}%);`;
  }
}

function hexToRgb(hex) {
  // Expand shorthand form (e.g. "03F") to full form (e.g. "0033FF")
  const shorthandRegex = /^#?([a-f\d])([a-f\d])([a-f\d])$/i;
  hex = hex.replace(shorthandRegex, (m, r, g, b) => {
    return r + r + g + g + b + b;
  });

  const result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
  return result
    ? [
      parseInt(result[1], 16),
      parseInt(result[2], 16),
      parseInt(result[3], 16),
    ]
    : null;
}

$(document).ready(() => {
  $('button.execute').click(() => {
    const rgb = hexToRgb($('input.target').val());
    if (rgb.length !== 3) {
      alert('Invalid format!');
      return;
    }

    const color = new Color(rgb[0], rgb[1], rgb[2]);
    const solver = new Solver(color);
    const result = solver.solve();

    let lossMsg;
    if (result.loss < 1) {
      lossMsg = 'This is a perfect result.';
    } else if (result.loss < 5) {
      lossMsg = 'The is close enough.';
    } else if (result.loss < 15) {
      lossMsg = 'The color is somewhat off. Consider running it again.';
    } else {
      lossMsg = 'The color is extremely off. Run it again!';
    }

    $('.realPixel').css('background-color', color.toString());
    $('.filterPixel').attr('style', result.filter);
    $('.filterDetail').text(result.filter);
    $('.lossDetail').html(`Loss: ${result.loss.toFixed(1)}. <b>${lossMsg}</b>`);
  });
});

.pixel {
  display: inline-block;
  background-color: #000;
  width: 50px;
  height: 50px;
}

<table>
  <tr valign="top">
    <td width="50%">
      <fieldset>
        <p>
          <label>Target color</label> <input class="target" type="text" placeholder="target hex" value="#00a4d6"/>
        </p>
        <button class="execute">Compute Filters</button>
      </fieldset>
      <p>Real pixel, color applied through CSS <code>background-color</code>:</p>
      <div class="pixel realPixel"></div>

      <p>Filtered pixel, color applied through CSS <code>filter</code>:</p>
      <div class="pixel filterPixel"></div>

      <p class="filterDetail"></p>
      <p class="lossDetail"></p>
    </td>
  </tr>
</table>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>