The arc function

• Arguments to the function
• An example
• How to draw a donut shape

The arc function is used whilst drawing paths to the canvas to draw a circle or a partial circle to the canvas.

Arguments to the function

• The X coordinate (of the center point)
• The Y coordinate (of the center point)
• The radius of the arc/circle
• The angle to start drawing ¹
• The angle to stop drawing at ¹
• A boolean indicating the direction to draw. true is anti-clockwise, false is clockwise

¹ Angles are measured in radians - not degrees. 1 radian is equal to (180 / Math.PI) degrees.

An example

<script>
    window.onload = function ()
    {
        var canvas  = document.getElementById('cvs');
        var context = canvas.getContext)'2d');

        context.beginPath();
    
        // This draws a whole circle to the canvas
        context.arc(125, 125, 100, 0, Math.PI * 2, false);
    
        context.stroke();
    }
</script>

How to draw a donut shape

At first, drawing a donut shape (a circle with a hole in the middle) isn't too taxing - just draw two circles, one smaller than the other which is coloured white. And you can indeed do this, but if you have anything at the back of the two filled circles then it won't show through.

There is a solution to this though and that's to draw the inner circle that has the smaller radius in an anti-clockwise direction. This way just the space between the two circles is filled.

In this example the canvas is first filled with a background colour and then the donut is drawn. As you can see, the background colour shows through the middle-hole after the donut is drawn.

<script>
    // Get the context
    context = document.getElementById('cvs2').getContext('2d');
    
    // Fill the whole canvas with a gray colour
    context.fillStyle = '#ddd';
    context.fillRect(0,0,300,300);
    
    // Start a new path
    context.beginPath();
    
    // Draw a circle with a radius of 100 in the
    // clockwise direction
    context.arc(150,150,100,0, 2 * Math.PI, false);
    
    // Now draw another circle with a smaller
    // radius and in the anti-clockwise direction
    context.arc(150,150,60,2 * Math.PI, 0, true);
    
    // Fill the resultant shape
    context.fillStyle = 'black';
    context.fill();
</script>