根据屏幕分辨率动态更改图像

Question

I have a section with a fullscreen background image that is loaded via a JavaScript data-attribute. As this is a responsive site, the same background image (1920 x 1080px) is scaled up and down depending on the screen size, which is not very performance-friendly, eg on mobile phones with smaller screen sizes.

HTML:

<!-- BEGIN PAGE TITLE -->
<section id="page-title" class="page-title-lg" data-bg-img="nature/full-20.jpg">
  <div class="container">
    <div class="page-title-wrapper">
      <div class="page-title-txt">
        <h1>Large Page Title</h1>
      </div>
    </div>
  </div>
</section>
<!-- END PAGE TITLE -->

JS:

// Image Background 
$('[data-bg-img]').each(function() {
      var dataImg = $(this).attr('data-bg-img');
      $(this).css('background-image', 'url(assets/img/' + dataImg + ')');
 });

Is there a way to dynamically detect the window size and load the appropriate image via different data-attributes (eg 2560 x 1440, 1920 x 1440, 1280 x 800, 640 x 960, etc.) ?

Answer 1

If have three different images for say tablet, desktop, and mobile devices you could create a simple system to automatically choose the correct image to load.

<section id="page-title" class="page-title-lg" data-bg data-bg-img-desktop="nature/full-20.jpg" data-bg-img-tablet="nature/full-20.jpg" data-bg-img-mobile="nature/full-20.jpg"></section>

Notice the four attributes: data-bg data-bg-img-desktop, data-bg-img-tablet, data-bg-img-mobile.

When the page loads, get the screen width and load the image that is closest to the screen size:

<script type="text/javascript">

var imageBgs = document.querySelectorAll('[data-bg]');
var screenWidth = window.innerWidth;

for(var i=0; i<imageBgs.length; i++) {
    if( screenWidth < 768 ){
        // Load mobile image
        imageBgs[i].style.backgroundImage = 'url('+imageBgs[i].getAttribute('data-bg-img-mobile')+')';
    } else if( screenWidth >= 768 && screenWidth <= 1024 ) {
        // Ipad
        imageBgs[i].style.backgroundImage = 'url('+imageBgs[i].getAttribute('data-bg-img-tablet')+')';
    } else {
        // desktop image
        imageBgs[i].style.backgroundImage = 'url('+imageBgs[i].getAttribute('data-bg-img-desktop')+')';
    }
}

</script>

Answer 2

Is there a reason you can't change the CSS directly?

There are a few ways you could go about this. One way would be to listen to the resize event, and update all images accordingly. However, you can also use jquery to create your media queries for each element during page load, and just let the browser handle switching everything out.

Here's a fiddle illustrating how to do that: https://jsfiddle.net/czr41bqp/

Essentially you would add a style in for each screen size you want to switch at and create your style element here and append it to your header. Of course, this requires that you have pre-generated all versions of the images and that the sizes are consistent with each image.

$(function() {
  //define a template with all media queries we want to target
  var mediaQueryTemplate = '\
    @media screen and (min-width:350px) { \
        #{{ID}} { background-image: url({{BASE_URL}}/350x150) } \
    } \
    @media screen and (min-width:640px) { \
        #{{ID}} { background-image: url({{BASE_URL}}/640x960) } \
    } \
    @media screen and (min-width:1280px) { \
        #{{ID}} { background-image: url({{BASE_URL}}/1280x800) } \
    } \
    @media screen and (min-width:1280px) { \
        #{{ID}} { background-image: url({{BASE_URL}}/1280x800) } \
    } \
    @media screen and (min-width:1920px) { \
        #{{ID}} { background-image: url({{BASE_URL}}/1920x1440) } \
    } \
    @media screen and (min-width:2560px) { \
        #{{ID}} { background-image: url({{BASE_URL}}/2560x1440) } \
    } \
  ';

  var $style = $('<style>');
  $style.appendTo('head');

  $('[data-bg-img]').each(function() {
    //generate all media queries from the template for each image and add them to the style tag

    var dataImg = $(this).attr('data-bg-img');
    var mediaQueries = mediaQueryTemplate.replace(/{{ID}}/g, this.id).replace(/{{BASE_URL}}/g, dataImg);

    $style.text($style.text() + mediaQueries);
  });
});

Answer 3

One approach would be to leave the "src" attribute blank and then assign it depending on the screen width.

windowSize = $("window").width();

if(windowSize >= 1000){
$("img").attr("src","bigFile.png")
}
if(windowSize <= 999 && windowSize >= 500){
$("img").attr("src","mediumFile.png")
}
if(windowSize < 499){
$("img").attr("src","mobileFile.png")
}

Alternatively you could use something like:

if(windowSize >= 1000){
    $("imageContainer").html("<img src="bigFile">);
}