Why is image preloading ineffective?

Question

My CMS project has a stylish web 2.0 login screen that fades over the screen using javascript. How come that even though I have made 120% sure that images are preloaded (I used the resource monitor in development tools) they still take a second to show up when my login screen appears. It completely destroys the fanciness! Take a look:

http://www.dahwan.info/Melior

When you click login, the screen is supposed to fade to dark using a 75% alpha 1px png image. Even though the image is preloaded, it doesn't show up until after the animation is done. However, if you click cancel and log in again, the animation flows smoothly and perfectly.

Can anyone think of a solution to this problem? I'm having it with the rest of my CMS' GUI as well. It's like there was no image preloading what so ever.

Thanks for answers

EDIT: Ah yes, I'm currently developing this CMS for Google Chrome 5.0.375.99, adding multi browser compatibility later. Sorry for leaving that out

Answer 1

The UI thread can only manage one task at a time -- it will either execute javascript or update the UI. If there is a lot of javascript parsing/executing before the code that preloads the image, that might be causing the delay.

Another suggestion is to disable clickability on the login link until after the image has been detected on the page.

To do so is fairly straightforward:
function disableBtn(el){
var btn = document.getElementById(el);
var btnId = btn.id;
btn.disabled = true;
}

To re-enable, set btn.disabled = false (after detecting that your image has been added to the DOM).

Answer 2

I have come up with a workaround to my problem. I have now tested this in three browsers on two different computers with perfect results. In short, in my image preloading function in javascript, i added each of the images into the DOM in an invisible Div tag.

$('#img_preload').append($('<img />').attr('src', img.src));

The images now being added to the Dom at page load, and according to my theory resting in the memory of my low-end computers, they appears instantly when my CMS needs them.

Thanks for your comments :)

Answer 3

You could also just preload them into an array. Your problem might be caused by what is known as "garbage collection". This is where the browser looks objects for that are consuming memory who no longer have an instance on the screen, and are not being referenced by anything else in memory.

If you preload images into your web age, they should be loaded into the cache, though. So, they should still re-appear when referenced again. However, images can also disappear if the cache expiration is not set long enough for these types of files.

Your problem could be browser-specific.... I have found that it is always best to create an "anchor" for pre-loaded content by placing them into an image array and then using the array to call up the images when needed instead of the image URL(URI).

Here is a quick-and-dirty article that covers this topic.

http://articles.techrepublic.com.com/5100-10878_11-5214317.html

Answer 4

A useful information about this problem:

The Codemonkey solution works because, by putting the images in a hidden div, the browser have to keep those images in memory and ready for a possible change of div's visibility. If the user needs to change de visibility of div from hidden to block, it has to be done instantly. This is why just load all images into an array doesn't work properly.