JQuery background image fading function causes hig

问题:

I have got a problem with the background fading script. The function causes high CPU usage (30-40%, tested on Intel i7-4810MQ). The problem is present only on Chrome and Opera. With Firefox everything work fine. It's quite a big problem for me, because when my website is open the laptop starts heating and the fan is getting louder. Here is the jsfiddle code: http://jsfiddle.net/jwdu8mkq/4/ JS:

 $(document).ready(function() {
                    var background = {};
                    background.num = 3;
                    background.min = 1;
                    background.max = 6;
                    background.firstShow = true;
                    background.swap = function() {
                        var swapFirst = false;
                        var swapSecond = false;
                        if($('.background.img1').attr('image-number') == this.num) {
                            $('.background.img1').fadeOut(2000);
                            swapSecond = true;
                        } else if($('.background.img2').attr('image-number') == this.num) {
                            $('.background.img2').fadeOut(2000);

                            swapFirst = true;
                        } else {
                            swapFirst = true;
                        }

                        this.num++;

                        if(this.num < this.min) {
                            this.num = this.min;
                        } else if(this.num > this.max) {
                            this.num = this.min;
                        }

                        if(swapFirst) {
                            $('.background.img1').css('background-image', 'url(\'http://lokeshdhakar.com/projects/lightbox2/images/image-' + this.num + '.jpg\')');
                            $('.background.img1').attr('image-number', this.num);
                            $('.background.img1').fadeIn(this.firstShow ? 0 : 2000);
                            this.firstShow = false;
                        } else if(swapSecond) {
                            $('.background.img2').css('background-image', 'url(\'http://lokeshdhakar.com/projects/lightbox2/images/image-' + this.num + '.jpg\')');
                            $('.background.img2').attr('image-number', this.num);
                            $('.background.img2').fadeIn(this.firstShow ? 0 : 2000);
                            this.firstShow = false;
                        }

                    }
                    setInterval(function() { background.swap() }, 6000);
                    background.swap();
                });

CSS:

.background {
    filter: blur(10px);
    -webkit-filter: blur(10px);
    -moz-filter: blur(10px);
    -o-filter: blur(10px);
    -ms-filter: blur(10px);
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;
    position: fixed;
    top: 0px;
    left: 0px;
    width: 100%;
    height: 100%;
    z-index: -1000;
    display: none;
}

HTML:

<div class="background img1"></div>
<div class="background img2"></div>

Is there any chance to optimize this code? Or maybe can I replace it with something similar that has the same effect?

Sorry if the question is obvious, I'm definitely not a good web designer.

回答1:

You might try using CSS transitions to do most of the lifting for you. If you define 6 classes each with a background then you can swap the classes and simplify things quite a lot.

This strategy allows you to swap classes rather than divs with a cross fade. The fade effect provided by a CSS transition.

For full effect you need to view a cycle so that the images are cached. For your proper implementation, you will want to pre-load the images.

$(document).ready(function() {
    var background = {};
    background.num = 3;
    background.max = 6;
    background.container = $(".background");
    background.swap = function() {
        this.container.removeClass("background_" + this.num);
        this.num = (this.num % this.max) + 1;
        this.container.addClass("background_" + this.num);
    }

    setInterval(function() { background.swap() }, 6000);
    background.swap();
});

.background {
    height: 600px;
    width: 600px;
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;

    -webkit-filter: blur(10px);
    -moz-filter: blur(10px);
    -o-filter: blur(10px);
    -ms-filter: blur(10px);
    filter: blur(10px);
	
    transition: all 0.5s ease-in-out;
}

.background_1 { background-image: url(http://lokeshdhakar.com/projects/lightbox2/images/image-1.jpg) }
.background_2 { background-image: url(http://lokeshdhakar.com/projects/lightbox2/images/image-2.jpg) }
.background_3 { background-image: url(http://lokeshdhakar.com/projects/lightbox2/images/image-3.jpg) }
.background_4 { background-image: url(http://lokeshdhakar.com/projects/lightbox2/images/image-4.jpg) }
.background_5 { background-image: url(http://lokeshdhakar.com/projects/lightbox2/images/image-5.jpg) }
.background_6 { background-image: url(http://lokeshdhakar.com/projects/lightbox2/images/image-6.jpg) }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="background background_3"></div>

This is the CPU use I see on my system in chrome. Your code for the left half and mine for the right.

回答2:

Well, a different approach might be to use a canvas then. This is a bit more complicated but conceptually mirrors your initial attempt. We will load up an array of images then cycle them by calling crossfadeTo(). There is a bunch of stuff you can do to improve on this, but it might get you going again. As an example, the first image crossfading to itself is a little wonky.

    function loadImages(imageURLs, images) {
        imageURLs.forEach(function(item){
            var img = new Image();
            img.onload = function(){ images.push(img); };
            img.src = item;
        });
    }

    var crossfadeTo = (function(myCanvas){
        var ctx = myCanvas.getContext('2d');
        var last = myCanvas.cloneNode(true).getContext('2d');
        var next = myCanvas.cloneNode(true).getContext('2d');

        var currentImage;
        var nextImage;
        var currentAlpha = 1;

        var crossfadeTo = function(toImage){
            currentAlpha = 1;
            nextImage = toImage;

            if (!currentImage) { currentImage = nextImage; }

            crossfade();
        }

        var crossfade = function() {
            last.clearRect(0, 0, myCanvas.width, myCanvas.height);
            last.globalAlpha = currentAlpha;
            last.drawImage(currentImage, 0, 0, currentImage.width, currentImage.height, 0, 0, myCanvas.width, myCanvas.height);

            next.clearRect(0, 0, myCanvas.width, myCanvas.height);
            next.globalAlpha = 1 - currentAlpha;
            next.drawImage(nextImage, 0, 0, nextImage.width, nextImage.height, 0, 0, myCanvas.width, myCanvas.height);

            ctx.clearRect(0, 0, myCanvas.width, myCanvas.height);
            ctx.drawImage(last.canvas, 0, 0);
            ctx.drawImage(next.canvas, 0, 0);

            currentAlpha -= 0.01;

            if (currentAlpha <= 0) {
                currentImage = nextImage;
            } else {
                requestAnimationFrame(crossfade);
            }
        };

        return crossfadeTo;
    })(document.getElementById("main"), images);

    var imageURLs = [
        "http://lokeshdhakar.com/projects/lightbox2/images/image-1.jpg",
        "http://lokeshdhakar.com/projects/lightbox2/images/image-2.jpg",
        "http://lokeshdhakar.com/projects/lightbox2/images/image-3.jpg",
        "http://lokeshdhakar.com/projects/lightbox2/images/image-4.jpg",
        "http://lokeshdhakar.com/projects/lightbox2/images/image-5.jpg",
        "http://lokeshdhakar.com/projects/lightbox2/images/image-6.jpg",
    ];
    var images = [];

    loadImages(imageURLs, images);

    var currentImageIndex = 0;
    setInterval(function(){
        if (images.length === 0 ){ console.log("not enough images yet..."); return; }
        crossfadeTo(images[currentImageIndex % images.length]);
        currentImageIndex = (currentImageIndex + 1) % images.length;
    }, 6000);

<canvas id="main" width="400" height="400"></canvas>

This solution uses almost no CPU under FF, Chrome and Edge.