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 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

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.

Return value

None (undefined).

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 it out. In other words, the arc is tangential to both segments.

HTML

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

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 = 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

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

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

Live demo

More sophisticated demo of the method. 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

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

Specification
HTML Standard # dom-context-2d-arcto-dev

Browser compatibility

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