skrollr (v 0.5.14)

Stand-alone parallax scrolling lib for mobile (Android + iOS) and desktop in just over 3.8k (minified + gzipped) for desktop. And another 6.5k for mobile support (which is optional).
Designer friendly. No JavaScript skills needed. Just plain CSS.
Actually, skrollr is much more than "just" parallax scrolling. It's a full-fledged scrolling animation library. In fact, you can use it and still have no parallax scrolling at all. But I wanted to sound hip and use some buzz-words. By the way, skrollr leverages HTML5 and CSS3 ;-)

In the wild

Further resources (tutorials etc.)

Want to get added? Just fork & pull request or tweet me @Prinzhorn

Documentation

First of all: look at the examples and read the source ;-). This might give you a feeling of how stuff works and you can see how some patterns can be implemented.

Abstract

skrollr allows you to animate any CSS property of any element depending on the horizontal scrollbar position. All you need to do is define key frames for each element at certain points in top scroll offset.
Other libraries require you to write JavaScript in order to define your animations. This introduces two main problems:

Animation and element are not at one place. In order to find out if any animations are defined for a given element, you have to scroll through many (sometimes thousands) of lines of JavaScript.
You have to learn a new syntax which is often very verbose and limited at the same time.

With skrollr, you put the definition of your key frames right where they belong (to the element) using a syntax you already know (plain CSS).
If you rather want the keyframes inside a separate file, take a look at skrollr-stylesheets.

Files you should know about

dist/skrollr.min.js - The skrollr core file. That's all you need for modern desktop browsers.
dist/skrollr.ie.min.js - For IE < 9, include it after the core using conditional comments. The plugin makes IE understand opacity, rgb() and hsl() (the ones with alpha are mapped to them) and it creates a very simple document.querySelector polyfill which only supports ID selectores (using getElementById). Needed if you want to use data-anchor-target.
dist/skrollr.mobile.min.js - Includes iScroll and a bridge script which initializes iScroll and makes it work with skrollr. Include it after the core when you need mobile support.
shim.html - Sample file to kickstart a project for desktop and mobile.
shim-desktop-only.html - Sample file for desktop only.

Let's get serious

If you're familiar with CSS, you already know the style attribute. In order to create an animation you would need several, at least two, of them. That's what skrollr does. You use the HTML5 data- attributes to define multiple sets of styles (we call each of them keyframe) and skrollr interpolates between them.

Let's change the background-color of a `div` starting at `#00f` when the scrollbar is at the top and ending with `#f00` when the user scrolled 500 pixels down

<div data-0="background-color:rgb(0,0,255);" data-500="background-color:rgb(255,0,0);">WOOOT</div>

View in browser

Lessons learned

Skrollr ensures that you can actually scroll down 500 pixels or more, even if there's not enough content. You can suppress this using the forceHeight option
You can't use #00f or #0000ff. You need to use rgb or hsl and explicitly decide which color space you want because they result in different animations (HSL is much cooler most of the time). Don't worry, the IE plugin teaches IE < 9 to display rgb and hsl correctly.

Now let's do a barrel roll at the same time

<div data-0="background-color:rgb(0,0,255);transform:rotate(0deg);" data-500="background-color:rgb(255,0,0);transform:rotate(360deg);">WOOOT</div>

View in browser

Lessons learned

Skrollr handles all these nasty CSS prefixes for you. Just -moz-relax and get yourself a cup of -webkit-coffee

Now let the rotation bounce like it were a hip-hop video

<div data-0="background-color:rgb(0,0,255);transform[bounce]:rotate(0deg);" data-500="background-color:rgb(255,0,0);transform[bounce]:rotate(360deg);">WOOOT</div>

View in browser

Lessons learned

Skrollr allows non-linear animations. The so called easing functions can be used per-property by putting them in square brakets behind the property. There's a built-in list of easing functions (see below in the JavaScript section) and you can use your own functions by using the easings options.

Now you may have noticed that using 500 as a keyframe position is kind of random and the look depends on your browser size.

