context.putImageData(imgData,x,y,dirtyX,dirtyY,dirtyWidth,dirtyHeight)

var dataURL = canvas.toDataURL(type, encoderOptions);

<!--HTML-->
<canvas id="canvas" width="600" height="600"></canvas>
<input id="handle" type="button" value="处理图片" />
<input id="create" type="button" value="生成图片" />
<div id="result"></div>

//JavaScript
window.onload = function(){
    var canvas = document.getElementById("canvas"),   //获取Canvas画布对象
        context = canvas.getContext('2d');  //获取2D上下文对象，大多数Canvas API均为此对象方法
    var image = new Image();  //定义一个图片对象
    image.src = 'imgs/img.jpg';  
    image.onload = function(){  //此处必须注意！后面所有操作均需在图片加载成功后执行，否则图片将处理无效
        context.drawImage(image,0,0);  //将图片从Canvas画布的左上角(0,0)位置开始绘制，大小默认为图片实际大小
        var handle = document.getElementById("handle");
        var create = document.getElementById("create");
        handle.onclick = function(){  // 单击“处理图片”按钮，处理图片
            var imgData = context.getImageData(0,0,canvas.width,canvas.height);   //获取图片数据对象
            var data = imgData.data;  //获取图片数据数组，该数组中每个像素用4个元素来保存，分别表示红、绿、蓝和透明度值
            var average = 0;
            for (var i = 0; i < data.length; i+=4) {
                average = Math.floor((data[i]+data[i+1]+data[i+2])/3);  //将红、绿、蓝色值求平均值后得到灰度值
                data[i] = data[i+1] = data[i+2] = average;  将每个像素点的色值重写
            }
            imgData.data = data;
            context.putImageData(imgData,0,0);  //将处理后的图像数据重写至Canvas画布，此时画布中图像变为黑白色
        };
        create.onclick = function(){  // 单击“生成图片”按钮，导出图片
            var imgSrc = canvas.toDataURL();  //获取图片的DataURL
            var newImg = new Image();
            var result = document.getElementById("result");
            newImg.src = imgSrc;  //将图片路径赋值给src
            result.innerHTML = '';
            result.appendChild(newImg);
        };
    };
};