Introducing `linear()`, an easing function in CSS that interpolates linearly between its points, allowing you to recreate bounce and spring effects.
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[Video](https://www.youtube.com/watch?v=oDcb3fvtETs)
## Easings in CSS
When animating or transitioning elements in CSS, you control the rate at which a value changes with an easing function using the `animation-timing-function` and `transition-timing-function` properties.
There are several keywords available as presets in CSS, namely `linear`, `ease`, `ease-in`, `ease-out`, and `ease-in-out`. To create your own easing curves use the `cubic-bezier()` function, or take a steps based approach using the `steps()` easing function.
> [!NOTE]
> **Note:** Learn more about these easing keywords and functions in [Learn CSS: Animations](https://web.dev/learn/css/animations#animation_timing_function).
When used appropriately, easings give an animated element a sense of weight as it appears to gather momentum.
Creating complex curves such as bounce or spring effects is not possible in CSS, but thanks to `linear()` you can now approximate them astonishingly well.
## An intro to `linear()`
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/CSS/Reference/Values/easing-function/linear)
A new way to define an easing in CSS is with `linear()`. This function accepts a number of stops, separated by commas. Each stop is a single number that ranges from 0 to 1. In between each stop the interpolation is done in a linear way, explaining the name of the function.
```css
animation-timing-function: linear(0, 0.25, 1);
```
These stops are by default spread equidistantly. In the preceding snippet, that means the output value of `0.25` will be used at the 50% mark.
Visualized, the graph for `linear(0, 0.25, 1)` looks like this:
If you don't want the stops to be spread equidistantly, you can optionally pass in a *stop length*. When passing in one value as a stop length, you define its starting point:
```css
animation-timing-function: linear(0, 0.25 75%, 1);
```
Here, an output value of `0.25` won't be used at the `50%` mark, but will at `75%`.
When specifying two values as a stop length, you define both its starting and ending point:
```css
animation-timing-function: linear(0, 0.25 25% 75%, 1);
```
An output value of 0.25 will be used from 25% to 75% in time.
## Create complex curves
While the previous examples are simpler easings, you can use `linear()` to recreate complex easing functions if you compromise some of the precision.
Take this bounce easing curve, a type of easing that cannot be expressed directly in CSS, defined using JavaScript:
```javascript
function easing(pos) {
const t = 7.5625;
const e = 2.75;
return pos < 1 / e
? t * pos * pos
: pos < 2 / e
? t * (pos -= 1.5 / e) * pos + 0.75
: pos < 2.5 / e
? t * (pos -= 2.25 / e) * pos + 0.9375
: t * (pos -= 2.625 / e) * pos + 0.984375;
}
```
While the code might not tell you much, a visualization might. Here's the output, visualized as a blue curve:
The curve can be simplified by adding a number of stops onto it. Here, each green dot indicates a stop:
When passed into `linear()`, the result is a curve that kinda looks like the original one, but is a bit rougher on the edges.
Compare the green animated box to the blue one, you can tell it's not as smooth.
But, if you add enough stops you can approximate the original curve quite well. Here's an updated version:
By merely doubling the number of stops, you already get a smooth result.
The code used to animate looks like this:
```css
animation-timing-function: linear(
/* Start to 1st bounce */
0, 0.004, 0.016, 0.035, 0.063 9.1%, 0.141, 0.25, 0.391, 0.563, 0.765, 1,
/* 1st to 2nd bounce */
0.891, 0.813 45.5%, 0.785, 0.766, 0.754, 0.75, 0.754, 0.766, 0.785, 0.813 63.6%, 0.891, 1 72.7%,
/* 2nd to 3rd bounce */
0.973, 0.953, 0.941, 0.938, 0.941, 0.953, 0.973, 1,
/* 3rd bounce to end */
0.988, 0.984, 0.988, 1
);
```
## A tool to help
Manually creating this list of stops would be very cumbersome. Thankfully, [Jake](https://developer.chrome.com/authors/jakearchibald) and [Adam](https://developer.chrome.com/authors/argyle) have created [a tool to help you convert an easing curve to its `linear()` counterpart](https://linear-easing-generator.netlify.app/).
 Screenshot of in action.
The tool takes a JavaScript easing function or SVG curve as its input, and outputs the simplified curve using `linear()`. Use the sliders to control the number of stops you want, and their precision.
At the top-right, you can also choose one of the presets: Spring, Bounce, Simple elastic, or Material Design emphasized easing are included.
## DevTools support
Available in DevTools is support to visualize and edit the result of `linear()`. Click the icon to show an interactive tooltip that lets you drag around the stops.
 Screenshot of Chrome DevTools's \`linear()\` editor.
This DevTools feature is available in DevTools from Chrome 114.