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Canvas element in HTML5

黄舟

Dec 22, 2016 pm 03:27 PM

The canvas element is used to draw graphics on web pages.

What is Canvas?

HTML 5’s canvas element uses JavaScript to draw images on the web page.

The canvas is a rectangular area that you can control every pixel of.

canvas has multiple ways to draw paths, rectangles, circles, characters, and add images.

Create Canvas element

Add canvas element to HTML 5 page, specify the element's id, width and height:

<canvas id="myCanvas" width="200" height="100"></canvas>

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Draw through JavaScript

The canvas element itself has no drawing capabilities. All drawing work must be done inside JavaScript:

<script type="text/javascript"> 
var c=document.getElementById("myCanvas"); 
var cxt=c.getContext("2d"); 
cxt.fillStyle="#FF0000"; 
cxt.fillRect(0,0,150,75); 
</script>

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JavaScript uses the id to find the canvas element:

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

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Then, create the context object:

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

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getContext("2d") The object is a built-in HTML 5 object with many Ways to draw paths, rectangles, circles, characters, and add images.

Example: Hover the mouse over the rectangle to see the coordinates. Try it yourself. The code is as follows:

<!DOCTYPE HTML>
<html>
<head>
<style type="text/css">
body {
	margin: 0px;
	font-size: 70%;
	font-family: verdana, helvetica, arial, sans-serif;
}
#coordiv {
	float: left;
	width: 199px;
	height: 99px;
	border: 1px solid #c3c3c3
}
</style>
<script type="text/javascript">  
function cnvs_getCoordinates(e) { 
	x=e.clientX; 
	y=e.clientY; 
	document.getElementById("xycoordinates").innerHTML="Coordinates: (" + x + "," + y + ")"; 
} 
function cnvs_clearCoordinates() { 
	document.getElementById("xycoordinates").innerHTML=""; 
} 
</script>
</head>
<body>
<p>把鼠标悬停在下面的矩形上可以看到坐标：</p>
<div id="coordiv" onmousemove="cnvs_getCoordinates(event)" onmouseout="cnvs_clearCoordinates()"></div>
<div id="xycoordinates"></div>
</body>
</html>

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