调用Web API接口FileReader中readAsDataURL方法读取数据（这个函数参数file标签获取到的文件路径），然后把数据加载到FileReader中（base64格式），之后就可以用Img来指定源时base64格式的数据，可以绘制图片。
<!DOCTYPE html>
<html>
<head>
<title>ReadImageDemo</title>
</head>
<body>
<input type="file" id="selectFiles" onchange="dealSelectFiles()" multiple webkitdirectory>
<canvas id="myCanvas" width=1440 height=900></canvas>
<script type="text/javascript">
var imgPosX = 0;
var imgWidth = 256;
function dealSelectFiles(){
/// get select files.
var selectFiles = document.getElementById("selectFiles").files;
for(var file of selectFiles){
console.log(file.webkitRelativePath);
/// read file content.
var reader = new FileReader();
reader.readAsDataURL(file);
reader.onloadend = function(){
/// deal data.
var img = new Image();
/// after loader, result storage the file content result.
img.src = this.result;
img.onload = function(){
var myCanvas = document.getElementById("myCanvas");
var cxt = myCanvas.getContext('2d');
cxt.drawImage(img, imgPosX, 0);
imgPosX += imgWidth;
}
}
}
}
</script>
</body>
</html>
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