Let's have the animation end when the top of the element reaches the top of the viewport (element leaves the viewport)

<div data-0="background-color:rgb(0,0,255);transform[bounce]:rotate(0deg);" data-top="background-color:rgb(255,0,0);transform[bounce]:rotate(360deg);">WOOOT</div>

View in browser

Lessons learned

Skrollr keyframes can either be absolute or relative.

That's the end of this short intro. The following sections will explain some more things in detail.
If you're not a fan of data-attributes or if you're planning a big website where you want a better and more flexible structure, take a look at skrollr-stylesheets.

Mobile support

Starting with version 0.5.0 skrollr officially supports mobile browsers including iOS and Android.

The Problem with mobile and the solution

(If you're not interested in the details, just scroll down a bit to see what you need to do for mobile support)
Some words on why this is an important milestone and why others failed: Mobile browsers try to save battery wherever they can. That's why mobile browsers delay the execution of JavaScript while you are scrolling. iOS in particular does this very aggressively and completely stops JavaScript. And in short that's the reason why many scrolling libraries either don't work on mobile devices or they come with their own scrollbar which is a usability nightmare on desktop. It was an important requirement while I developed skrollr that I don't force you to scroll the way I want it. skrollr on desktop uses a native scrollbar and you can scroll the way you want to (keyboard, mouse, etc.).
You just told me it doesn't work on mobile, but why does it? The answer is simple. When using skrollr on mobile you don't actually scroll. Using the excellent iScroll library scrolling is faked using the touch events on mobile browsers and preventing the native scrolling. Support for iScroll is built into the core of skrollr and you don't need to include it yourself. skrollr.mobile.min.js contains iScroll together with a small bridge-script which does everything you need to have it work with skrollr. Just follow the steps in the next section.

What you need in order to support mobile browsers

Take a look at the shim.html in the root of this project.

You need to include skrollr.mobile.min.js after the skrollr core. The shim.html contains a funky regular expression to only include it on mobile browsers. This does not only prevent unnecessary bytes to be transfered to desktop browsers, it's also very important because including skrollr.mobile.min.js on desktop browsers breaks scrolling there. Feel free to use a simpler check.
The element with the id skrollr-body needs to be the very first element inside the body element.

If you wonder why this is, scroll up to the previous section (you didn't just scroll down here, right?).

Absolute vs relative mode

Being only able to define key frames in absolute values is simply insufficient for some cases. For example if you don't know where an element will exactly be in the document. That's why there are two modes for key frames, namely absolute and relative mode.

absolute mode (or document mode)

The key frames are defined as absolute values describing how much the document has been scrolled down.
The syntax is data-[offset]-[anchor], where offset can be any integer (0 is default) and anchor can be either start (default) or end. Either offset or anchor can be ommited in some situations. Here are some examples of key frames and their meaning.

data-0 = data-start = data-0-start: When the scroll top is 0.
data-100 = data-100-start: When the scroll top is 100.
data--100 = data--100-start: When the scroll top is -100 (sounds like nonsense, but keep in mind that interpolation will be relative to this point).
data-end = data-0-end: When offset is 0, but counting from the bottom of the document instead of from the top. In short: when you reach the bottom of the page.
data-100-end: 100px before we reach the bottom.
data--100-end: 100px after we reach the bottom (again, it's up to you whether you need it).

relative mode (or viewport mode)

Instead of defining key frames relative to the document (i.e. absolute), we are able to define them depending on the position of any element in relation to the viewport.
The syntax is data-[offset]-(viewport-anchor)-[element-anchor], where offset can again be any integer and defaults to 0. Both viewport-anchor (mandatory) and element-anchor (optional) can be one of top, center or bottom. If element-anchor is ommitted, the value of viewport-anchor will be taken (just like with background-position). Here are some examples of key frames and their meaning.

data-top = data-0-top = data-top-top = data-0-top-top: When the element's top is aligned with the top of the viewport.
data-100-top = data-100-top-top: When the element's top is 100px above the top of the viewport.
data--100-top = data--100-top-top: When the element's top is 100px below the top of the viewport.
data-top-bottom= data-0-top-bottom: When the bottom of the element is at the top of the viewport (it's just not visible).
data-center-center = data-0-center-center: When the element is at the center of the viewport.
data-bottom-center = data-0-bottom-center: When the element's center is at the bottom of the viewport, thuss the upper half of the element is visible.

