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

To draw directly into an element's background using JavaScript in our CSS, we define a paint worklet using the registerPaint() function, tell the document to include the worklet using the paintWorklet addModule() method, then include the image we created using the CSS paint()  function.

We create our PaintWorklet called 'hollowHighlights' using the registerPaint() function:

registerPaint('hollowHighlights', class {

  static get inputProperties() { return ['--boxColor']; }

  static get inputArguments() { return ['*','<length>']; }

  static get contextOptions() { return {alpha: true}; }

  paint(ctx, size, props, args) {
		const x = 0;
		const y = size.height * 0.3;
		const blockWidth = size.width * 0.33;
		const blockHeight = size.height * 0.85;

		const theColor = props.get( '--boxColor' );
		const strokeType = args[0].toString();
		const strokeWidth = parseInt(args[1]);

		console.log(theColor);
		
		if ( strokeWidth ) {
			ctx.lineWidth = strokeWidth;
		} else {
			ctx.lineWidth = 1.0;
		}

		if ( strokeType === 'stroke' ) {
			ctx.fillStyle = 'transparent';
			ctx.strokeStyle = theColor;
		} else if ( strokeType === 'filled' ) {
			ctx.fillStyle = theColor;
			ctx.strokeStyle = theColor;
		} else {
			ctx.fillStyle = 'none';
			ctx.strokeStyle = 'none';
		}

	
		ctx.beginPath();
		ctx.moveTo( x, y );
		ctx.lineTo( blockWidth, y );
		ctx.lineTo( blockWidth + blockHeight, blockHeight );
		ctx.lineTo( x, blockHeight );
		ctx.lineTo( x, y );
		ctx.closePath();
		ctx.fill();
		ctx.stroke();
		
		for (let i = 0; i < 4; i++) {
			let start = i * 2;
			ctx.beginPath();
			ctx.moveTo( blockWidth + (start * 10) + 10, y);
			ctx.lineTo( blockWidth + (start * 10) + 20, y);
			ctx.lineTo( blockWidth + (start * 10) + 20 + blockHeight, blockHeight);
			ctx.lineTo( blockWidth + (start * 10) + 10 + blockHeight, blockHeight);
			ctx.lineTo( blockWidth + (start * 10) + 10, y);
			ctx.closePath();
			ctx.fill();
			ctx.stroke();
		}
  } 
});

We then include the paintWorklet:

  CSS.paintWorklet.addModule('https://mdn.github.io/houdini-examples/cssPaint/intro/worklets/hilite.js');

Then we can use the <image> with the CSS paint() function:

li {
   --boxColor: hsla(55, 90%, 60%, 1.0);
   background-image: paint(hollowHighlights, stroke, 2px);
}

li:nth-of-type(3n) { 
   --boxColor: hsla(155, 90%, 60%, 1.0);
   background-image: paint(hollowHighlights, filled,  3px);
}

li:nth-of-type(3n+1) { 
   --boxColor: hsla(255, 90%, 60%, 1.0);
   background-image: paint(hollowHighlights, stroke, 1px);
}

We've included a custom property in the selector block defining a boxColor. Custom properties are accessible to the PaintWorklet.

Specifications

Specification Status Comment
CSS Painting API Level 1 Working Draft Initial definition.

Browser compatibility

See the browser compatibility data for each CSS Painting API Interfaces.

See Also