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,//縮放比例   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      }; } //將截取的圖片畫在小的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();//開始新的路徑     rect(rubberbandRectangle.left,      rubberbandRectangle.top,      rubberbandRectangle.width,      rubberbandRectangle.height);   context.fillStyle='rgba(,,,.)';            addRectanglePath();   context.fill();//填充路徑   context.fillStyle='rgba(,,,)';    captureCanvasPixels();//將選取的圖像copy到預覽canvas中   context.beginPath();   context.strokeStyle = '#';   context.lineWidth = .;   context.arc(rubberbandRectangle.left+rubberbandRectangle.width,rubberbandRectangle.top+rubberbandRectangle.height,,,Math.PI*,true);   context.fill();//填充路徑   context.stroke();//填充路徑   context.restore(); } function rect(x, y, w, h, direction){   if(direction){//逆時針     context.moveTo(x, y);     context.lineTo(x, y + h);     context.lineTo(x + w, y + h);     context.lineTo(x + w, y);   }else{//順時針     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('對不起，您的瀏覽器不支持');   } } image.src = ''; image.onload = function () {   console.log(image);   var w,h;   //計算圖片縮放比例   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>    <!--預覽-->    <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>    <!--上傳-->    <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>