By default the element is the element where the key frames are defined on (self), but can be any element on the page. You can optionally specify which element you want by using the data-anchor-target and any CSS selector. The first element on the page matching the selector will be used. data-anchor-target requires IE 8 or greater.
Examples: data-anchor-target="#foo" or data-anchor-target=".bar:not(.bacon) ~ span > a[href]"
Note: If you need to support IE 7, then you may only use IDs as anchor-targets, i.e. #foo. The IE plugin maps querySelector to getElementById.
Here's an infographic for better understanding of anchors (click to open PDF):

Important: All those values will be calculated up-front and transformed to absolute mode. So if either the element's box height changes (height, padding, border) or the elements position within the document, you probably need to call refresh() (see documentation in JavaScript section below). Window resizing is handled by skrollr.

Hash navigation

In case you want to use hash links, e.g. <a href="#section-about">About</a> you need to know the following:

If you animate top, margin-top or anything that moves the element up/down, the browser won't be able to jump to the correct position and you may end up somewhere else
If you're using skrollr on mobile they won't work at all, because we're not using native scrolling there

But we've got you covered. Download the dist/skrollr.menu.min.js file and include it right after the skrollr.min.js file. Then you need to call skrollr.menu.init(s) passing the skrollr instance as first parameter and optionally some options. Here's a full example.

var s = skrollr.init(/*other stuff*/);

//The options (second parameter) are all optional. The values shown are the default values.
skrollr.menu.init(s, {
    animate: true, //skrollr will smoothly animate to the new position using `animateTo`.
    duration: 500, //How long the animation should take in ms.
    easing: 'sqrt' //The easing function to use.
});

And in order to fix the problem with the wrong offset, you are able to specify the target scroll position right at the link, e.g.

<a href="#section-about" data-menu-top="500">About</a>

This link will cause the page to scroll to 500. But you should let the the href point to the actual target because if skrollr or js are disabled, the links will still work.

Working with constants

I was lying to you. The syntax for absolute mode is not data-[offset]-[anchor] and for relative mode it's not data-[offset]-(viewport-anchor)-[element-anchor]. In both cases offset can be preceeded by a constant which can be passed to the ìnit method. The name of the constant needs to be preceeded with an underscore.
Example:

skrollr.init({
    constants: {
        foobar: 1337
    }
});

<div data-_foobar="left:0%;" data-_foobar--100="left:50%;" data-_foobar-100="left:100%;"></div>

<!--Equal to-->

<div data-1337="left:0%;" data-1237="left:50%;" data-1437="left:100%;"></div>

Valid characters for a constant are [a-z0-9_].

CSS classes

skrollr will add a skrollr class to the HTML element when calling init and will remove a no-skrollr class if present. This allows fallback CSS rules to create a good user experience on unsupported devices or when JavaScript or skrollr are disabled.
All elements under skrollr's control (elements with appropriate data-attributes) will get the skrollable class.
In addition we add the rendered or unrendered class, depending on whether an element is currently being styled by skrollr, that means the current scroll offset is in between the key frames of that element, or not.

Filling missing values

Imagine the following animation

<div data-100="left:0%;" data-200="top:0%;" data-300="left:50%;" data-400="top:50%;"></div>

One could expect left to have a value of 25% at keyframe 200. That is not the case. By design skrollr only interpolates values between key frames which are direct neighbors. What actually happens is that skrollr internally fills out all holes once from left and then from right. So the above is equivalent to

<div data-100="left:0%;top:0%;" data-200="left:0%;top:0%;" data-300="left:50%;top:0%;" data-400="left:50%;top:50%;"></div>

Preventing interpolation

The reason why skrollr is so lightweight and powerfull is because it literally interpolates every number it can find. If you want to prevent some side effect, you can suppress interpolation for a specific value by prepending an exclamation point.
Example:

<!-- This will get your image url f***** up because there's no "kitten1.4561799.jpg" and the like -->
<div data-0="background-image:url(kitten1.jpg);" data-100="background-image:url(kitten2.jpg)"></div>

<!-- Better -->
<div data-0="background-image:!url(kitten1.jpg);" data-100="background-image:!url(kitten2.jpg)"></div>

Note: The values for both keyframes (at least the if they contain a number) need to be prefixed if you want to avoid skrollr throwing an exception at you!

