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="處理圖片" />
<input id="create" type="button" value="生成圖片" />
<div id="result"></div>

//JavaScript
window.onload = function(){
    var canvas = document.getElementById("canvas"),   //獲取Canvas畫布對象
        context = canvas.getContext('2d');  //獲取2D上下文對象，大多數Canvas API均為此對象方法
    var image = new Image();  //定義一個圖片對象
    image.src = 'imgs/img.jpg';  
    image.onload = function(){  //此處必須注意！后面所有操作均需在圖片加載成功后執行，否則圖片將處理無效
        context.drawImage(image,0,0);  //將圖片從Canvas畫布的左上角(0,0)位置開始繪制，大小默認為圖片實際大小
        var handle = document.getElementById("handle");
        var create = document.getElementById("create");
        handle.onclick = function(){  // 單擊“處理圖片”按鈕，處理圖片
            var imgData = context.getImageData(0,0,canvas.width,canvas.height);   //獲取圖片數據對象
            var data = imgData.data;  //獲取圖片數據數組，該數組中每個像素用4個元素來保存，分別表示紅、綠、藍和透明度值
            var average = 0;
            for (var i = 0; i < data.length; i+=4) {
                average = Math.floor((data[i]+data[i+1]+data[i+2])/3);  //將紅、綠、藍色值求平均值后得到灰度值
                data[i] = data[i+1] = data[i+2] = average;  將每個像素點的色值重寫
            }
            imgData.data = data;
            context.putImageData(imgData,0,0);  //將處理后的圖像數據重寫至Canvas畫布，此時畫布中圖像變為黑白色
        };
        create.onclick = function(){  // 單擊“生成圖片”按鈕，導出圖片
            var imgSrc = canvas.toDataURL();  //獲取圖片的DataURL
            var newImg = new Image();
            var result = document.getElementById("result");
            newImg.src = imgSrc;  //將圖片路徑賦值給src
            result.innerHTML = '';
            result.appendChild(newImg);
        };
    };
};