1. 新建一个html页面，在body tag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。
<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>
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。