var canvas = document.getElementById('canvas'),
  context = canvas.getContext('d'),
  canvas = document.getElementById('canvas_dp'),
  context = canvas.getContext('d'),
  image = new Image(),//document.getElementById('myimg'),
  imageData,
  scale,//缩放比例
  rubberbandRectangle = {left:,top:,width:,height:},
  resize = ;
  oldRubberbandRectangle = {};
  dragging = false,
  extending = false,
  mousedown = {};
 // Functions.....................................................
 function windowToCanvas(canvas, x, y) {
  var canvasRectangle = canvas.getBoundingClientRect();
  return {
    x: x - canvasRectangle.left,
    y: y - canvasRectangle.top
   };
 }
 //将截取的图片画在小的canvas中
 function captureCanvasPixels() {
  context.drawImage(image,rubberbandRectangle.left/scale,rubberbandRectangle.top/scale,rubberbandRectangle.width/scale,rubberbandRectangle.height/scale,,,,);
 }
 function drawRubberband() {
  context.save();
  context.beginPath();//开始新的路径
   rect(rubberbandRectangle.left,
   rubberbandRectangle.top,
   rubberbandRectangle.width,
   rubberbandRectangle.height);
  context.fillStyle='rgba(,,,.)';     
  addRectanglePath();
  context.fill();//填充路径
  context.fillStyle='rgba(,,,)'; 
  captureCanvasPixels();//将选取的图像copy到预览canvas中
  context.beginPath();
  context.strokeStyle = '#';
  context.lineWidth = .;
  context.arc(rubberbandRectangle.left+rubberbandRectangle.width,rubberbandRectangle.top+rubberbandRectangle.height,,,Math.PI*,true);
  context.fill();//填充路径
  context.stroke();//填充路径
  context.restore();
 }
 function rect(x, y, w, h, direction){
  if(direction){//逆时针
   context.moveTo(x, y);
   context.lineTo(x, y + h);
   context.lineTo(x + w, y + h);
   context.lineTo(x + w, y);
  }else{//顺时针
   context.moveTo(x, y);
   context.lineTo(x + w, y);
   context.lineTo(x + w, y + h);
   context.lineTo(x, y + h);
  }
  context.closePath();
 }
 function addRectanglePath(){
  rect(,,canvas.width,canvas.height,true);
 }
 function startDragging(loc){
  mousedown.x = loc.x;
  mousedown.y = loc.y;
  oldRubberbandRectangle.left = rubberbandRectangle.left;
  oldRubberbandRectangle.top = rubberbandRectangle.top;
 }
 function updateRubberbandRectangle(loc){
  var left = oldRubberbandRectangle.left + loc.x-mousedown.x;
  var top = oldRubberbandRectangle.top + loc.y - mousedown.y;
  rubberbandRectangle.left = (left<) ? : left;
  rubberbandRectangle.top = (top < ) ? : top;
  if(rubberbandRectangle.left + rubberbandRectangle.width > image.width * scale)rubberbandRectangle.left = image.width * scale - rubberbandRectangle.width; 
  if(rubberbandRectangle.top + rubberbandRectangle.height > image.height * scale)rubberbandRectangle.top = image.height * scale - rubberbandRectangle.height;
  drawRubberband();
 }
 function startExtendSelection(loc){
  mousedown.x = loc.x;
  mousedown.y = loc.y;
  oldRubberbandRectangle.width = rubberbandRectangle.width;
  oldRubberbandRectangle.height = rubberbandRectangle.height;
 }
 function extendSelection(loc){
  var width = parseInt(oldRubberbandRectangle.width) + parseInt(loc.x)-parseInt(mousedown.x);
  var height = parseInt(parseInt(oldRubberbandRectangle.height) * parseInt(width)/parseInt(oldRubberbandRectangle.width));
  rubberbandRectangle.width = width;
  rubberbandRectangle.height = height;
  drawRubberband();
 }
 function clearRubberbandRectangle(){
  context.clearRect(, , canvas.width, canvas.height);
  context.putImageData(imageData, ,);
 }
 // Event handlers...............................................
