开发者社区> 问答> 正文

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>

展开
收起
小旋风柴进 2016-06-07 13:43:10 2586 0
1 条回答
写回答
取消 提交回答
  • 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
问答排行榜
最热
最新

相关电子书

更多
《零基础HTML入门教程》 立即下载
天猫 HTML5 互动技术实践 立即下载
天猫HTML5互动技术实践 立即下载