HTML5组件Canvas实现图像灰度化(步骤+实例效果)

HTML5教程手机阅读

　　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

　　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：

<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" />

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