所以我正在寻找一种方法,允许我首先加载低分辨率版本(min.bg.jpg),一旦网站加载,使用高分辨率版本 – 甚至一个分辨率接近设备的分辨率使用(NNNxNNN.bg.jpg或只是bg.jpg).
背景是使用CSS设置,就像每个人都期望的一样.它适用于身体和整个声明如下所示:
- body {
- background: url("/cdn/theme/images/bg.jpg");
- color: white;
- height: 100%;
- width: 100%;
- background-repeat: no-repeat;
- background-position: 50% 50%;
- background-attachment: fixed;
- }
现在,我想改为使用min.bg.jpg代替第一次加载,然后像这样:
- jQuery(function(){
- jQuery("body").[...]
- });
我以哪种方式异步下载新的背景,然后将其作为新的CSS背景图像插入?
为了显示一些差异,这里是我正在测试的主要和迷你版本的一个例子:
- Ingwie@Ingwies-Macbook-Pro.local ~/Work/BIRD3/cdn/theme/images $file *.jpg
- bg.jpg: JPEG image data,EXIF standard
- min.bg.jpg: JPEG image data,JFIF standard 1.01
- Ingwie@Ingwies-Macbook-Pro.local ~/Work/BIRD3/cdn/theme/images $du -h *.jpg
- 1,0M bg.jpg
- 620K min.bg.jpg
解决方法
CSS:
- #div_whatever {
- position: whatever;
- background-repeat: no-repeat;
- background-position: whatever whatever;
- background-image: url(dir/image.jpg);
- /* image.jpg is a low-resolution at 30% quality. */
- }
- #img_highQuality {
- display: none;
- }
HTML:
- <img id="img_highQuality" src="dir/image.png">
- <!-- img.png is a full-resolution image. -->
- <div id="div_whatever"></div>
JQUERY:
- $("#img_highQuality").off().on("load",function() {
- $("#div_whatever").css({
- "background-image" : "url(dir/image.png)"
- });
- });
- // Side note: I usually define CSS arrays because
- // I inevitably want to go back and add another
- // property at some point.
怎么了:
>一个低分辨率版本的背景快速加载.
>同时,更高分辨率的版本被加载为隐藏的图像.
>当高分辨率图像被加载时,jQuery用高分辨率版本交换div的低分辨率图像.
PURE JS VERSION
这个例子对于将一个元素更改为多个元素将是有效的.
CSS:
- .hidden {
- display: none;
- }
- #div_whatever {
- position: whatever;
- background-repeat: no-repeat;
- background-position: whatever whatever;
- background-image: url(dir/image.jpg);
- /* image.jpg is a low-resolution at 30% quality. */
- }
HTML:
- <div id="div_whatever"></div>
- <img id="img_whatever" class="hidden" src="dir/image.png" onload="upgradeImage(this);">
JAVASCRIPT:
- function upgradeImage(object) {
- var id = object.id;
- var target = "div_" + id.substring(4);
- document.getElementById(target).style.backgroundImage = "url(" + object.src + ")";
- }
更新/增强(1/31/2017)
这个增强是gdbj的优点,我的解决方案导致图像路径在三个位置被指定.虽然我没有使用gdbj的addClass()技术,但是以下jQuery代码被修改以提取图像路径(而不是将其连接到jQuery代码中).更重要的是,该版本允许多个低分辨率高分辨率图像替换.
CSS
- .img_highres {
- display: none;
- }
- #div_whatever1 {
- width: 100px;
- height: 100px;
- background-repeat: no-repeat;
- background-position: center center;
- background-image: url(PATH_TO_LOW_RES_PHOTO_1);
- }
- #div_whatever2 {
- width: 200px;
- height: 200px;
- background-repeat: no-repeat;
- background-position: center center;
- background-image: url(PATH_TO_LOW_RES_PHOTO_2);
- }
HTML
- <div id="div_whatever1"></div>
- <img id="img_whatever1" class="img_highres" src="PATH_TO_HIGH_RES_PHOTO_1">
- <div id="div_whatever2"></div>
- <img id="img_whatever2" class="img_highres" src="PATH_TO_HIGH_RES_PHOTO_2">
JQUERY
- $(function() {
- $(".img_highres").off().on("load",function() {
- var id = $(this).attr("id");
- var highres = $(this).attr("src").toString();
- var target = "#div_" + id.substring(4);
- $(target).css("background-image","url(" + highres + ")");
- });
- });
发生什么事:
>根据他们的CSS为每个div加载低分辨率图像背景图像设置. (请注意,CSS还将div设置为预期尺寸.)>同时,分辨率较高的照片正在进行中作为隐藏图像加载(所有共享类名img_highres).>每次img_highres照片都会触发一个jQuery函数完成加载.> jQuery函数读取图像src路径,和改变相应div的背景图像.在里面在上面的例子中,命名约定是可见div的“div_ [name]”和“img_ [同名]”用于加载的高分辨率图像背景.