js 压缩图片 H5

原理 用 canvas的 toDataURI （type , int ）  如果

type

参数的值为

image/jpeg或

image/webp

,则第二个参数的值如果在0.0和1.0之间的话,会被看作是图片质量参数

 dataURI转 blob的代码是 copy 来的

压缩代码

/*common*/

var canvasSupported = isCanvasSupported()

function dataURItoBlob(dataURI) {
// convert base64 to raw binary data held in a string
var byteString
,mimestring

if(dataURI.split(',')[0].indexOf('base64') !== -1 ) {
byteString = atob(dataURI.split(',')[1])
} else {
byteString = decodeURI(dataURI.split(',')[1])
}

mimestring = dataURI.split(',')[0].split(':')[1].split(';')[0]

var content = new Array();
for (var i = 0; i < byteString.length; i++) {
content[i] = byteString.charCodeAt(i)
}

return new Blob([new Uint8Array(content)], {type: mimestring});
}

function imgScale (src , scale,cbk) {
if (!src) return cbk(false)
var _canvas = document.createElement('canvas')
var tImg = new Image
tImg.onload = function(){
var _context = _canvas.getContext('2d');
_context.drawImage(tImg,0,0);
var type = 'image/jpeg'
src = _canvas.toDataURL(type , scale)
var blob = dataURItoBlob(src)
cbk(blob)
/*
var r = _canvas.mozGetAsFile('f' , type)
cbk(r)
*/
};
tImg.src = src

}

function isCanvasSupported(){
var elem = document.createElement('canvas');
return !!(elem.getContext && elem.getContext('2d'));
}

exports.support = canvasSupported

/* opt {scale :0-1}*/
exports.zip = function(files ,opt,cbk){
opt = opt || {}
var scale = opt.scale
if (!canvasSupported) return cbk(files)
if (!scale || 1 == scale ) return cbk(files)
var files_count = files.length
,ret = []

for (var i = 0 ,j = files.length ; i<j ; i++){
var fReader = new FileReader();
fReader.onload = function (e){
var result = e.target.result
imgScale(result , scale ,function(file){
file && ret.push(file)
files_count--
if (files_count <= 0 ) cbk && cbk(ret)

})
};
fReader.readAsDataURL(files[i]);
}
}


使用

var compress = require('app/compress')

if (compress.support){
compress.zip(files , {'scale':opt.compress} , function(files){
console.log(files)
//返回的 blob 对象可以 append 到 FormData对象上 用 ajax 上传
})
}