html5-canvasHTML5 Canvas element

ipivanov No Comments JavaScript 41

The HTML 5 canvas specification has introduced a slew of new and exciting technologies to the web. Here we’ll expand on just one: the Canvas API. It’s for drawing pretty, whizzy things.

The first thing to know is that canvas is a new element in the HTML 5 specification. To begin drawing on the canvas, you grab hold of it and retrieve a particular context – either 2d or webgl:


var canvas = document.querySelector('canvas'),
    ctx = canvas.getContext('2d');

From there you can begin drawing, filling and stroking shapes and text on the canvas. Here’s an example that draws a simple square on a canvas.


<canvas></canvas>

<script>
    var canvas = document.querySelector('canvas'),
        ctx = canvas.getContext('2d');
    ctx.fillRect(10, 10, 10, 10);
</script>

Try this out on a page to see a little black square!

The getContext function retrieved the correct context, which is an object whose methods are used to draw, fill and generally modify the canvas. Above we’re using fillRect, which takes the x and y position of the top left corner of the rectangle as the first two arguments, and the width and height dimensions as the latter two.

Animation

canvas works a bit like a real canvas – pixels are drawn and layered on top of each other until you erase – clear – them. This means that if you want to animate something moving across a canvas you have to repeatedly clear the canvas (or a section of it) and redraw the scene.

In the next example we’ve animating the square bouncing around the canvas, which automatically resizes to fill the screen. loop is the business part of the code. TheclearRect function is used to clear the entire canvas before each frame.


var canvas = document.querySelector('canvas'),
    ctx = canvas.getContext('2d');

var resize = function () {
  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;
};

window.addEventListener('resize', resize);

window.addEventListener('load', function () {

  resize();

  var pos, vel;
  pos = {
    x: 10,
    y: 10
  };
  vel = {
    x: 1,
    y: 1
  };

  var loop = function () {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    pos.x += vel.x;
    pos.y += vel.y;
    if (pos.x < 5 || pos.x > canvas.width - 5) {
      vel.x *= -1;
    }
    if (pos.y < 5 || pos.y > canvas.height - 5) {
      vel.y *= -1;
    }
    ctx.fillRect(pos.x - 5, pos.y - 5, 10, 10);
  };

  setInterval(loop, 1000 / 60);
});

There’s lots to explore with canvas, this was just a very shallow look at it. There are many specialized resources on the web so, to find out more, Google is your best friend.

You can see the above example in the pen below:

See the Pen QjYZza by Ivan Ivanov (@ipivanov) on CodePen.12906

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