const
  img = document.querySelector('img'),
  canvas = document.getElementById('canvas'),
  ctx = canvas.getContext('2d')

img.onload = () => makeItBlue(img)

function makeItBlue(img) {
  // handy variable for the width and height of the image
  const {width:w, height:h} = img;
  // set the canvas to the same size as the image
  canvas.width = w
  canvas.height = h
  
  // draw the image to the canvas and grab its raw ImageData
  ctx.drawImage(img, 0, 0, w, h)
  const imgData = ctx.getImageData(0,0,w,h)
  
  // make every pixel significantly bluer
  for (const pixel of pixels(imgData)) {
      // how much space is left in the blue channel?
      const dblue = 255 - pixel.b
      // increase blue to take half of that space
      pixel.b += dblue/2
  }

  // write the updated imageData to the canvas
  ctx.putImageData(imgData,0,0)
}


/**
 * Simple generator function that makes it easy to iterate
 * over an ImageData array one pixel at a time and make
 * updates
 * 
 * NOTE: This function is designed to make modifying ImageData
 * easier, so any changes made to the yielded pixel objects
 * will be written back to the original ImageData object!
 * 
 * @generator
 * @param {ImageData} imgData
 * @yields {Pixel}  The next pixel in the image
 */
function* pixels(imgData) {
    const pixels = imgData.data
    for (let i=0; i<pixels.length/4; i++) {
        const p = i*4
        const pixel = {
            r: pixels[p],
            g: pixels[p+1],
            b: pixels[p+2],
            a: pixels[p+3]
        }
        yield pixel
        // push pixel updates back to imgData object
        pixels[p]   = pixel.r
        pixels[p+1] = pixel.g
        pixels[p+2] = pixel.b
        pixels[p+3] = pixel.a
    }
}

/**
 * @typedef {Object} Pixel
 * @description   A single pixel in an ImageData object
 * @property {Number} r   The red channel; 8-bit unsigned integer
 * @property {Number} g   The green channel; 8-bit unsigned integer
 * @property {Number} b   The blue channel; 8-bit unsigned integer
 * @property {Number} a   The alpha channel; 8-bit unsigned integer
 */