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<input id="input_upload" type="file"/ >
<canvas id="canvas" style="border:1px solid #c3c3c3;" ></canvas>
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
$('input[type=file]').change(function(){
var file=this.files[0];
var reader=new FileReader();
var image=new Image();
reader.readAsDataURL(file);
reader.onload=function(){
// 通过 reader.result 来访问生成的 DataURL
var url=reader.result;
image.src=url;
alert(image.width);
alert(image.height);
image.height /=4;
image.width /=4;
canvas.setAttribute("width", image.width+"px");
canvas.setAttribute("height", image.height+"px");
alert(image.naturalWidth);
alert(image.naturalHeight);
context.drawImage(image,0,0,image.width,image.height);
};
});