canvas实现整个页面画成一张图片并进行分享，解决获取线上图片报错问题

川长思鸟来 2022-03-12 15:20 544阅读 0赞

最近项目中遇到将整个页面都分享给朋友的需求，我们都知道如果长按页面的话只能发送其中的一个图片。所以最终选择用canvas来画一张图片。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM3NDgxNTEy_size_16_color_FFFFFF_t_70][]

这个是需求图

使用canvas画图不难，网上有很多案例，我这里要说的是canvas的一个坑：

canvas有同源策略的问题，这个之前还真的不知道，在这个需求中我需要获取个人的微信头像，但是当我在本地测试的时候会报错：![20190306131201157.png][]

也就是说canvas不允许跨域访问。怎么解决这个问题呢？

重点代码：

这个代码中包含了如何获取微信头像，以及解决跨域问题（注意需要线上测试才能看到微信头像），还有canvas将微信头像变圆的方法。

下面放上整个案例的代码以供参考：

<!DOCTYPE html>
    <html lang="en">
    
    	<head>
    		<meta charset="UTF-8">
    		<meta name="viewport" content="width=device-width, initial-scale=1,maximum-scale=1,user-scalable=no">
    		<meta name="apple-mobile-web-app-capable" content="yes">
    		<meta name="format-detection" content="telephone=no" />
    		<meta name="apple-mobile-web-app-status-bar-style" content="black">
    		<link rel="stylesheet" href="css/common.css">
    		<title>二维码邀请</title>
    	</head>
    	<style>
    		.QRcodeInvitation {
    			/*overflow: hidden;*/
    			background: url("./images/invitation_back.png") no-repeat;
    			background-size: 100% 100%;
    			position: relative;
    		}
    		
    		.QRcodeInvitation> canvas {
    			width: 100%;
    			height: 100%;
    			border-radius: 10px;
    		}
    		
    		.QRcodeInvitation> img {
    			display: none;
    		}
    		
    		#qrcodeCanvas {
    			display: none;
    		}
    		
    		#canvasImg {
    			width: 100%;
    			height: 100%;
    			background: #fff;
    			overflow: hidden;
    			display: block!important;
    			position: absolute;
    			top: 0;
    			left: 0;
    		}
    		
    		#user_imgs {
    			width: 68px;
    			height: 69px;
    			border-radius: 50%;
    			margin-bottom: 20px;
    		}
    	</style>
    
    	<body>
    		<div class="QRcodeInvitation">
    			<canvas id="canvas">请升级浏览器</canvas>
    			<img id="canvasImg" src="" alt="">
    			<img id="user_imgs" src="" alt="">
    			<img id="invite_friends" src="./images/invite_friends.png" alt="">
    			<img id="code" src="./images/code.png" alt="">
    			<div id="qrcodeCanvas"></div>
    			<img id="base" src="" alt="">
    			<img id="invitation_back" src="./images/inviteBj.png" alt="">
    		</div>
    	</body>
    
    </html>
    <script src="js/jquery.min.js"></script>
    <script src="js/layer/layer.js"></script>
    <script src="js/common.js"></script>
    <script src="js/config.js"></script>
    <script src="./js/utf.js"></script>
    <script src="./js/jquery.qrcode.js"></script>
    
    <script>
    	var user_id = $_GET['user_id'];
    	var thumbnail_image_url = $_GET['thumbnail_image_url'];
    	console.log(thumbnail_image_url);
    	var cav = $('canvas')[0],
    		ctx = cav.getContext('2d')
    	var img = new Image()
    		// 允许跨域，后端需要在响应头添加'Access-Control-Allow-Origin: *'(cors解决跨域)
    	img.crossOrigin = '*'
    	img.onload = function() {
    		var r=38;
            var x=10;
            var y=25;
            var d=10;
              ctx.save();
            var d =2 * r;
            var cx = x + r;
            var cy = y + r;
            ctx.arc(cx, cy, r, 0, 2 * Math.PI);
            ctx.clip();
            ctx.drawImage(img, x, y, d, d);
            ctx.restore();
    			try {
    				// 读取图片base64 [不传会默认读取为image/png格式，不可压缩] [如果把png图片读取为image/jpeg，那么透明区域会变黑，可大幅度压缩质量] [如果读取为image/webp格式，只有谷歌支持读取和显示这种格式]
    				var base64 = cav.toDataURL()
    				console.log(base64)
    					// canvas 转换为 img
    				$('#user_imgs')[0].src = base64
    			} catch(err) {
    				// 报错'Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported.'表示图片需同源
    				console.warn('图片需同源')
    			}
    		}
    		// src最后赋值，为了兼容ie8
    	img.src = thumbnail_image_url;
    	if(thumbnail_image_url != '') {
    		img.src = thumbnail_image_url
    	} else {
    		img.src = './images/head-portrait.png'
    	}
    	$("#qrcodeCanvas").qrcode({
    		render: "canvas", //设置渲染方式，有table和canvas，使用canvas方式渲染性能相对来说比较好
    		text: commonsUrl + "api/gxsc/invite/others/register?user_id=" + user_id, //扫描二维码后显示的内容,可以直接填一个网址，扫描二维码后自动跳向该链接
    		width: "200", //二维码的宽度
    		height: "200", //二维码的高度
    		background: "#ffffff", //二维码的后景色
    		foreground: "#000000", //二维码的前景色
    		src: './images/scLogo.png' //二维码中间的图片
    	})
    	window.onload = function() {
    		var loding = layer.load(0, {
    			offset: '35%'
    		});
    		var imgData = document.getElementById("qrcodeCanvas").children[0].toDataURL("image/jpeg");
    		//      if (thumbnail_image_url != "") {
    		//      $('#user_imgs').attr('src', thumbnail_image_url);
    		//  	};
    		$('#base').attr('src', imgData);
    		document.documentElement.style.fontSize = (document.documentElement.clientWidth / 750) * 100 + 'px';
    		var height = $(this).height();
    		$('.QRcodeInvitation').css('height', height + 'px');
    		var user_imgs = document.getElementById('user_imgs');
    		var invite_friends = document.getElementById('invite_friends');
    		var codeImg = document.getElementById('base');
    		var invitation_back = document.getElementById('invitation_back')
    
