HTML5 Canvas, 2nd Edition

Dynamically Resizing the Canvas

In the code we developed in this chapter, we created a reference to the Canvas object on the HTML page—with the id canvasOne—and used it to retrieve the 2D context of the Canvas object:

var theCanvas = document.getElementById("canvasOne");
var context = theCanvas.getContext("2d");

While the 2D context is very important because we used it to draw directly onto the canvas, we did not spend any time discussing the Canvas object itself. In this chapter, we use the width property of the Canvas object to center text on the canvas. However, the Canvas object also includes another property named height, and both of these properties can be used to dynamically resize the Canvas object on demand. Why would you want to do this? There could be many uses, including the following:

• Updating the canvas to the exact size of a loaded video object

• Dynamically animating the canvas after the page is loaded

• Other, more creative uses like the one we will experiment with next

Resizing the canvas on the fly is quite easy. To do it, simply set the width and height properties of the Canvas object, and then redraw the canvas contents:

Canvas.width = 600;
Canvas.height = 500;
drawScreen();

The Canvas 2D API describes this function as a way to “scale” the canvas, but in practice, this does not appear to be true. Instead, the contents of the canvas are simply redrawn at the same size and same location on a larger canvas. Furthermore, if you don’t redraw the canvas content, it appears to ...