CanvasRenderingContext2D.addHitRegion()

The CanvasRenderingContext2D method addHitRegion() adds a hit region to the bitmap.

Canvas hit regions make hit detection easy. They let you route events to DOM elements, and make it possible for users to explore the canvas without seeing it.

Syntax

void ctx.addHitRegion(options);

Options

The options argument is optional. When provided, it is an Object which can contain the following properties:

path

A Path2D object describing the area of the hit region. If not provided, the current path is used.

fillRule

The algorithm by which to determine if a point is inside or outside the hit region.
Possible values:

• nonzero: The non-zero winding rule. Default rule.
• evenodd: The even-odd winding rule.

id

The ID for this hit region to reference it for later use in events, for example.

parentID

The ID of the parent region for cursor fallback and navigation by accessibility tools.

cursor

The cursor to use when the mouse is over this region (defaults to inherit). Inherits the cursor of the parent hit region, if any, or the canvas element's cursor.

control

An element (descendant of the canvas) to which events are to be routed. Defaults to null.

label

A text label for accessibility tools to use as a description of the region, if there is no control. Defaults to null.

role

An ARIA role for accessibility tools to determine how to represent this region, if there is no control. Defaults to null.

Examples

Using the addHitRegion method

This example demonstrates the addHitRegion() method.

HTML

<canvas id="canvas"></canvas>

JavaScript

const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');

canvas.addEventListener('mousemove', function(event) {
  if(event.region) {
    alert('ouch, my eye :(');
  }
});

ctx.beginPath();
ctx.arc(100, 100, 75, 0, 2 * Math.PI);
ctx.lineWidth = 5;
ctx.stroke();

// Eyes
ctx.beginPath();
ctx.arc(70, 80, 10, 0, 2 * Math.PI);
ctx.arc(130, 80, 10, 0, 2 * Math.PI);
ctx.fill();
ctx.addHitRegion({id: "eyes"});

// Mouth
ctx.beginPath();
ctx.arc(100, 110, 50, 0, Math.PI);
ctx.stroke();

Edit the code below to see your changes update live in the canvas. (If you don't see the full smiley, check the browser compatibility table to see if your current browser supports hit regions already; you might need to activate a preference.)

<canvas id="canvas" width="400" height="200" class="playable-canvas"></canvas>
<div class="playable-buttons">
  <input id="edit" type="button" value="Edit" />
  <input id="reset" type="button" value="Reset" />
</div>
<textarea id="code" class="playable-code" style="height:250px">
ctx.beginPath();
ctx.arc(100, 100, 75, 0, 2 * Math.PI, false);
ctx.lineWidth = 5;
ctx.stroke();

// eyes
ctx.beginPath();
ctx.arc(70, 80, 10, 0, 2 * Math.PI, false);
ctx.arc(130, 80, 10, 0, 2 * Math.PI, false);
ctx.fill();
ctx.addHitRegion({id: "eyes"});

// mouth
ctx.beginPath();
ctx.arc(100, 110, 50, 0, Math.PI, false);
ctx.stroke();</textarea>

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var textarea = document.getElementById("code");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;

function drawCanvas() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  eval(textarea.value);
}

reset.addEventListener("click", function() {
  textarea.value = code;
  drawCanvas();
});

edit.addEventListener("click", function() {
  textarea.focus();
});

canvas.addEventListener("mousemove", function(event){
  if(event.region) {
    alert("ouch, my eye :(");
  }
});

textarea.addEventListener("input", drawCanvas);
window.addEventListener("load", drawCanvas);

Specifications

Canvas hit regions have been removed from the WHATWG Living Standard, although discussions about future standardization are ongoing. See https://github.com/whatwg/html/issues/3407 for more information.

Browser compatibility

See also

• CanvasRenderingContext2D.removeHitRegion()
• CanvasRenderingContext2D.clearHitRegions()