canvas实现放大镜效果实现HTML5
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> 
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