Tutorial: Introduction to canvas 2/4

In the previous tutorial, we have seen what is a canvas, how to include a canvas in your web pages and how to access it using JavaScript. We have also discussed about canvas accessibility and the current support in browsers. We also finished the first part of this tutorial by discussing how to draw a simple rectangle on a canvas.

In this second part of this tutorial, I am going to discuss how to draw other shapes than a rectangle on a canvas using paths.

Think of a path as a collection of pixels going from a starting point to an ending point. A path can also be composed of subpaths.

Four methods are used when dealing with paths:
  • beginPath(): To start a new path
  • closePath(): To close the current path
  • stroke(): To stroke the path
  • fill(): To fill the path

When we will draw on the canvas, the default color is black. We can change that by using CSS colors with the two following context properties: fillStyle and strokeStyle. For instance ctx.fillStyle='rgb(255,0,0)'; would define the filling color as red.

Drawing straight lines

Consider these two main methods to draw straight lines:
  • moveTo(x,y): Starting point of the line we want to draw. Think of this method as lifting the "pencil" to the specified coordinate.
  • lineTo(x,y): Think of this method as drawing the path from the previous specified point to this one.

Here is how you would draw a path using three lines:

var ctx=document.getElementById("mycanvas").getContext('2d');

Drawing circles or arcs

To draw a circle or an arc, we are going to use the arc(x, y, radius, startAngle, endAngle) method:
  • x and y being the coordinates of the center of the circle/arc in pixels
  • radius being the radius of your circle/arc in pixel
  • startAngle being the starting angle of the circle/arc in radians
  • endAngle being the ending angle of the circle/arc in radians


var ctx=document.getElementById("mycanvas").getContext('2d');
ctx.arc(200,100,50,0,Math.PI*2); //Here for instance, this will represent a full circle.
ctx.fill(); //Again, we have the choice to either draw a filled circle or a stroked circle.

Drawing text

We have seen how to draw shapes and lines made of paths.
  • Drawing text is still drawing a path
  • Like other shapes we have just discussed, we can have a filled text or stroked text

Let us see how it works:

var ctx=document.getElementById("mycanvas").getContext('2d');
ctx.textAlign="center"; //is the position of the text regarding to a specified point.
ctx.font="bold 30px verdana"; //has the same syntax as the CSS font rule.
ctx.fillText("Hello World!", 266.5, 150);

fillText has the following arguments: a text string to draw then the x coordinate and the y coordinate of the point where we want to draw the text.

Stay tuned and check out the third part of this tutorial coming soon which will discuss how to draw images on a canvas and how to work with pixels on the canvas.

Published December 23, 2010