Dynamic Background Scrolling

Well, this was fun, thanks!

I hope you don't mind me taking the liberty to use percentages to make my life a little bit easier and possibly the script slightly more robust since I can reliably use floats with percentages.

What I did is make the layout, html and css comply with the rules you need for the bg to be animated properly, they stayed largely the same from what you had.

Then it was just a question of figuring out the calculations needed with the right properties to figure out the percentage you were from the top, the *20 is actually the amount of space 'left' to fill by the background image in percentages (as the background height is 80%).

They I moved the calculations to a function so I could call that on scroll and on window resize, making sure it's initiated on any event that modifies the window somehow...

Didn't do extensive testing but it worked in Chrome and I'm tired :p

I believe this is what you are looking for:

http://jsfiddle.net/sg3s/RSqrw/15/ See edit 2

If you wanted this the other way arround just make the page background start at the top and modify that:

http://jsfiddle.net/sg3s/RSqrw/14/ See edit 2

Edit:

As a bonus, and since I had never actually written jquery script as a 'plugin', I decided to convert this into one. What I came up with should be easy to implement and use!

http://jsfiddle.net/sg3s/RSqrw/52/ See Edit 3

Functionality successfully tested in Chrome, Firefox 3.6, IE9 + compatibility mode

Edit 2:

Reading the question again checking if I did it right I noticed I didn't quite do what you want, so I updated the link in the first edit which gives you a plugin in which you can have several options for the scrolling background. It retains my 'old' interpetation while also doing what you want... Read comments in code for some extra descriptions.

Edit 3:

As I went to work today I was bothered with the fact that my plugin 'try' was a little bloated. And as you mentioned in the comment it didn't quite fit the requirements.

So I rewrote it to only do what you want and not much more, tested in Chrome Firefox, IE9 +compat etc etc.. This script is a lot cleaner.

http://jsfiddle.net/sg3s/vZxHW/

You can chose to make the background stick to the top or bottom if the height fits in the window. Nothing else, but that is already more than enough to do some pretty cool stuff :p

An exact solution: Fiddle: http://jsfiddle.net/srGHE/2/show/

View source

Thanks for the challenge. See below for the solution, which is complying with all requirements, including recommended yet optional (with steps on how to remove these) features. I only show the changed parts of your page, with an explanation after each section (CSS, HTML and JavaScript):

CSS (changes):

html,body{
    margin: 0;
    height: 100%;
    padding: 0;
}
body{
    background-color: #70d4e3;  
}
#background { /*Previously: .background*/
    /*Removed: margin-top: 45px;
      No other changes*/
}
#banner /*Previously: .banner; no other changes */
#content /*Previously: .content; no other changes */
#innerContent /*Previously: .innerContent; no other changes */

Explanation of CSS revisions:

1. margin-top:45px at the background is unnecessary, since you're absolutely positioning the element.
2. All of the elements which are unlikely to appear more than once should be selected via the id (#) selector. This selector is more specific than the class selector.

HTML (changes): All of the class attributes have been replaced by id. No other changes have been made. Don't forget to include the JQuery framework, because I've implemented your wishes using JQuery.

JavaScript (new):
Note: I have added a feature which you didn't request, but seems logical. The code will automatically reserve sufficient margin at the left side of the window in order to always display the background. Remove anything between the marked comments if you don't want this feature.

$(document).ready(function(){
    //"Static" variables
    var background = $("#background");
    var marginTop = parseFloat(background.css("margin-top")) || 0;
    var bannerWidth = $("#banner").width(); /*Part of auto left-margin */
    var extraContWidth = (bannerWidth - $("#content").width())/2; /*Same as above*/

    function fixBG(){
        var bodyWidth = $("body").width();
        var body_bg_width_ratio = bodyWidth/1920;
        var bgHeight = body_bg_width_ratio * 926; //Calcs the visible height of BG
        
        var height = $(document).height();
        var docHeight = $(window).height();
        var difHeight = bgHeight - docHeight;
        var scrollDif = $(document).scrollTop() / (height - docHeight) || 0;
        
        /*Start of automatic left-margin*/
        var arrowWidth = body_bg_width_ratio * 115; //Arrow width
        if(bodyWidth - bannerWidth > arrowWidth*2){
            $("body > div").css("margin-left", "auto");
        } else {
            $("body > #banner").css("margin-left", arrowWidth+"px");
            $("body > #content").css("margin-left", (arrowWidth+extraContWidth)+"px");
        }
        /*End of automatic left-margin*/
        
        if(difHeight > 0){
            background.css({top:(-scrollDif*difHeight-marginTop)+"px", bottom:""});
        } else {
            background.css({top:"", bottom:"0"});
        }
    }
    $(window).resize(fixBG);
    $(window).scroll(fixBG);
    fixBG();
});

Explanation of the JavaScript code
The size of the background is determined by calculating the ratio of the background and document width. The width property is used, because it's the most reliable method for the calculation.

Then, the height of the viewport, document body and background is calculated. If applicable, the scrolling offset is also calculated, to prepare the movement of the background, if necessary.

Optionally, the code determines whether it's necessary to adjust the left margin (to keep the background visible at a narrow window).

Finally, if the background arrow has a greater height than the document's body, the background is moved accordingly, taking the scrolling position into account. The arrow starts at the top of the document, and will move up as the user scrolls (so that the bottom side of the arrow will be the bottom of the page when the user has fully scrolled down). If it's unnecessary to move the background, because it already suits well, the background will be positioned at the bottom of the page.

When the page has finished loading, this functionality is added to the Resize and scroll events, so that the background is always at the right location.

If you've got any other questions, feel free to ask them.