Improving Performance on Background Parallax Scrolling

Question

Hello StackOverflow Community,

what I am trying to achieve is a header that can be moved with the mouse. You klick into the header and drag the mouse and the elements inside the header will move with different speeds.

I achieved the parallaxing part but the performance is not really good. It is partially a bit laggy while dragging the backgrounds.

My question now is: what can be changed in the code to get a performance boost?

That's the part of the code that takes care of parallaxing. On every mousemove a each loop is executed which I think is the reason for the performance beeing so laggy:

var dragging = false;
var clickMouseX;

//Our object for the layers
//each layer has a different scrolling speed
var movingObjects = {
    '#header-l1' : {'speed': 1},
    '#header-l2' : {'speed': 1.4},
    '#header-l3' : {'speed': 1.85},
    '#header-l4' : {'speed': 2.2},
};

$('#header-wrapper').mousedown(function(e){
    dragging = true;

    //Get initial mouse position when clicked
    clickMouseX = e.pageX;

        $(this).mousemove(function(mme){
            //execute only if mousedown
            if(dragging){
                //iterate through all layers which have to be parallaxed
                $.each(movingObjects, function(el, opt){
                    var element = $(el);
                    //get difference of initial mouse position and current mouse position
                    var diff = clickMouseX - mme.pageX;
                    //scroll-position left speed 1
                    if(diff < 0) diff = -1;
                    //scroll position right speed 1
                    if(diff >= 0) diff = 1;
                    //get current position of layer
                    currLeft = parseInt(element.css('left'));
                    //get current layer width
                    elWidth = element.width();

                    //if right border is reached don't scroll further
                    if(currLeft < -(elWidth - 810)){
                        element.css('left', -(elWidth - 810));
                    }           
                    //so do with left border
                    if(currLeft > 0){
                        element.css('left', 0);
                    }
                    //parallax it! Subtract the scroll position speed multiplied by the speed of the desired
                    //layer from the current left property
                    element.css('left', parseInt(element.css('left')) - diff*opt.speed);    
                });
            }
        });


    /* Cursor */
    $(this).css('cursor', 'pointer');
    return false;
});

I also put a fiddle up: http://jsfiddle.net/yWGDz/

Thanks in advance, Thomas

P.S. maybe someone even finds out why layer two and three have the same scroll speed while having different speeds defined.

Answer 1

I worked at this a bit, and came up with this: http://jsfiddle.net/amqER/2/

This works a lot faster than the original (especially in firefox, where it performs a whole lot better, chrome it's still pretty slow). I also changed up some of the logic in your code, to make it make more sense.

A list of things that I did:

Minify your pngs

2 of your png files were over 2 megs, so I threw them into a png compressor (tinypng) and that reduced the size a lot. This helps with loading time and overall snappiness.

Re-use values as much as possible

In your original code, you wrote to and then subsequently read from the css left property a couple times in your code. Doing this is going to make it a lot slower. Instead, I kept an left property, and would only touch $.css when I absolutely needed to. Likewise for reading each element's width each update.

Also, like I said, I modified your logic to (I think) make more sense, given what you were trying to accomplish. It calculates a new diff each update, and tries to move according to that. Also, it doesn't try to keep moving once one of the images falls off (which yours does if you move all the way to the right, and it looks really weird). You can also look at this: http://jsfiddle.net/amqER/5/, which maybe is more like the control scheme you wanted.