# CanvasRenderingContext2D.arcTo()

The `CanvasRenderingContext2D``.arcTo()` method of the Canvas 2D API adds a circular arc to the current sub-path, using the given control points and radius. The arc is automatically connected to the path's latest point with a straight line, if necessary for the specified parameters.

This method is commonly used for making rounded corners.

Note: Be aware that you may get unexpected results when using a relatively large radius: the arc's connecting line will go in whatever direction it must to meet the specified radius.

## Syntax

```void ctx.arcTo(x1, y1, x2, y2, radius);
```

### Parameters

`x1`
The x-axis coordinate of the first control point.
`y1`
The y-axis coordinate of the first control point.
`x2`
The x-axis coordinate of the second control point.
`y2`
The y-axis coordinate of the second control point.
`radius`
The arc's radius. Must be non-negative.

## Examples

### How arcTo works

One way to think about `arcTo()` is to imagine two straight segments: one from the starting point to a first control point, and another from there to a second control point. Without `arcTo()`, these two segments would form a sharp corner: `arcTo()` creates a circular arc that fits this corner and smooths is out. In other words, the arc is tangential to both segments.

#### HTML

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

#### JavaScript

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

// Tangential lines
ctx.beginPath();
ctx.strokeStyle = 'gray';
ctx.moveTo(200, 20);
ctx.lineTo(200, 130);
ctx.lineTo(50, 20);
ctx.stroke();

// Arc
ctx.beginPath();
ctx.strokeStyle = 'black';
ctx.lineWidth = 5;
ctx.moveTo(200, 20);
ctx.arcTo(200,130, 50,20, 40);
ctx.stroke();

// Start point
ctx.beginPath();
ctx.fillStyle = 'blue';
ctx.arc(200, 20, 5, 0, 2 * Math.PI);
ctx.fill();

// Control points
ctx.beginPath();
ctx.fillStyle = 'red';
ctx.arc(200, 130, 5, 0, 2 * Math.PI); // Control point one
ctx.arc(50, 20, 5, 0, 2 * Math.PI);   // Control point two
ctx.fill();
```

#### Result

In this example, the path created by `arcTo()` is thick and black. Tangent lines are gray, control points are red, and the start point is blue.

### Creating a rounded corner

This example creates a rounded corner using `arcTo()`. This is one of the method's most common uses.

#### HTML

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

#### JavaScript

The arc begins at the point specified by `moveTo()`: (230, 20). It is shaped to fit control points at (90, 130) and (20, 20), and has a radius of 50. The `lineTo()` method connects the arc to (20, 20) with a straight line. Note that the arc's second control point and the point specified by `lineTo()` are the same, which produces a totally smooth corner.

```const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
const p0 = { x: 230, y: 20  }
const p1 = { x: 90,  y: 130 }
const p2 = { x: 20,  y: 20  }

const labelPoint = function (p) {
const offset = 15;
ctx.fillText('(' + p.x + ',' + p.y + ')', p.x + offset, p.y + offset);
}

ctx.beginPath();
ctx.moveTo(p0.x, p0.y);
ctx.arcTo(p1.x, p1.y, p2.x, p2.y, 50);
ctx.lineTo(p2.x, p2.y);

labelPoint(p0);
labelPoint(p1);
labelPoint(p2);

ctx.stroke();
```

### Result of a large radius

If you use a relatively large radius, the arc may appear in a place you didn't expect. In this example, the arc's connecting line goes above, instead of below, the coordinate specified by `moveTo()`. This happens because the radius is too large for the arc to fit entirely below the starting point.

#### HTML

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

#### JavaScript

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

ctx.beginPath();
ctx.moveTo(180, 90);
ctx.arcTo(180,130, 110,130, 130);
ctx.lineTo(110, 130);
ctx.stroke();
```

### Live demo

More sophisticated demo of the method. You can play around with range input to see how arc changes.

#### HTML

```<div>
</div>
<canvas id="canvas"></canvas>
```

#### JavaScript

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

control.oninput = () => {
controlOut.textContent = r = control.value;
};

const mouse = { x: 0, y: 0 };

let   r  = 100; // Radius
const p0 = { x: 0, y: 50 };

const p1 = { x: 100, y: 100 };
const p2 = { x: 150, y: 50 };
const p3 = { x: 200, y: 100 };

const labelPoint = function (p, offset, i = 0){
const {x, y} = offset;
ctx.beginPath();
ctx.arc(p.x, p.y, 2, 0, Math.PI * 2);
ctx.fill();
ctx.fillText(`\${i}:(\${p.x}, \${p.y})`, p.x + x, p.y + y);
}

const drawPoints = function (points){
for (let i = 0; i < points.length; i++) {
var p = points[i];
labelPoint(p, { x: 0, y: -20 } , i)
}
}

// Draw arc
const drawArc = function ([p0, p1, p2], r) {
ctx.beginPath();
ctx.moveTo(p0.x, p0.y);
ctx.arcTo(p1.x, p1.y, p2.x, p2.y, r);
ctx.lineTo(p2.x, p2.y);
ctx.stroke();
}

let t0 = 0;
let rr = 0; // the radius that changes over time
let a  = 0; // angle
let PI2 = Math.PI * 2;
const loop = function (t) {
t0 = t / 1000;
a  = t0 % PI2;
rr = Math.abs(Math.cos(a) * r);

ctx.clearRect(0, 0, canvas.width, canvas.height);

drawArc([p1, p2, p3], rr);
drawPoints([p1, p2, p3]);
requestAnimationFrame(loop);
}

loop(0);```

## Specifications

Specification Status Comment
HTML Living Standard
The definition of 'CanvasRenderingContext2D.arcTo' in that specification.
Living Standard

## Browser compatibility

Update compatibility data on GitHub
Desktop Mobile Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Chrome Full support Yes Edge Full support 12 Firefox Full support 1.5 IE Full support Yes Opera Full support Yes Safari Full support Yes WebView Android Full support Yes Chrome Android Full support Yes Firefox Android Full support 4 Opera Android Full support Yes Safari iOS Full support Yes Samsung Internet Android Full support Yes

Full support
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