decode-text-from-img-rgb-channels

function decode(imageURL) {
  "use strict"
  
  function createShadowCanvas(width, height) {
    var canvas = document.createElement('canvas')
    canvas.width = width
    canvas.height = height
    
    return canvas
  }
  
  function wrapCanvas(canvas) {
    var context = canvas.getContext('2d')
    
    return {
      context: context,
      getData: function() { return context.getImageData(0, 0, canvas.width, canvas.height).data; },
      draw: function(img) { return context.drawImage(img, 0, 0); },
    }
  }

  function clearTrailingData(msgChunks) {
    // no need for "leftover" byte; utf-16 uses 2 bytes so
    // when we have a hanging byte it's for sure not being used; discard it
    if (msgChunks.length % 2 !== 0) {
      msgChunks.pop()
    }

    // Go from end and clear all 2byte pairs that are empty
    var partsToTrim = 0

    for (var i = msgChunks.length - 1; i >= 0; i -= 2) {
      if (msgChunks[i] === 0 && msgChunks[i - 1] === 0) {
        partsToTrim += 2
      } else {
        break
      }
    }

    msgChunks.splice(-partsToTrim)

    return msgChunks
  }

  
  function extractMessage(imageData) {
    var msgChunks = []
    var msg = ''

    for (var i = 0; i < imageData.length; i += 4) {
      msgChunks.push(
        imageData[i],
        imageData[i + 1],
        imageData[i + 2]
      )
    }

    msgChunks = clearTrailingData(msgChunks)

    for (var i = 0; i < msgChunks.length; i += 2) {
      msg += String.fromCharCode(
        (msgChunks[i] << 8) + msgChunks[i + 1]
      )
    }

    return msg
  }

  return new Promise(function(resolve, reject) {
    var img = new Image();
    img.crossOrigin = 'Anonymous' // enable cross origin content
    img.onload = function() {
      var wrappedCanvas = wrapCanvas(
        createShadowCanvas(img.width, img.height)
      )

      wrappedCanvas.draw(img)
      resolve(extractMessage(wrappedCanvas.getData()))
    }

    img.src = imageURL;
  })
}