http://blog.csdn.NET/qazwsx2345/article/details/21827553
主要用了两个HTML5的 API，一个file，一个canvas，压缩主要使用cnavas做的，file是读取文件，之后把压缩好的照片放入内存，最后内存转入表单下img.src，随着表单提交。
照片是自己用单反拍的，5M多，压缩下面3张分别是600多kb，400多kb，300kb的最后那张失真度很大了，压缩效率蛮高的。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<title>File API Test</title>
<script type="text/javascript" src="js/jquery-1.11.0.min.js"></script>
<script type="text/javascript" src="js/JIC.js"></script>
<style>
#test{
display: none;
}
</style>
</head>
<body>
<input type="file" id="fileImg" >
<form>
<img src="" id="test" alt="">
</form>
<script>
function handleFileSelect (evt) {
// var filebtn = document.getElementById(id);
// console.log(filebtn);
// var files = filebtn.target.files;
// console.log(filebtn.target);
// console.log(files);
var files = evt.target.files;
for (var i = 0, f; f = files[i]; i++) { // Only process image files.
if (!f.type.match('image.*')) {
continue;
} var reader = new FileReader(); // Closure to capture the file information.
reader.onload = (function(theFile) {
return function(e) {
// Render thumbnail.
// console.log(evt.target.files[0]);
// console.log(e.target);
console.log(e.target.result);
var i = document.getElementById("test");
i.src = event.target.result;
console.log($(i).width());
console.log($(i).height());
$(i).css('width',$(i).width()/10+'px');
//$(i).css('height',$(i).height()/10+'px');
console.log($(i).width());
console.log($(i).height());
var quality = 50;
i.src = jic.compress(i,quality).src;
console.log(i.src);
i.style.display = "block";
};
})(f); // Read in the image file as a data URL.
reader.readAsDataURL(f);
}
} document.getElementById('fileImg').addEventListener('change', handleFileSelect, false);
</script>
</body>
</html> var jic = {
/**
* Receives an Image Object (can be JPG OR PNG) and returns a new Image Object compressed
* @param {Image} source_img_obj The source Image Object
* @param {Integer} quality The output quality of Image Object
* @return {Image} result_image_obj The compressed Image Object
*/ compress: function(source_img_obj, quality, output_format){ var mime_type = "image/jpeg";
if(output_format!=undefined && output_format=="png"){
mime_type = "image/png";
} var cvs = document.createElement('canvas');
//naturalWidth真实图片的宽度
cvs.width = source_img_obj.naturalWidth;
cvs.height = source_img_obj.naturalHeight;
var ctx = cvs.getContext("2d").drawImage(source_img_obj, 0, 0);
var newImageData = cvs.toDataURL(mime_type, quality/100);
var result_image_obj = new Image();
result_image_obj.src = newImageData;
return result_image_obj;
},
function ****(***){}
}