What is canvas?

The <canvas> tag in HTML5 is used to draw images (via scripts, usually JavaScript). However, the <canvas> element itself has no drawing ability. It is only a container of graphics. We must use scripts to complete the actual drawing task.

Create canvas element

Add the canvas element to the HTML 5 page and specify the ID, width, and height of the element:

<canvas id="myCanvas" width="200" height="200">
<p>Your browser does not support canvas! </p>
</canvas>

Draw with JavaScript

Canvas element itself has no drawing ability. All the drawing work must be done in javascript:

<script type="text/javascript">
var canvas=document.getElementById("myCanvas");
var cxt=canvas.getContext("2d");
cxt.fillStyle="red";
cxt.fillRect(0,0,150,75);
</script>

Where JavaScript uses id to find the canvas element:

var canvas=document.getElementById("myCanvas");

Then, create the context object:

var cxt=canvas.getContext("2d");

getContext("2d") objects are built-in HTML5 objects, which have many ways to draw paths, rectangles, circles, characters and add images.

Next, draw a red rectangle in two lines of code:

cxt.fillStyle="red";
cxt.fillRect(0,0,150,75); 

It is dyed red by fillStyle method, and the shape, position and size are specified by fillRect method.


Understanding of coordinates

The fillRect method in the code has parameters (0,0150,75). This means drawing a 150x75 rectangle on the canvas, starting at the top left corner (0,0).

As shown in the following figure, the X and Y coordinates of the canvas are used to locate the painting on the canvas.

canvas坐标图