Utilisez Canvas pour implémenter la fonction d'interception et de téléchargement pour imiter les compétences avatar_javascript de Sina Weibo

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<) &#63; : left;
   rubberbandRectangle.top = (top < ) &#63; : 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>