<script> class FogParticle {

   constructor(ctx, canvasWidth, canvasHeight) {
       this.ctx = ctx
       this.canvasWidth = canvasWidth
       this.canvasHeight = canvasHeight
       this.x = 0
       this.y = 0
   }

   setPosition(x, y) {
       this.x = x
       this.y = y
   }

   setVelocity(x, y) {
       this.xVelocity = x
       this.yVelocity = y
   }

   setImage(image) {
       this.image = image
   }

   render() {
       if (!this.image) return

       this.ctx.drawImage(
           this.image,
           this.x - this.image.width / 2,
           this.y - this.image.height / 2,
           400,
           400
       )

       this.x += this.xVelocity
       this.y += this.yVelocity

       // Check if has crossed the right edge
       if (this.x >= this.canvasWidth) {
           this.xVelocity = -this.xVelocity
           this.x = this.canvasWidth
       }
       // Check if has crossed the left edge
       else if (this.x <= 0) {
           this.xVelocity = -this.xVelocity
           this.x = 0
       }

       // Check if has crossed the bottom edge
       if (this.y >= this.canvasHeight) {
           this.yVelocity = -this.yVelocity
           this.y = this.canvasHeight
       }
       // Check if has crossed the top edge
       else if (this.y <= 0) {
           this.yVelocity = -this.yVelocity
           this.y = 0
       }
   }

}

class Fog {

   constructor({ selector, density = 50, velocity = 2, particle, bgi } = {}) {
       const canvas = document.querySelector(selector)
       const bcr = canvas.parentElement.getBoundingClientRect()
       this.ctx = canvas.getContext('2d')
       this.canvasWidth = canvas.width = bcr.width
       this.canvasHeight = canvas.height = bcr.height

       this.particleCount = density
       this.maxVelocity = velocity
       this.particle = particle
       this.bgi = bgi

       this._createParticles()
       this._setImage()

       if (!this.bgi) return

       const img = new Image()
       img.onload = () => {
           const size = coverImg(img, this.canvasWidth, this.canvasHeight)
           this.bgi = { img, w: size.w, h: size.h }
           this._render()
       }
       img.src = this.bgi
   }

   _createParticles() {
       this.particles = []

       const random = (min, max) => Math.random() * (max - min) + min

       for (let i = 0; i < this.particleCount; i++) {
           const particle = new FogParticle(this.ctx, this.canvasWidth, this.canvasHeight)

           particle.setPosition(
               random(0, this.canvasWidth),
               random(0, this.canvasHeight)
           )
           particle.setVelocity(
               random(-this.maxVelocity, this.maxVelocity),
               random(-this.maxVelocity, this.maxVelocity)
           )

           this.particles.push(particle)
       }
   }

   _setImage() {
       if (!this.particle) return

       const img = new Image()
       img.onload = () => this.particles.forEach(p => p.setImage(img))
       img.src = this.particle
   }

   _render() {
       if (this.bgi) {
           this.ctx.drawImage(this.bgi.img, 0, 0, this.bgi.w, this.bgi.h)
       } else {
           this.ctx.fillStyle = "rgba(0, 0, 0, 1)"
           this.ctx.fillRect(0, 0, this.canvasWidth, this.canvasHeight)
       }

       this.particles.forEach(p => p.render())

       requestAnimationFrame(this._render.bind(this))
   }

}

class Eraser {

   constructor({ bgCanvas, brushCanvas, bgi, radius = 120 } = {}) {
       bgCanvas = this.bgCanvas = document.querySelector(bgCanvas)
       this.brushCanvas = document.querySelector(brushCanvas)
       this.bgCtx = this.bgCanvas.getContext('2d')
       this.brushCtx = this.brushCanvas.getContext('2d')

       this.parentElement = this.bgCanvas.parentElement
       const bcr = this.parentElement.getBoundingClientRect()
       this.canvasWidth = this.bgCanvas.width = this.brushCanvas.width = bcr.width
       this.canvasHeight = this.bgCanvas.height = this.brushCanvas.height = bcr.height

       this.brushRadius = radius

       this.bgi = new Image()
       this.bgi.onload = this._attachEvents.bind(this)
       this.bgi.src = bgi

       this.utils = {
           distanceBetween(point1, point2) {
               return Math.sqrt(Math.pow(point2.x - point1.x, 2) + Math.pow(point2.y - point1.y, 2))
           },
           angleBetween(point1, point2) {
               return Math.atan2(point2.x - point1.x, point2.y - point1.y)
           },
           getMousePos(e) {
               const bcr = bgCanvas.getBoundingClientRect()
               return {
                   x: e.clientX - bcr.left,
                   y: e.clientY - bcr.top
               }
           }
       }
   }

   _attachEvents() {
       this.parentElement.addEventListener('mousemove', this._onMouseMove.bind(this))
       this.parentElement.addEventListener('mouseleave', this._onMouseLeave.bind(this))
   }

   _onMouseMove(e) {
       const currentPoint = this.utils.getMousePos(e)
       this.lastPoint = this.lastPoint || currentPoint

       const dist = this.utils.distanceBetween(this.lastPoint, currentPoint)
       const angle = this.utils.angleBetween(this.lastPoint, currentPoint)

       for (let ii = 0; ii < dist; ii += 5) {
           const x = this.lastPoint.x + (Math.sin(angle) * ii)
           const y = this.lastPoint.y + (Math.cos(angle) * ii)

           const brush = this.brushCtx.createRadialGradient(x, y, 0, x, y, this.brushRadius)
           brush.addColorStop(0, 'rgba(0, 0, 0, 1)')
           brush.addColorStop(.3, 'rgba(0, 0, 0, .1)')
           brush.addColorStop(1, 'rgba(0, 0, 0, 0)')

           this.brushCtx.fillStyle = brush
           this.brushCtx.fillRect(
               x - this.brushRadius,
               y - this.brushRadius,
               this.brushRadius * 2,
               this.brushRadius * 2
           )
       }

       this.lastPoint = currentPoint

       this.bgCtx.globalCompositeOperation = 'source-over'
       const size = coverImg(this.bgi, this.canvasWidth, this.canvasHeight)
       this.bgCtx.drawImage(this.bgi, 0, 0, size.w, size.h)
       this.bgCtx.globalCompositeOperation = 'destination-in'
       this.bgCtx.drawImage(this.brushCanvas, 0, 0)
   }

   _onMouseLeave() {
       this.lastPoint = null
   }

}

const coverImg = (img, width, height) => {

   const ratio = img.width / img.height
   let w = width
   let h = w / ratio
   if (h < height) {
       h = height
       w = h * ratio
   }
   return { w, h }

}

// const bgi = 'http://saijogeorge.com/test/assets/images/mybackground.png' const bgi = 'http://www.wallpaperup.com/uploads/wallpapers/2013/02/20/42433/big_thumb_a4789e7543f7b7467e6b1efcba41eaa0.jpg'

new Fog({

   selector: '#fog',
   particle: 'http://asista.pl/f/fog-particle.png',
   density: 80,
   bgi,

})

</script>

   <canvas id="fog">
   <canvas id="bg">
   <canvas id="brush">