CanvasRenderingContext2D: arcTo() method

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 example if the starting point and control points are in a line.

This method is commonly used for making rounded corners.

Syntax

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.

Usage notes

Assume P₀ is the point on the path when arcTo() is called, P₁ = (x1, y1) and P₂ = (x2, y2) are the first and second control points, respectively, and r is the radius specified in the call:

• If r is negative, an IndexSizeError exception is raised.
• If r is 0, arcTo() behaves as if P₀, P₁, and P₂ are collinear (in a line).
• In the case of all of the points being collinear, a line from P₀ to P₁ is drawn unless the points P₀ and P₁ are coincident (having the same coordinates), in which case nothing is drawn.

These conditions can be created in the Constructing an arcTo() path example below to see the results.

Return value

None (undefined).

Exceptions

IndexSizeError DOMException

Thrown if radius is a negative value.

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 at this corner and smooths it 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 = (p) => {
  const offset = 10;
  ctx.fillText(`(${p.x},${p.y})`, p.x + offset, p.y + offset);
};

ctx.beginPath();
ctx.lineWidth = 4;
ctx.font = "1em sans-serif";
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

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

Constructing an arcTo() path

The demo shows the semi-infinite lines and circle with center C tangent to the lines at T₁ and T₂ used to determine the path rendered by arcTo().

Note that arcTo will create a straight line from P₀ to P₁ when all points are in a line. Additionally, nothing is drawn by arcTo if P₀ and P₁ have the same coordinates.

Besides being able to set the arc radius with the slider, the initial point P₀ and control points P₁ and P₂ can be moved by dragging them with the mouse with the left button down. The numeric values can also be edited, and the arrow keys can be used to change an underlined element that is in focus.

Animating arcTo() drawing

For this example, you can play around with the arc radius to see how the path changes. The path is drawn from the starting point p0 using arcTo() with control points p1 and p2 and a radius that varies from 0 to the maximum radius selected with the slider. Then a lineTo() call completes the path to p2.

HTML

<div>
  <label for="radius">Radius: </label>
  <input name="radius" type="range" id="radius" min="0" max="100" value="50" />
  <label for="radius" id="radius-output">50</label>
</div>
<canvas id="canvas"></canvas>

JavaScript

const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
const controlOut = document.getElementById("radius-output");
const control = document.getElementById("radius");
control.oninput = () => {
  controlOut.textContent = radius = control.value;
};

const p1 = { x: 100, y: 100 };
const p2 = { x: 150, y: 50 };
const p3 = { x: 200, y: 100 };
let radius = control.value; // match with init control value

function labelPoint(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);
}

function drawPoints(points) {
  points.forEach((p, i) => {
    labelPoint(p, { x: 0, y: -20 }, `p${i}`);
  });
}

// Draw arc
function drawArc([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();
}

function loop(t) {
  const angle = (t / 1000) % (2 * Math.PI);
  const rr = Math.abs(Math.cos(angle) * radius);

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

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

loop(0);

Result

Specifications

Browser compatibility

See also

• The interface defining this method: CanvasRenderingContext2D