本文实例讲述了JavaScript+HTML5 canvas实现放大镜效果。分享给大家供大家参考，具体如下：
效果：
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>www.qb5200.com canvas放大镜</title>
<style>
#copycanvas {
border: 1px solid #000;
display: none;
}
#square {
width: 90px;
height: 90px;
background-color: #cc3;
border: 1px solid #f00;
opacity: 0.5;
position: absolute;
z-index: 999;
display: none;
cursor: crosshair;
}
</style>
</head>
<body>
<canvas id="canvas" width="450" height="676"></canvas>
<canvas id="copycanvas" width="300" height="300"></canvas>
<div id="square"></div>
<script>
var canvas = document.getElementById('canvas'), //获取canvas对象
context = canvas.getContext('2d'), //获取上下文
copycanvas = document.getElementById('copycanvas'), //获取copycanvas
copycontext = copycanvas.getContext('2d'),
square = document.getElementById('square'), //获取透明框
squaredata = {}, //用来保存选择框数据
box = canvas.getBoundingClientRect();
//getBoundingClientRect方法可以获取元素上、下、左、右分别相对浏览器的坐标位置
//创建图像对象，并加载
image = new Image();
image.src = "3.jpg";
image.onload = function(){
context.drawImage(image,0,0,canvas.width,canvas.height);
};
canvas.onmouseover = function(e){
var x = e.clientX, //获取鼠标实时坐标
y = e.clientY;
createSquare(x,y); //保存透明选择框属性
};
window.onmousemove = function(e){
var x = e.clientX,
y = e.clientY;
//判断鼠标是否移出canvas
if(x >= canvas.offsetLeft && x <= canvas.offsetLeft + canvas.width &&
y >= canvas.offsetTop && y <= canvas.offsetTop + canvas.height){
createSquare(x,y);
}else{
hideSquare();
hideCanvas();
}
}
function showSquare(){
square.style.display = 'block';
}
function hideSquare(){
square.style.display = 'none';
}
function showCanvas(){
copycanvas.style.display = "inline";
}
function hideCanvas(){
copycanvas.style.display = "none";
}
function createSquare(x,y){
//控制选择框不移动出canvas
x = x - 45 < canvas.offsetLeft ? canvas.offsetLeft:x - 45;
y = y - 45 < canvas.offsetTop ? canvas.offsetTop:y - 45;
x = x + 90 < box.right ? x:box.right - 90;
y = y + 90 < box.bottom ? y:box.bottom - 90;
squaredata.left = x;
squaredata.top = y;
moveSquare(x,y);
}
function moveSquare(x,y){
square.style.left = x + "px";
square.style.top = y + "px";
showCanvas();
showSquare();
copy();
}
function copy(){
copycontext.drawImage(
canvas,
squaredata.left - box.left,
squaredata.top - box.top,
90,
90,
0,
0,
copycanvas.width,
copycanvas.height
);
}
</script>
</body>
</html>
感兴趣的朋友可使用在线HTML/CSS/JavaScript代码运行工具：http://tools.softyun.net/code/HtmlJsRun测试一下运行效果。
希望本文所述对大家JavaScript程序设计有所帮助。
