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

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.οnchange=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();

};

html

CarlZhang

以上代码很简单吧，附有注释，有不同见解的朋友，欢迎给我留言，共同交流学习进步。欲了解更多有关canvas头像截取上传问题，请持续关注本站，本站每天都有新的内容更新。