如何使用 JavaScript 在 HTML5 Canvas 中绘制圆圈？

Question

如何使用最少的 JavaScript 代码在 HTML5 Canvas 中绘制一个简单的圆圈？

Answer 1

Here is how to draw a circle using JavaScript in HTML5:

 const canvas = document.getElementById('myCanvas'); const context = canvas.getContext('2d'); const centerX = canvas.width / 2; const centerY = canvas.height / 2; const radius = 70; context.beginPath(); context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false); context.fillStyle = 'green'; context.fill(); context.lineWidth = 5; context.strokeStyle = '#003300'; context.stroke();

 body { margin: 0px; padding: 0px; }

 <canvas id="myCanvas" width="578" height="200"></canvas>

Answer 2

I took the answers above and turned them into a useful function:

function drawCircle(ctx, x, y, radius, fill, stroke, strokeWidth) {
  ctx.beginPath()
  ctx.arc(x, y, radius, 0, 2 * Math.PI, false)
  if (fill) {
    ctx.fillStyle = fill
    ctx.fill()
  }
  if (stroke) {
    ctx.lineWidth = strokeWidth
    ctx.strokeStyle = stroke
    ctx.stroke()
  }
}

Then, use it like so to draw a black circle with a red stroke (width 2) at coordinates 50,50, with radius 25:

let ctx = canvas.getContext('2d')
drawCircle(ctx, 50, 50, 25, 'black', 'red', 2)

Answer 3

Creating shapes is easier using new Canvas Path2D , which uses the same canvas drawing API, and allow separation of the declaration from drawing itself, and reuse of complex geometry:

 var canvas = document.getElementById('myCanvas'); var context = canvas.getContext('2d'); var centerX = canvas.width / 2; var centerY = canvas.height / 2; var radius = 70; let circle = new Path2D(); // <<< Declaration circle.arc(centerX, centerY, radius, 0, 2 * Math.PI, false); context.fillStyle = 'blue'; context.fill(circle); // <<< pass circle to context context.lineWidth = 10; context.strokeStyle = '#000066'; context.stroke(circle); // <<< pass circle here too

 body { margin: 0px; padding: 0px; }

 <canvas id="myCanvas" width="578" height="200"></canvas>

Answer 4

First of all, you need to get the canvas context:-

const canvas = document.querySelector('#canvas');
const ctx = canvas.getContext('2d');

Now let us define the position and radius:-

const X = canvas.width / 2;
const Y = canvas.height / 2;
const radius = 45;

Now let us set the color and line width:-

ctx.lineWidth = 3;
ctx.strokeStyle = '#FF0000';

Now, to draw the circle, we have to use the arc method and set the angle to 2 X π

ctx.beginPath();
ctx.arc(X, Y, radius, 0, 2 * Math.PI, false);
ctx.stroke();

Answer 5

Base on Sam's answer, I combined all the parameters into one object so that it is more readable when using the function.

function drawCircle(obj) {
obj.ctx.beginPath();
obj.ctx.arc(obj.x, obj.y, obj.radius, 0, 2 * Math.PI, false);
if (obj.fill) {
    obj.ctx.fillStyle = obj.fill;
    obj.ctx.fill();
}
if (obj.stroke) {
    obj.ctx.lineWidth = obj.strokeWidth;
    obj.ctx.strokeStype = obj.stroke;
    obj.ctx.stroke();
}
}

To use:

let ctx = canvas.getContext('2d');    
drawCircle({
            ctx: ctx,
            x: 100,
            y: 100,
            radius: 20,
            fill: "green",
        });