Preloading CSS Images

Question

I have a hidden contact form which is deployed clicking on a button  Its fields are set as CSS background images  and they always appears a bit later than the div that have been toggled   I was using this snippet in the  lt head gt  section  but with no luck  after I cleared the cache      lt script gt    document  ready function             pic   new Image            pic2   new Image            pic3   new Image            pic src   lt  php bloginfo  template directory      gt  images inputs input1 png           pic2 src   lt  php bloginfo  template directory      gt  images inputs input2 png           pic3 src   lt  php bloginfo  template directory      gt  images inputs input3 png        lt  script gt    I m using jQuery as my library  and it would be cool if I could use it as well for arranging this issue   Thanks for your thoughs

User · Answer

Preloading images using HTML  lt link gt  Tag  I believe most of the visitors of this question are looking for the answer of  How can I preload an image before the page s render starts   and the best solution for this problem is using  lt link gt  tag because  lt link gt  tag is capable to block the further rendering of the page  See preemptive  These two value options of rel  relationship between the current document and the linked document  attribute are most relevant with the issue    prefetch   load the given resource while page rendering preload   load the given resource before page rendering starts   So if you want to load a resource  in this case it s an image  before the rendering process of  lt body gt  tag starts  use    lt link rel  preload  as  image  href  IMAGE URL  gt    and if you want to load a resource while  lt body gt  is rendering but you are planning to use it later on dynamically and don t wanna bother the user with loading time  use    lt link rel  prefetch  href  RESOURCE URL  gt

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For preloading background images set with CSS  the most efficient answer i came up with was a modified version of some code I found that did not work       hidden   each function       var backgroundImage     this  css  background-image      if  backgroundImage     none         tempImage   new Image        tempImage src   backgroundImage            The massive benefit of this is that you don t need to update it when you bring in new background images in the future  it will find the new ones and preload them

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how about loading that background image somewhere hidden   That way it will be loaded when the page is opened and wont take any time once the form is created using ajax   body   background   ffffff url  img tree png   no-repeat -100px -100px

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If you re reusing these bg images anywhere else on your site for form inputs  you probably want to use an image sprite   That way you can centrally manage your images  instead of having pic1  pic2  pic3  etc         Sprites are generally faster for the client  since they are only requesting one  albeit slightly larger  file from the server instead of multiple files   See SO article for more benefits   CSS image sprites  Then again  this might not be helpful at all if you re just using these for one form and you really only want to load them if the user requests the contact form   might make sense though   http   www alistapart com articles sprites

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You could use this jQuery plugin waitForImage or you could put you images into an hidden div or  width 0 and height 0  and use onload event on images    If you only have like 2-3 images you can bind events and trigger them in a chain so after every image you can do some code

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The only way is to Base64 encode the image and place it inside the HTML code so that it doesn t need to contact the server to download the image    This will encode an image from url so you can copy the image file code and insert it in your page like so     body     background-image url data image png base64 iVBORw0KGgoAAAANSUhEUgAAADIA

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When there is no way to modify CSS code and preload images with CSS rules for  before or  after pseudo elements another approach with JavaScript code traversing CSS rules of loaded stylesheets can be used  In order to make it working scripts should be included after stylesheets in HTML  for example  before closing body tag or just after stylesheets   getUrls         const urlRegExp    url                                  let urls           for  let i   0  i  lt  document styleSheets length  i              let cssRules   document styleSheets i  cssRules          for  let j   0  j  lt  cssRules length  j                  let cssRule   cssRules j               if   cssRule selectorText                    continue                             for  let k   0  k  lt  cssRule style length  k                      let property   cssRule style k                       urlMatch   cssRule style property  match urlRegExp                   if  urlMatch     null                        urls push urlMatch 2                                                        return urls     preloadImages         return new Promise resolve   gt            let urls   getUrls                loadedCount   0           const onImageLoad        gt                loadedCount                if  urls length     loadedCount                    resolve                                      for  var i   0  i  lt  urls length  i                  let image   new Image                image src   urls i               image onload   onImageLoad                       document addEventListener  DOMContentLoaded        gt        preloadImages   then      gt               CSS images are loaded here

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I can confirm that my original code seems to work  I was casually sticking to an image with a wrong path   Here s a test   http   paragraphe org slidetoggletest test html   lt script gt      var pic   new Image        var pic2   new Image        var pic3   new Image        pic src  images inputs input1 png       pic2 src  images inputs input2 png       pic3 src  images inputs input3 png    lt  script gt

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http   css-tricks com snippets css css-only-image-preloading   Technique  1  Load the image on the element s regular state  only shift it away with background position  Then move the background position to display it on hover    grass   background  url images grass png  no-repeat -9999px -9999px     grass hover   background-position  bottom left      Technique  2  If the element in question already has a background-image applied and you need to change that image  the above won t work  Typically you would go for a sprite here  a combined background image  and just shift the background position  But if that isn t possible  try this  Apply the background image to another page element that is already in use  but doesn t have a background image    random-unsuspecting-element   background  url images grass png  no-repeat -9999px -9999px     grass hover   background  url images grass png  no-repeat

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Preloading images using CSS only In the below code I am randomly choosing the body element  since it is one of the only elements guaranteed to exist on the page  For the  quot trick quot  to work  we shall use the content property which comfortably allows setting multiple URLs to be loaded  but as shown  the   after pseudo element is kept hidden so the images won t be rendered  body  after     position absolute  width 0  height 0  overflow hidden  z-index -1     hide images    content url img1 png  url img2 png  url img3 gif  url img4 jpg        load images    Demo  it s better to use a sprite image to reduce http requests    if there are many relatively small sized images  and make sure the images are hosted where HTTP2 is used

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If the page elements and their background images are already in the DOM  i e  you are not creating changing them dynamically   then their background images will already be loaded  At that point  you may want to look at compression methods

User · Answer

try with this   var c new Image  Path to the background image    c onload function         render the form     With this code you preload the background image and render the form when it s loaded

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