想要在html5 canvas画布中打开一张本地图片，代码有什么问题
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<script type="text/javascript">
var canvas=document.getElementById("demo");
var cxt=canvas.getContext("2d");
//打开图片
function preImg(sourceId, targetId) {
cxt.clearRect(0,0,400,400);
if (typeof FileReader === 'undefined') {
alert('Your browser does not support FileReader...');
return;
}
var reader = new FileReader();
reader.onload = function(e) {
var img = document.getElementById(targetId);
var cxt=document.getElementById("demo").getContext("2d")
img.src = this.result;
img.onload=function()
{
cxt.drawImage(img,0,0);
}
}
reader.readAsDataURL(document.getElementById(sourceId).files[0]);
}
</script>
</head>
<body>
<form action="">
<input type="file" name="imgOne" id="imgOne" onchange="preImg(this.id,'imgPre');" />
<img id="imgPre" src="" style="display: block;" />
</form>
<canvas id="demo" width="400" height="400" style="border:1px solid red"></canvas>
</body>
</html>
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调整循序，script放canvas后面，要不获取不到对象
<form action=""> <input type="file" name="imgOne" id="imgOne" onchange="preImg(this.id,'imgPre');" /> <img id="imgPre" src="" style="display: block;" /> </form> <canvas id="demo" width="400" height="400" style="border:1px solid red"></canvas> <script type="text/javascript"> var canvas = document.getElementById("demo"); var cxt = canvas.getContext("2d"); //打开图片 function preImg(sourceId, targetId) { cxt.clearRect(0, 0, 400, 400); if (typeof FileReader === 'undefined') { alert('Your browser does not support FileReader...'); return; } var reader = new FileReader(); reader.onload = function (e) { var img = document.getElementById(targetId); var cxt = document.getElementById("demo").getContext("2d") img.src = this.result; img.onload = function () { cxt.drawImage(img, 0, 0); } } reader.readAsDataURL(document.getElementById(sourceId).files[0]); }2019-07-17 19:29:53赞同 展开评论 打赏
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