最近看到微博头像上传功能很感兴趣，于是就使用canvas写了一个，本文写的不好还请见谅。本程序目前在谷歌浏览器和火狐浏览器测试可用，ie浏览器无法支持。 

因为ie的安全机制不允许img使用本地路径，所以若想ie支持本程序，必须先将图片上传，然后给img对象上传后的图片地址。

我在这里没写，是因为暂时没写上传功能的后端代码,并且还不确定有没有更好的解决办法。

如下是新浪的

如下是我做的截取部分

代码：

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(); };

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