下面来实现一个放大镜的效果，鼠标按下并移动时，显示当前图片区域的放大效果，抬起后效果消失。放大镜效果主要使用离屏canvas的技术，离屏canvas放置的是图片的放大版，而普通canvas则放置图片的正常版
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<canvas id="drawing" style="border:1px solid black">
<p>The canvas element is not supported!</p>
</canvas>
<canvas id="drawingOff" style="display:none">
<p>The canvas element is not supported!</p>
</canvas>
</body>
<script type="text/javascript">
if(drawing.getContext){
var cxt = drawing.getContext('2d');
var cxtOff = drawingOff.getContext('2d');
var W,H;
var scale = 1.5;
var img = new Image();
img.src="https://www.qiquanji.com/data/img/dmj/201905051557020391812974.jpg";
img.onload = function(){
W = img.width;
H = img.height;
drawing.width = W/scale;
drawing.height = H/scale;
drawingOff.width = W;
drawingOff.height = H;
cxt.drawImage(img,0,0,W/scale,H/scale);
cxtOff.drawImage(img,0,0);
drawing.onmousedown = function(e){
e = e || event;
var x0 = this.offsetLeft;
var y0 = this.offsetTop;
drawMagnifier(e);
drawing.onmousemove = function(e){
drawMagnifier(e);
}
document.onmouseup = function(e){
cxt.clearRect(0,0,W/scale,H/scale);
cxt.drawImage(img,0,0,W/scale,H/scale);
drawing.onmousemove = null;
}
function drawMagnifier(e){
cxt.clearRect(0,0,W/scale,H/scale);
cxt.drawImage(img,0,0,W/scale,H/scale);
var x = (e.clientX-x0);
var y = (e.clientY-y0);
var r = 40;
var dx = x - r;
var dy = y - r;
var sx = x*scale - r;
var sy = y*scale - r;
cxt.save();
cxt.beginPath();
cxt.arc(x,y,r,0,Math.PI*2);
cxt.lineWidth = 4;
cxt.strokeStyle = '#069';
cxt.stroke();
cxt.clip();
cxt.drawImage(drawingOff,sx,sy,2*r,2*r,dx,dy,2*r,2*r);
cxt.restore();
}
}
}
}
</script>
</html>
原文链接：https://www.qiquanji.com/post/7033.html
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