用HTML5 Canvas组件实现图像灰度化

来源 | 看云

HTML5已经发布很长一段时间了，发现HTML5中的画布组件功能真的很强大，今天我们用Canvas来实现简单相框和图像灰度化。一起来看看HTML5 Canvas是怎么做到的吧！

1、新建一个html页面，在body标签之间加入

<canvas id="myCanvas" >Gray Filter</canvas>

2、添加一段最简单的JavaScript代码：

window.onload = function() {      var canvas = document.getElementById("myCanvas");     // do something here!!}

从Canvas对象获取重新定位对象某些上下文的代码如下：

var context = canvas.getContext("2d");

在html页面中加入一幅图像的html代码如下：

<img id="imageSource" src="hanjiaren.jpg" alt="Canvas Source" />

var image = document.getElementById("imageSource");

将得到的图像绘制在画布对象中的代码如下：

context.drawImage(image, 0, 0);

从Canvas对象中获取图像编码数据的代码如下：

var canvasData = context.getImageData(0, 0, canvas.width, canvas.height);

读取数值与实现灰度计算的代码如下：

for ( var x = 0; x < canvasData.width; x++) {          for ( var y = 0; y < canvasData.height; y++) {  
            // Index of the pixel in the array              var idx = (x + y * canvasData.width) * 4;              var r = canvasData.data[idx + 0];                  var g = canvasData.data[idx + 1];                  var b = canvasData.data[idx + 2];  
                // calculate gray scale value                  var gray = .299 * r + .587 * g + .114 * b;  
            // assign gray scale value              canvasData.data[idx + 0] = gray; // Red channel              canvasData.data[idx + 1] = gray; // Green channel              canvasData.data[idx + 2] = gray; // Blue channel              canvasData.data[idx + 3] = 255; // Alpha channel  
            // add black border              if(x < 8 || y < 8 || x > (canvasData.width - 8) || y > (canvasData.height - 8))               {                  canvasData.data[idx + 0] = 0;                  canvasData.data[idx + 1] = 0;                  canvasData.data[idx + 2] = 0;              }          }  }

其中计算灰度公式为灰色= 0.299×红色+ 0.578×绿色+ 0.114 *蓝色
读取出来的像素值顺序为RGBA分别代表红色，绿色，蓝色，alpha通道

处理完成的数据要重新载入到Canvas中。代码如下：

context.putImageData(canvasData, 0, 0);

程序最终的效果如下：

完全源代码如下：

<html>  <head>  <script>  window.onload = function() {          var canvas = document.getElementById("myCanvas");          var image = document.getElementById("imageSource");  
        // re-size the canvas deminsion          canvas.width  = image.width;          canvas.height = image.height;  
        // get 2D render object          var context = canvas.getContext("2d");          context.drawImage(image, 0, 0);          var canvasData = context.getImageData(0, 0, canvas.width, canvas.height);          alert(canvasData.width.toString());          alert(canvasData.height.toString());  
        // gray filter          for ( var x = 0; x < canvasData.width; x++) {              for ( var y = 0; y < canvasData.height; y++) {  
                // Index of the pixel in the array                  var idx = (x + y * canvasData.width) * 4;                  var r = canvasData.data[idx + 0];                  var g = canvasData.data[idx + 1];                  var b = canvasData.data[idx + 2];  
                // calculate gray scale value                  var gray = .299 * r + .587 * g + .114 * b;  
                // assign gray scale value                  canvasData.data[idx + 0] = gray; // Red channel                  canvasData.data[idx + 1] = gray; // Green channel                  canvasData.data[idx + 2] = gray; // Blue channel                  canvasData.data[idx + 3] = 255; // Alpha channel  
                // add black border                  if(x < 8 || y < 8 || x > (canvasData.width - 8) || y > (canvasData.height - 8))                   {                      canvasData.data[idx + 0] = 0;                      canvasData.data[idx + 1] = 0;                      canvasData.data[idx + 2] = 0;                  }              }          }          context.putImageData(canvasData, 0, 0); // at coords 0,0      };  </script>  </head>  <body>      <h2>Hello World!</h2>      <img id="imageSource" src="hanjiaren.jpg" alt="Canvas Source" />      <br />      <canvas id="myCanvas" >Gray Filter</canvas>  </body>  </html>