CSS Painting API
================

**Experimental:** **This is an experimental technology**  
Check the Browser compatibility table carefully before using this in production.

The CSS Painting API — part of the CSS Houdini umbrella of APIs — allows developers to write JavaScript functions that can draw directly into an element's background, border, or content.

Concepts and usage
------------------

Essentially, the CSS Painting API contains functionality allowing developers to create custom values for `paint()`, a CSS `<image>` function. You can then apply these values to properties like `background-image` to set complex custom backgrounds on an element.

For example:


```css
aside {
  background-image: paint(myPaintedImage);
}
```


The API defines a `worklet` that can be used to programmatically generate an image that responds to computed style changes. To find out more about how this is used, consult Using the CSS Painting API.

Interfaces
----------

`PaintWorkletGlobalScope`

The global execution context of the paint worklet.

`PaintRenderingContext2D`

Implements a subset of the `CanvasRenderingContext2D` API. It has an output bitmap that is the size of the object it is rendering to.

`PaintSize`

Returns the read-only values of the output bitmap's width and height.

Examples
--------

The following example creates a list of items with a background image that rotates between three different colors and three widths. In a supporting browser you will see something like the image below.

![The width and color of the background image changes based on the custom properties](/en-US/docs/Web/API/CSS_Painting_API/guide/boxbg.png)

To achieve this we'll define two custom CSS properties, `--boxColor` and `--widthSubtractor`.

### The paint worklet

In our worklet, we can reference these custom properties.


```js
registerPaint(
  "boxbg",
  class {
    static get contextOptions() {
      return { alpha: true };
    }

    /\*
 use this function to retrieve any custom properties (or regular properties, such as 'height')
 defined for the element, return them in the specified array
 \*/
    static get inputProperties() {
      return ["--boxColor", "--widthSubtractor"];
    }

    paint(ctx, size, props) {
      /\*
 ctx -> drawing context
 size -> paintSize: width and height
 props -> properties: get() method
 \*/

      ctx.fillStyle = props.get("--boxColor");
      ctx.fillRect(
        0,
        size.height / 3,
        size.width \* 0.4 - props.get("--widthSubtractor"),
        size.height \* 0.6,
      );
    }
  },
);
```


We used the `inputProperties()` method in the `registerPaint()` class to get the values of two custom properties set on an element that has `boxbg` applied to it and then used those within our `paint()` function. The `inputProperties()` method can return all properties affecting the element, not just custom properties.

### Using the paint worklet

#### HTML


```html
<ul>
  <li>item 1</li>
  <li>item 2</li>
  <li>item 3</li>
  <li>item 4</li>
  <li>item 5</li>
  <li>item 6</li>
  <li>item 7</li>
  <li>item 8</li>
  <li>item 9</li>
  <li>item 10</li>
  <li>item 11</li>
  <li>item 12</li>
  <li>item 13</li>
  <li>item 14</li>
  <li>item 15</li>
  <li>item 16</li>
  <li>item 17</li>
  <li>item</li>
</ul>
```


#### CSS

In our CSS, we define the `--boxColor` and `--widthSubtractor` custom properties.


```css
li {
  background-image: paint(boxbg);
  --boxColor: hsl(55 90% 60% / 100%);
}

li:nth-of-type(3n) {
  --boxColor: hsl(155 90% 60% / 100%);
  --widthSubtractor: 20;
}

li:nth-of-type(3n + 1) {
  --boxColor: hsl(255 90% 60% / 100%);
  --widthSubtractor: 40;
}
```


#### JavaScript

In our `<script>` we register the worklet:


```js
CSS.paintWorklet.addModule("boxbg.js");
```


#### Result

While you can't play with the worklet's script, you can alter the custom property values in DevTools to change the colors and width of the background image.

Specifications
--------------

| Specification |
| --- |
| CSS Painting API Level 1 # paintworkletglobalscope |

Browser compatibility
---------------------

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

See also
--------

* Using the CSS Painting API
* CSS Typed Object Model API
* Houdini APIs