    		function draw() {
    			var canvas = document.getElementById('canvas');
    			var ctx = canvas.getContext('2d');
    			//  计算画布的宽度
    			width = canvas.offsetWidth,
    				//  计算画布的高度
    				height = canvas.offsetHeight,
    				//  设置宽高
    				canvas.width = width;
    			canvas.height = height;
    			var cvwidth = parseInt(width * .8);
    			var cvheight = parseInt(height * .6);
    			var left = parseInt(width * .1);
    			var top = parseInt(height * .15);
    			ctx.drawImage(invitation_back, 0, 0, width, height);
    			//绘制一个圆角矩形
    			strokeRoundRect(ctx, left, top, cvwidth, cvheight, 10);
    
    			//绘制并填充一个圆角矩形
    			fillRoundRect(ctx, left, top, cvwidth, cvheight, 10, '#fff');
    			//			ctx.drawImage(user_img, left + 15, top - 30, cvwidth - 15, cvheight * .4);
    			ctx.drawImage(user_imgs, left + 95, top - 60, 300, 150);
    			ctx.drawImage(invite_friends, left + 15, top + 65, cvwidth - 15, cvheight * .4);
    			ctx.drawImage(codeImg, (cvwidth / 2) - (cvheight * .20 / 2), height * .42, height * .23, height * .23);
    			ctx.font = '13px Arial';
    			ctx.fillStyle = '#000';
    			ctx.fillText('长按识别二维码', (width / 2) - 50, height * .7);
    			layer.close(loding);
    		}
    
    		function fillRoundRect(cxt, x, y, width, height, radius, /*optional*/ fillColor) {
    			//圆的直径必然要小于矩形的宽高
    			if(2 * radius > width || 2 * radius > height) {
    				return false;
    			}
    
    			cxt.save();
    			cxt.translate(x, y);
    			//绘制圆角矩形的各个边
    			drawRoundRectPath(cxt, width, height, radius);
    			cxt.fillStyle = fillColor || "#fff"; //若是给定了值就用给定的值否则给予默认值
    			cxt.fill();
    			cxt.restore();
    		}
    
    		function strokeRoundRect(cxt, x, y, width, height, radius, /*optional*/ lineWidth, /*optional*/ strokeColor) {
    			//圆的直径必然要小于矩形的宽高
    			if(2 * radius > width || 2 * radius > height) {
    				return false;
    			}
    
    			cxt.save();
    			cxt.translate(x, y);
    			//绘制圆角矩形的各个边
    			drawRoundRectPath(cxt, width, height, radius);
    			cxt.lineWidth = lineWidth || 2; //若是给定了值就用给定的值否则给予默认值2
    			cxt.strokeStyle = strokeColor || "#fff";
    			cxt.stroke();
    			cxt.restore();
    		}
    
    		function drawRoundRectPath(cxt, width, height, radius) {
    			cxt.beginPath(0);
    			//从右下角顺时针绘制，弧度从0到1/2PI
    			cxt.arc(width - radius, height - radius, radius, 0, Math.PI / 2);
    
    			//矩形下边线
    			cxt.lineTo(radius, height);
    
    			//左下角圆弧，弧度从1/2PI到PI
    			cxt.arc(radius, height - radius, radius, Math.PI / 2, Math.PI);
    
    			//矩形左边线
    			cxt.lineTo(0, radius);
    
    			//左上角圆弧，弧度从PI到3/2PI
    			cxt.arc(radius, radius, radius, Math.PI, Math.PI * 3 / 2);
    
    			//上边线
    			cxt.lineTo(width - radius, 0);
    
    			//右上角圆弧
    			cxt.arc(width - radius, radius, radius, Math.PI * 3 / 2, Math.PI * 2);
    
    			//右边线
    			cxt.lineTo(width, height - radius);
    			cxt.closePath();
    		}
    
    		setTimeout(function() {
    			draw();
    			var type = 'jpg';
    			download(type);
    		}, 500)
    
    		//图片下载操作,指定图片类型
    		function download(type) {
    			var canvas = document.getElementById('canvas');
    			//设置保存图片的类型
    			var imgdata = canvas.toDataURL(type);
    			//将mime-type改为image/octet-stream,强制让浏览器下载
    			var fixtype = function(type) {
    				type = type.toLocaleLowerCase().replace(/jpg/i, 'jpeg');
    				var r = type.match(/png|jpeg|bmp|gif/)[0];
    				return 'image/' + r;
    			}
    			imgdata = imgdata.replace(fixtype(type), 'image/octet-stream')
    			$('#canvasImg').attr('src', imgdata).css('display', 'black')
    
    		}
    	}
    </script>

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM3NDgxNTEy_size_16_color_FFFFFF_t_70]: /images/20220312/15fceab0de3f49a1aba13003c2ce3fd8.png
[20190306131201157.png]: /images/20220312/3ad38082dd704755911f5a0d1747c7a6.png