HTML5组件Canvas实现图像灰度化(步骤+实例效果)

HTML5组件Canvas实现图像灰度化(步骤+实例效果)

新建一个html页面，在body tag之间加入 

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

添加一段最简单的JavaScript 脚本 

<pre name="code" class="javascript">window.onload = function() { 
var canvas = document.getElementById("myCanvas"); 
<span style="white-space:pre"> </span>// TODO: do something here 
}

从Canvas对象获取绘制对象上下文Context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" />

从html img对象中获取image 对象的javascript代码如下：

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

将得到的图像绘制在Canvas对象中的代码如下： 

context.drawImage(image, 0, 0);

从Canvas对象中获取图像像素数据的代码如下： 

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

读取像素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 gray color = 0.299 × red color + 0.578 × green color + 0.114 * blue color 
读取出来的像素值顺序为RGBA 分别代表red color, green color, blue color, alpha channel 

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： 

 
 
<script> 
window.onload = function() { 
var canvas = document.getElementById("myCanvas"); 
var image = document.getElementById(&quot;imageSource&quot;); 
// re-size the canvas deminsion 
canvas.width = image.width; 
canvas.height = image.height; 
// get 2D render object 
var context = canvas.getContext(&quot;2d&quot;); 
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 &lt; canvasData.width; x++) { 
for ( var y = 0; y &lt; 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 &lt; 8 || y &lt; 8 || x &gt; (canvasData.width - 8) || y &gt; (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> 
 
 
Hello World!
 
<img id="imageSource" src="hanjiaren.jpg" alt="Canvas Source" /> 

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

代码中的文件可以替换任意你想要看到的图片文件 
HTML5, 原来如此神奇。程序在google浏览器中测试通过， 
最后的忠告，千万不要在本地尝试运行上面的代码，google浏览器的安全检查会自动阻止从浏览器中读写非domain的文件 
最好在tomcat或者任意个web container的server上发布以后从google浏览器查看效果即可。

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