JSFiddle - React, Tailwind, and code Playground

HTML

<canvas id="c" width="800" height="600" style='cursor: pointer;'></canvas>

JavaScript

// Grab the Canvas and Drawing Context
var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
var _winLevel = false;
var window_level = 512
var window_width = 512;
var downWinW = null;
var downWinL = null;
var downPosX = null;
var downPosY = null;
var _IsMouseDown = false;
// Create an image element
  var img = document.createElement('IMG');

$(function(){
  
img.crossOrigin = "Anonymous";

  // When the image is loaded, draw it
  img.onload = function () {
      ctx.drawImage(img, 0, 0);
      canvas.addEventListener('mousedown', CanvasMouseDownEvent, false);
      canvas.addEventListener('mouseup', CanvasMouseUpEvent, false);
      canvas.addEventListener('mousemove', CanvasMouseMoveEvent, false);
  }


  img.src='http://i.imgur.com/KNgakND.jpg';
  //img.src='http://i.imgur.com/bIQyjlR.jpg';
});


var slope = 1;
var intercept = -1024;

function ReDrawImage(){

    if (_winLevel == true) {
    		var imageData = ctx.getImageData(0, 0, img.width, img.height);
        var data = imageData.data;
        
         var pixels = imageData.data
              , len = pixels.length;
            var a = 256 * slope / window_width
              , b = 256 * ((intercept - window_level) / window_width);
        
        for (i = 0; i < len; i += 4) {
          //pixval = a * (pixels[i] * 256 + pixels[i + 1]) + b;
          //pixels[i] = pixels[i + 1] = pixels[i + 2] = pixval

          if (pixels[i + 3] == 0)
            continue;

          var pixelIndex = i;
          var red = pixels[pixelIndex];  // red   color
          var green = pixels[pixelIndex + 1];  // green color
          var blue = pixels[pixelIndex + 2];  // blue  color
          var alpha = pixels[pixelIndex + 3];

          var pixelValue = a * (red * 256 + green + b);
          pixels[i] = pixels[i + 1] = pixels[i + 2] = pixelValue;
          pixels[i + 3] = 0xff;
          //console.log(pixelValue == pixval);

        }
        
        ctx.save();
   
        ctx.clearRect(0, 0,...