CSS Painting API

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:

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

The API defines PaintWorklet, 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

PaintWorklet: Programmatically generates an image where a CSS property expects a file. Access this interface through CSS.paintWorklet.
PaintWorkletGlobalScope: The global execution context of the paintWorklet.
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.

Dictionaries

PaintRenderingContext2DSettings: A dictionary providing a subset of CanvasRenderingContext2D settings.

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

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.

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

<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.

li {
   background-image: paint(boxbg);
   --boxColor: hsla(55, 90%, 60%, 1.0);
}

li:nth-of-type(3n) {
   --boxColor: hsla(155, 90%, 60%, 1.0);
   --widthSubtractor: 20;
}

li:nth-of-type(3n+1) {
   --boxColor: hsla(255, 90%, 60%, 1.0);
   --widthSubtractor: 40;
}

JavaScript

In our <script> we register the worklet:

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 (CSS Painting API 1) # paintworkletglobalscope