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.

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');

ctx.beginPath();
ctx.moveTo(230, 20);
ctx.arcTo(90,130, 20,20, 50);
ctx.lineTo(20, 20);
ctx.stroke();

Result

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();

Result

Specifications

Browser compatibilityUpdate compatibility data on GitHub

See also

• The interface defining this method: CanvasRenderingContext2D