parallax scrolling is a bit jumpy

Question

I've a small bit of parallax on a site I'm working on, it's working almost fine but the foreground divs are a bit jumpy when I scroll down the page.

Near the top of the page I have a div called #top-banner, it has a fixed background image, sitting within this div are two more within a row, the fisrt div / column has an image of a model & the second div has just text.

Below the #top-banner div is a div with a background image of a waterline, the desired effext is to have the waterline to cover the #top-banner as the user scrolls down, to make it seem as if the model, text & background are being covered by water.

I've got it working by using jQuery to change the css bottom property to make it seem that the two columns divs are moving down the page beneath the waterline at a similar speed to the scroll when the user scrolls down the page. I've set the speeds/increments to be slightly different to create a parallax effect.

It's working pretty well but is a bit jumpy, I've also tried to use the jQuery animate function but that is even more jumpy.

HTML

<section id="top-banner">
    <div class="row">
        <div class="col-2 prlx-1">
            <img src="model.png"/>
        </div>
        <div class="r-col-2 prlx-2">
            <h3>Lorem Ipsum</h1> 
            <p>More Ipsum</p>
        </div>                            
</section>   

<section id="hp-water-line"></section>

CSS

#hp-top-banner {
  background: url(bg.png);
  height: 600px;
  background-attachment: fixed;
  background-origin: initial;
  background-clip: initial;
  background-size: cover;
  overflow: hidden;
  width: 100%;
  position: relative;
}

#hp-water-line {
  background: url(water-line.png) no-repeat transparent;
  min-height: 92px;
  margin: 0 auto;
  width: 100%;
  position: relative;
  top: -15px;
  background-size: cover;
}

JS

$(document).ready(function(){

    function parallax(){
        var prlx_effect_1= -((window.pageYOffset / 4) *2.25 );
        $('.prlx-1').css({"position": "relative","bottom":prlx_effect_1, "transition": "0s ease-in-out"});
           // jQ('.prlx-1').css({"position": "relative"});
           // jQ('.prlx-1').animate({"bottom":prlx_effect_1},"fast");

        var prlx_effect_2= -(window.pageYOffset / 5 );
        $('.prlx-2').css({"position": "relative","bottom":prlx_effect_2, "transition": "0s ease-in-out"});

    }

    window.addEventListener("scroll", parallax, false);

});

Updated JS based on Prinzhorn Comment

var requestAnimationFrame = window.requestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.msRequestAnimationFrame ||
    window.oRequestAnimationFrame;

function onScroll() {
    requestAnimationFrame(parallax);
}

function parallax(){
    var prlx_effect_1= +(window.pageYOffset *.7).toFixed(2); // .55 is a good speed but slow
    var prlx_str_1 = "translate3d(0, "+prlx_effect_1+"px, 0)";
    jQ('.prlx-1').css({
        "transform":prlx_str_1,
        "-ms-transform":prlx_str_1,
        "-webkit-transform":prlx_str_1
    });

    var prlx_effect_2= +(window.pageYOffset * 1 ).toFixed(2); // .33 is a good speed but slow
    var prlx_str_2 = "translate3d(0, "+prlx_effect_2+"px, 0)";
    jQ('.prlx-2').css({
        "transform":prlx_str_2,
        "-ms-transform":prlx_str_2,
        "-webkit-transform":prlx_str_2
    });

    requestAnimationFrame(parallax);

}

window.addEventListener("scroll", onScroll, false);

Answer 1

I used to build parallax sites in a similar way,by using jquery to adjust the background-position or margins, however, i read this article a few months back which really changed the way I approach them.

He suggest using CSS translateZ and perspective to move containers or imagery forward and backwards into the 3 dimensional space to create 'real' parralax. This creates more fluid animations, and also renders better on mobile devices. I also personally find this much easier to execute.

I.E.

.parallax {
    perspective: 1px;
    height: 100vh;
    overflow-x: hidden;
    overflow-y: auto;
}
.parallax__layer {
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
}
.parallax__layer--base {
    transform: translateZ(0);
}
.parallax__layer--back {
    transform: translateZ(-1px);
}

The only issue is, that using real 3Dimensional layers, means you have to be smart with your Z-Index to make sure your layers are not overlapping at the wrong places.

The article has an excellent demo, which you can view the side profile of the 3D space to see how the layers are distributed in the z-axis. Just click the 'debug' button in the top left corner.

http://keithclark.co.uk/articles/pure-css-parallax-websites/