Canvas 擦抹模糊玻璃特效

// defining variables

// c and h are canvases, f and b are contexts

let c, h, f, b, img, mouseX = null,

    mouseY = null,

    array = [],

    startTime = 0,

    over500msElapsed = true

$("body").mousemove((e) => {

  mouseX = e.clientX

  mouseY = e.clientY

  startTime = Date.now()

  over500msElapsed || onImageLoad()

})

// handle mouse leaving window (set null) or resize (rebuild canvases)

$(window)

  .on("blur mouseout", function() {

  mouseX = mouseY = null

}).on("resize", function() {

  c && c.parentNode && c.parentNode.removeChild(c)

  setUpCanvases()

})

setUpCanvases()

function setUpCanvases() {

  const bodyWidth = $("body").width()

  const bodyHeight = $("body").height()

  c = document.createElement("canvas")

  c.width = bodyWidth

  c.height = bodyHeight

  c.style.position = "absolute"

  c.style.top = 0

  c.style.left = 0

  $("body").append(c)

  h = document.createElement("canvas")

  h.width = bodyWidth

  h.height = bodyHeight

  // set up contexts

  if (c.getContext && c.getContext("2d") && (f = c.getContext("2d"),

                                             b = h.getContext("2d"), b.lineCap = "round", b.shadowColor = "#000", !img)) {

    // loads image (never added to DOM) so that it can be added to canvas (browser only needs to download it once)

    img = new Image

    // add listener before setting source so it will definitely capture the event

    $(img).one("load", onImageLoad)

    $(img).attr("src", "https://static.canva.com/static/images/bg_tiles_color.2.jpg")

  }

}

function onImageLoad() {

  let currentTime = Date.now()

  over500msElapsed = currentTime > startTime + 500 ? false : true

  // push to start of array

  mouseX && over500msElapsed && array.unshift({

    time: currentTime,

    x: mouseX,

    y: mouseY + $("body").scrollTop()

  })

  // trims array - removes all items older than 1s

  for (let i = 0; i < array.length; i++) {

    if (currentTime - array[i].time > 1000) {

      array.length = i

    }

  }

  if (array.length > 0) {

    requestAnimationFrame(onImageLoad)

  }

  // clear canvas2 by its own width and height

  b.clearRect(0, 0, h.width, h.height)

  // loop through each item in array

  for (i = 1; i < array.length; i++) {

    const thisItem = array[i]

    const lastItem = array[i - 1]

    // fading stroke over time

    const lineOpacity = Math.max(1 - (currentTime - thisItem.time) / 1000, 0)

    b.strokeStyle = `rgba(0,0,0,${lineOpacity})`

    b.lineWidth = 80

    b.beginPath()

    b.moveTo(lastItem.x, lastItem.y)

    b.lineTo(thisItem.x, thisItem.y)

    b.stroke()

  }

  // adjusting for canvas ratio

  let imageRatio1 = c.width

  let imageRatio2 = c.width / img.naturalWidth *

    img.naturalHeight

  imageRatio2 < c.height && (imageRatio2 = c.height, imageRatio1 = c.height / img.naturalHeight * img.naturalWidth)

  // drawing the images

  // draw image onto f

  f.drawImage(img, 0, 0, imageRatio1, imageRatio2)

  // set "destination-in" temporarily: when f and h overlap, f is kept (h acts as a mask for the image)

  f.globalCompositeOperation = "destination-in"

  // add h as mask

  f.drawImage(h, 0, 0)

  // reset to normal composite operation

  f.globalCompositeOperation = "source-over"

}