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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

HTML

            
              <main class="container">
  <div class="row">
    <h1 class="col-xs-12">Canvas Grayscale Sliders</h1>
  </div>
  
  <div class="row">
    <canvas id="c" width="500" height="400"></canvas>
  </div>

  <form class="row">
    <div class="form-group col-xs-4">
      <label for="red">Red: <span>100%</span></label>
      <input type="range" id="red" min="0" max="100" value="100">
    </div>
    <div class="form-group col-xs-4">
      <label for="green">Green: <span>100%</span></label>
      <input type="range" id="green" min="0" max="100" value="100">
    </div>
    <div class="form-group col-xs-4">
      <label for="blue">Blue: <span>100%</span></label>
      <input type="range" id="blue" min="0" max="100" value="100">
    </div>
  </form>
</main>
            
          
!

CSS

            
              .container {
  max-width: 500px;
}
body {
  background: #403E3E;
  color: #fff;
}
.preview {
  max-width: 100%;
}
canvas {
  border: 1px solid #000;
}
            
          
!

JS

            
              var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
var readPixels;
var writePixels;

// sliders
var redControl = document.getElementById('red');
var greenControl = document.getElementById('green');
var blueControl = document.getElementById('blue');

// create image object
var imageObj = new Image();

// set image source
// I have to load an external data URI (puppy.js) to make the Pen work
imageObj.src = img;

imageObj.onload = function() {
  // resize and center the image
  var scaleDiff;
  var xOffset = 0;
  var yOffset = 0;
  if (this.width > this.height) {
    scaleDiff = canvas.height / this.height;
    xOffset = ( (this.width*scaleDiff) - canvas.width)/2;
  } else if (this.height > this.width) {
    scaleDiff = canvas.width / this.width;
    yOffset = ( (this.height*scaleDiff) - canvas.height)/2;
  } else {
    // square, width = height
    scaleDiff = canvas.height / this.height;
  }

  // apply the scale and center info
  ctx.drawImage(this, 0-xOffset, 0-yOffset, this.width*scaleDiff, this.height*scaleDiff);

  // we need two copies
  // readPixels is a reference to the original image
  readPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
  writePixels = ctx.getImageData(0, 0, canvas.width, canvas.height);

  applyGrayscale();
}

var applyGrayscale = function() {

  // slider inputs
  var rScale = redControl.value/100;
  var gScale = greenControl.value/100;
  var bScale = blueControl.value/100;

  // var adjustment = (rAdj + gAdj + bAdj)/(rAdj + gAdj + bAdj);
  var pixelLength = readPixels.data.length;
  for (var i = 0; i < pixelLength; i++ ) {
    if (i % 4 == 0) {
      // get values
      var r = readPixels.data[i] * rScale;
      var g = readPixels.data[i + 1] * gScale;
      var b = readPixels.data[i + 2] * bScale;

      // find average
      var averageValues = (r + g + b);
      var averageScale = (rScale + gScale + bScale);
      if (averageScale < 1) {
        averageScale = 1;
      }
      var average = averageValues / averageScale;

      // put average back
      writePixels.data[i] = average; //red
      writePixels.data[i + 1] = average; //green
      writePixels.data[i + 2] = average; //blue
    }
  };

  ctx.putImageData(writePixels, 0, 0);
};

redControl.onchange = function() {
  var label = document.querySelector('label[for="red"] span');
  label.innerHTML = this.value + '%';
  applyGrayscale();
}
greenControl.onchange = function() {
  var label = document.querySelector('label[for="green"] span');
  label.innerHTML = this.value + '%';
  applyGrayscale();
}
blueControl.onchange = function() {
  var label = document.querySelector('label[for="blue"] span');
  label.innerHTML = this.value + '%';
  applyGrayscale();
}

            
          
!
999px

Console