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.
arcTo(x1, y1, x2, y2, radius)
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');
ctx.beginPath();
ctx.strokeStyle = 'gray';
ctx.moveTo(200, 20);
ctx.lineTo(200, 130);
ctx.lineTo(50, 20);
ctx.stroke();
ctx.beginPath();
ctx.strokeStyle = 'black';
ctx.lineWidth = 5;
ctx.moveTo(200, 20);
ctx.arcTo(200,130, 50,20, 40);
ctx.stroke();
ctx.beginPath();
ctx.fillStyle = 'blue';
ctx.arc(200, 20, 5, 0, 2 * Math.PI);
ctx.fill();
ctx.beginPath();
ctx.fillStyle = 'red';
ctx.arc(200, 130, 5, 0, 2 * Math.PI);
ctx.arc(50, 20, 5, 0, 2 * Math.PI);
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.
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
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
More sophisticated demo of the method. You can play around with range input to see how arc changes.
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 = r = control.value;
};
const mouse = { x: 0, y: 0 };
let r = 100;
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)
}
}
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;
let a = 0;
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);
Result
