這篇文章主要介紹JavaScript+HTML5使用canvas實現放大鏡效果的示例，文中介紹的非常詳細，具有一定的參考價值，感興趣的小伙伴們一定要看完！
具體如下：
效果：
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>www.jb51.net canvas放大鏡</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'), //獲取canvas對象
context = canvas.getContext('2d'), //獲取上下文
copycanvas = document.getElementById('copycanvas'), //獲取copycanvas
copycontext = copycanvas.getContext('2d'),
square = document.getElementById('square'), //獲取透明框
squaredata = {}, //用來保存選擇框數據
box = canvas.getBoundingClientRect();
//getBoundingClientRect方法可以獲取元素上、下、左、右分別相對瀏覽器的坐標位置
//創建圖像對象，并加載
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, //獲取鼠標實時坐標
y = e.clientY;
createSquare(x,y); //保存透明選擇框屬性
};
window.onmousemove = function(e){
var x = e.clientX,
y = e.clientY;
//判斷鼠標是否移出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){
//控制選擇框不移動出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>1、能夠嵌入動態文本于HTML頁面。2、對瀏覽器事件做出響應。3、讀寫HTML元素。4、在數據被提交到服務器之前驗證數據。5、檢測訪客的瀏覽器信息。6、控制cookies，包括創建和修改等。7、基于Node.js技術進行服務器端編程。
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