Limitations

There are some limitations of skrollr you should be aware of.

All numeric values have to have the same unit. It's not possible to animate from 0% to 100px. skrollr won't complain, but results are undefined.
Animations between values which are composed of multiple numeric values like margin:0 0 0 0; are only possible for the same number of values. margin:0px 0px 0px 0px; to margin:0px 100px 50px 3px; is fine, but not margin:10px; to margin:5px 10px;.
Animations between CSS transforms only work when they use the same functions in same order. From rotate(0deg) scale(1) to rotate(1000deg) scale(5) is fine.
Color animations don't support named values like "red" or hex values like "#ff0000". Instead, you have to use rgb(), rgba(), hsl() and hsla(). Don't worry, there's a skrollr plugin for IE < 9 to support hsl() (without "a"!) and to fall rgba back to rgb.
Color animations only work for same color functions. hsl() to hsl() or hsla() is fine, but not rgb() to hsl(). Which makes sense, because animating from the same colors in rgb space and in hsl space results in different animations (hsl gives you the nice rainbow stuff).

But feel free to send in a pull request to fix any of them. Just keep in mind that keeping skrollr as lightweight as possible has high priority.

JavaScript

On the JavaScript part there's not much to do (you can, if you want to!). So if you only know CSS and HTML, perfect.

skrollr.init([options])

All there is to do is to call skrollr.init([options]); which returns an instance of the singleton skrollr class. Subsequent calls to init() will just return the same skrollr instance again.
Possible options for init() are

smoothScrolling=true

Smooth scrolling smoothens your animations. When you scroll down 50 pixel the animations will transition instead of jumping to the new position.
The global setting can be overridden per element by setting data-smooth-scrolling to on or off.

constants={}

An object containing integers as values. The keys can contain [a-z0-9_]. They do not need a leading underscore.
Example: data-_myconst-200 and skrollr.init({constants: {myconst: 300}}) result in data-500.

scale=1

By default skrollr uses the largest key frame and makes document height + viewport height this high, thus the max possible scroll top offset. If your animation runs too fast or too slow, just adjust the scale value.
When forceHeight is set to false, scale is ignored.
scale affects constants as well.
scale does only affect key frames in absolute mode, e.g. data-500 but not data-top.

forceHeight=true

true: Make sure the document is high enough that all key frames fit inside. Example: You use data-1000, but the content only makes the document 500px high. skrollr will ensure that you can scroll down the whole 1000px. Or if you use relative mode, e.g. data-top-bottom, skrollr will make sure the bottom of the element can actually reach the top of the viewport.
false: Don't manipulate the document and just keep the natural scrollbar.

beforerender

A listener function getting called each time right before we render everything. The function will be passed an object with the following properties:

{
    curTop: 10, //the current scroll top offset
    lastTop: 0, //the top value of last time
    maxTop: 100, //the max value you can scroll to. curTop/maxTop will give you the current progress.
    direction: 'down' //either up or down
}

Returning false will prevent rendering.

render

A listener function getting called right after we finished rendering everything. The function will be passed the same parameters as beforerender

easing

An object defining new easing functions or overwriting existing ones. Easing functions get just one argument, which is a value between 0 and 1 (the percentage of how much of the animation is done). The function should return a value between 0 and 1 as well, but for some easings a value less than 0 or greater than 1 is just fine.
An easing function basically transforms the timeline for an animation. When the animation should be 50% done, you can transform it to be 90% done or whatever your function does.
Example:

skrollr.init({
    easing: {
        //This easing will sure drive you crazy
        wtf: Math.random,
        inverted: function(p) {
            return 1 - p;
        }
    }
});

You can now use the easing functions like any other.
skrollr ships with some built in functions:

linear: The default. Doesn't need to be specified.
quadratic: To the power of two. So 50% looks like 25%.
cubic: To the power of three. So 50% looks like 12.5%
begin/end: They always return 0 or 1 respectively. No animation.
swing: Slow at the beginning and accelerates at the end. So 25% -> 14.6%, 50% -> 50%, 75% -> 85.3%
sqrt: Square root. Starts fast, slows down at the end.
bounce: Bounces like a ball. See https://www.desmos.com/calculator/tbr20s8vd2 for a graphical representation.