 canvas.onmousedown = function(e){
  e.preventDefault();
  var loc = windowToCanvas(canvas, e.clientX, e.clientY);
  if(rubberbandRectangle.left < loc.x && rubberbandRectangle.top < loc.y && (rubberbandRectangle.left + rubberbandRectangle.width) > loc.x && (rubberbandRectangle.top + rubberbandRectangle.height) > loc.y){
   dragging = true;
   startDragging(loc);
  }else if((rubberbandRectangle.left + rubberbandRectangle.width - ) < loc.x && (rubberbandRectangle.top + rubberbandRectangle.height - ) < loc.y && (rubberbandRectangle.left + rubberbandRectangle.width +) > loc.x && (rubberbandRectangle.top + rubberbandRectangle.height + ) > loc.y){
   extending = true;
   startExtendSelection(loc);
  }
 }
 canvas.onmousemove = function (e) {
 e.preventDefault();
 var loc = windowToCanvas(canvas, e.clientX, e.clientY);
  if(rubberbandRectangle.left < loc.x && rubberbandRectangle.top < loc.y && (rubberbandRectangle.left + rubberbandRectangle.width) > loc.x && (rubberbandRectangle.top + rubberbandRectangle.height) > loc.y){
   canvas.style.cursor='move';
  }else if((rubberbandRectangle.left + rubberbandRectangle.width - ) < loc.x && (rubberbandRectangle.top + rubberbandRectangle.height - ) < loc.y && (rubberbandRectangle.left + rubberbandRectangle.width +) > loc.x && (rubberbandRectangle.top + rubberbandRectangle.height + ) > loc.y){
   canvas.style.cursor='nw-resize';
  }else{
   canvas.style.cursor='';
  }
 if (dragging) {
  clearRubberbandRectangle();
  updateRubberbandRectangle(loc);
  }
 if(extending){
  canvas.style.cursor='nw-resize';
  clearRubberbandRectangle();
  extendSelection(loc);
 }
 }
 canvas.onmouseup = function(e){
  e.preventDefault();
  dragging = false;
  extending = false;
 }
 // Initialization..............................................
 var myfileInput = document.getElementById('myfileInput');
 myfileInput.onchange=function(){
  setImage(myfileInput);
 };
 function setImage(fileObj){
  if (fileObj.files && fileObj.files[]) {
   //火狐下，谷歌下都是支持的
   image.src = window.URL.createObjectURL(fileObj.files[]);
  } else {
   alert('对不起，您的浏览器不支持');
  }
 }
 image.src = '';
 image.onload = function () {
  console.log(image);
  var w,h;
  //计算图片缩放比例
  if(image.width>canvas.width){
   console.log();
   w = canvas.width;
   h = canvas.width*image.height/image.width
  }else if(image.height>canvas.height){
   console.log();
   h = canvas.height;
   w = canvas.height*image.width/mage.height
  }else if(image.width/image.height >= canvas.width/canvas.height){
   console.log();
   w = canvas.width;
   h = canvas.width*image.height/image.width;
  }else if(image.width/image.height < canvas.width/canvas.height){
   console.log();
   w = canvas.height*image.width/image.height
   h = canvas.height;
  }
  scale = w/image.width;
  context.clearRect(,,canvas.width,canvas.height);
 context.drawImage(image, , ,w, h); 
 console.log( w+':'+h+'###'+canvas.width+':'+canvas.height);
 imageData= context.getImageData(, , canvas.width, canvas.height);
  drawRubberband();
 };

<!DOCTYPE html>
<html>
 <head>
  <meta charset="utf-">
  <title>CarlZhang</title>
 </head>
 <body>
  <div name="container" style="height:px;width:px;border:#BB px solid;text-align:center">
   <canvas id="canvas" style="z-index: ; " height="" width=""></canvas>
  </div>
  <!--预览-->
  <div name="display" style="height:px;width:px;border:#BB px solid;position:absolute;left:px;top:px;">
   <canvas id="canvas_dp" style="z-index: ; " height="" width=""></canvas>
  </div>
  <!--上传-->
  <input id="myfileInput" type="file" accept="image/gif, image/jpeg, image/x-png"/>
  <script src="js/cavas_img_upload.js" type="text/javascript" charset="utf-"></script>
 </body>
</html>