灰度

代码运行

<!DOCTYPE HTML>
<html>
  <head>
    <title>灰度</title>
    <style>
      body {
        margin: 0px;
        padding: 0px;
      }
    </style>
  </head>
  <body>
    <canvas id="myCanvas" width="578" height="400"></canvas>
    <script>
      function drawImage(imageObj) {
        var canvas = document.getElementById('myCanvas');
        var context = canvas.getContext('2d');
        var x = 69;
        var y = 50;

context.drawImage(imageObj, x, y);

var imageData = context.getImageData(x, y, imageObj.width, imageObj.height);
        var data = imageData.data;

for(var i = 0; i < data.length; i  = 4) {
          var brightness = 0.34 * data[i]   0.5 * data[i   1]   0.16 * data[i   2];
          // red
          data[i] = brightness;
          // green
          data[i   1] = brightness;
          // blue
          data[i   2] = brightness;
        }

// overwrite original image
        context.putImageData(imageData, x, y);
      }
      
      var imageObj = new Image();
      imageObj.onload = function() {
        drawImage(this);
      };
      imageObj.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
    </script>
  </body>
</html>