Note: Your easing functions should return 1 for input of 1. After the keyframe is passed, skrollr sets the values to the values of this keyframe. So if you function returns .8 for input of 1, your elements will jump at the end. But you can also use this on purpose, like the "inverted" function in the above example. The element will do everything in reverse, but at end the jumps to the end position.

Public API

Calling init() returns an instance of skrollr which exposes a public api.

refresh([elements])

Reparses all given elements. Useful when

elements in relative mode change and need to be updated
data-attributes are manipulated dynamically
new elements are added to the DOM and should be controlled by skrollr

When no elements are given, all elements in the document will be parsed again. In fact, when calling skrollr.init() skrollr uses refresh() without parameters internally.
Time consuming operation, should not be called on every rendering.

relativeToAbsolute(element, viewportAnchor, elementAnchor)

returns an integer which represents the absolute scroll position which correlates to the relative anchor.
element must be a DOM element.
viewportAnchor and elementAnchor must be one of top, center or bottom
Example:

var offset = s.relativeToAbsolute(document.getElementById('foo'), 'top', 'bottom');

//offset contains the scroll position at which #foo's bottom is at the top of the viewport.
//if you now use setScrollTop(offset) or animateTo(offset) #foo's bottom will be perfectly aligned with the top of the viewport.

getScrollTop()

Returns the current scroll offset in pixels. Normalizes different browser quirks and returns the iScroll y-position in case of skrollr.mobile.

setScrollTop(top[, force = false])

Sets the top offset using window.scrollTo(0, top) on dektop or iscroll.scrollTo(0, -top) when using skrollr.mobile.
When force is set to true, skrollr will jump to the new position without any kind of transition. By default the global smoothScrolling setting applies.

animateTo(top[, options])

Animates the scroll position from current position to top. Possible options are

duration

How long the animation should run in milliseconds. The default is 1000 or one second.

easing

The name of an easing function. The same functions can be used as for property animations. Default is linear .

done

A function to be called after the animation finished. When you pass a top value, which is the same as the current, then the function will be called immediately. The function gets a boolean argument interrupted which indicates if the animation was iterrupted by stopAnimateTo or finished to the end.

stopAnimateTo()

Stops the animation and calls the done callback passing true as interrupted arguments.

isAnimatingTo()

Returns if an animation caused by animateTo is running.

on(name, fn)

Set a listener function for one of the events described in the options section (beforerender, render). Only one listener can be attached at a given time. This method overwrites the current listener, if any.

off(name)

Removes the listener for the given event.

Stand-alone parallax scrolling lib for mobile (Android + iOS) and desktop in just over 3.8k (minified + gzipped) for desktop.

skrollr (v 0.5.14)

In the wild

Further resources (tutorials etc.)

Documentation

Abstract

Files you should know about

Let's get serious

Let's change the background-color of a div starting at #00f when the scrollbar is at the top and ending with #f00 when the user scrolled 500 pixels down

Lessons learned

Now let's do a barrel roll at the same time

Lessons learned

Now let the rotation bounce like it were a hip-hop video

Lessons learned

Let's have the animation end when the top of the element reaches the top of the viewport (element leaves the viewport)

Lessons learned

Mobile support

The Problem with mobile and the solution

What you need in order to support mobile browsers

Absolute vs relative mode

absolute mode (or document mode)

relative mode (or viewport mode)

Hash navigation

Working with constants

CSS classes

Filling missing values

Preventing interpolation

Limitations

JavaScript

skrollr.init([options])

smoothScrolling=true

constants={}

scale=1

forceHeight=true

beforerender

render

easing

Public API

refresh([elements])

relativeToAbsolute(element, viewportAnchor, elementAnchor)

getScrollTop()

setScrollTop(top[, force = false])

animateTo(top[, options])

duration

easing

done

stopAnimateTo()

isAnimatingTo()

on(name, fn)

off(name)

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Let's change the background-color of a `div` starting at `#00f` when the scrollbar is at the top and ending with `#f00` when the user scrolled 500 pixels down