javascript - Draw a circle in the middle of canvas - Stack Overflow

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Having -

HTML -

<canvas  id="myCanvas" >
        </canvas>

        <script>

              var canvas = document.getElementById('myCanvas');
              var context = canvas.getContext('2d');
              var centerX = canvas.width / 2;
              var centerY = canvas.height / 2;
              var radius = 30;

              context.beginPath();
              context.arc(centerX,centerY, radius, 0, 2 * Math.PI);
              context.fillStyle = 'blue';
              context.fill();



        </script>

CSS-

#myCanvas {
    border:2px solid;
    display:inline-block;
    width:500px;
    height:400px;

}

/

but I get not accurate circle . How can I make it accurate circle ?

Having -

HTML -

<canvas  id="myCanvas" >
        </canvas>

        <script>

              var canvas = document.getElementById('myCanvas');
              var context = canvas.getContext('2d');
              var centerX = canvas.width / 2;
              var centerY = canvas.height / 2;
              var radius = 30;

              context.beginPath();
              context.arc(centerX,centerY, radius, 0, 2 * Math.PI);
              context.fillStyle = 'blue';
              context.fill();



        </script>

CSS-

#myCanvas {
    border:2px solid;
    display:inline-block;
    width:500px;
    height:400px;

}

http://jsfiddle/urielz/3E656/

but I get not accurate circle . How can I make it accurate circle ?

• javascript
• css
• canvas

Share Improve this question Follow asked Apr 9, 2014 at 10:04 URL87URL87 11k3636 gold badges111111 silver badges177177 bronze badges

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4 Answers 4

Sorted by: Reset to default 6

Demo Fiddle

If you just want set the canvas size in CSS, change your code to:

<canvas id="myCanvas"></canvas>
<script>
    var canvas = document.getElementById('myCanvas');
    var style = window.getComputedStyle(canvas);
    if (canvas.getContext) {
        canvas.width = parseInt(style.getPropertyValue('width'));
        canvas.height = parseInt(style.getPropertyValue('height'));
    }

    var context = canvas.getContext('2d');
    var centerX = canvas.width / 2;
    var centerY = canvas.height / 2;
    var radius = 30;

    context.beginPath();
    context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
    context.fillStyle = 'blue';
    context.fill();
    context.lineWidth = 5;
    context.strokeStyle = '#003300';
    context.stroke();

    this.arc(x, y, radius, 0, Math.PI * 2, false);
    this.fill();
</script>

You need to set explicitly the size of your canvas, otherwise it will get the default size: 300x150.

you should do something like this

<canvas id="myCanvas" width="500" height="400"></canvas>

or via javascript (before getting the context)

canvas.width = 500;
canvas.height = 400;
var context = canvas.getContext('2d');

You need to set the width of your canvas, like so:

<canvas id="myCanvas" width="500" height="400"></canvas>

Place below code after begin path:

    var centerX = canvas.width;
    var centerY = canvas.height;
context.arc(centerX*0.5,centerY*0.5,10,0 ,radius, 0, 2 * Math.PI);

Css:

#myCanvas {
    border:2px solid;
    display:inline-block;
    width:600px;
    height:300px;

}

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