<!--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(shù)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(zhí)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(shù)據對象
      var data = imgData.data; //獲取圖片數(shù)據數(shù)組，該數(shù)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(shù)據重寫至Canvas畫布，此時畫布中圖像變?yōu)楹诎咨?
    };
    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);
    };
  };
};