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新建一個(gè)html頁面，在body tag之間加入 復(fù)制代碼 代碼如下:<canvas id="myCanvas" >Gray Filter</canvas> 添加一段最簡單的JavaScript 腳本 復(fù)制代碼 代碼如下:<pre name="code" class="javascript">window.onload = function() { var canvas = document.getElementById("myCanvas"); <span style="white-space:pre"> </span>// TODO: do something here } 從Canvas對(duì)象獲取繪制對(duì)象上下文Context的代碼如下： 復(fù)制代碼 代碼如下:var context = canvas.getContext("2d"); 在html頁面中加入一幅圖像的html代碼如下 復(fù)制代碼 代碼如下:<img id="imageSource" src="hanjiaren.jpg" alt="Canvas Source" /> 從html img對(duì)象中獲取image 對(duì)象的javascript代碼如下： 復(fù)制代碼 代碼如下:var image = document.getElementById("imageSource"); 將得到的圖像繪制在Canvas對(duì)象中的代碼如下： 復(fù)制代碼 代碼如下:context.drawImage(image, 0, 0); 從Canvas對(duì)象中獲取圖像像素?cái)?shù)據(jù)的代碼如下： 復(fù)制代碼 代碼如下:var canvasData = context.getImageData(0, 0, canvas.width, canvas.height); 讀取像素值與實(shí)現(xiàn)灰度計(jì)算的代碼如下： 復(fù)制代碼 代碼如下: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; } } } 其中計(jì)算灰度公式為 gray color = 0.299 × red color + 0.578 × green color + 0.114 * blue color 讀取出來的像素值順序?yàn)镽GBA 分別代表red color, green color, blue color, alpha channel 處理完成的數(shù)據(jù)要重新載入到Canvas中。代碼如下： context.putImageData(canvasData, 0, 0); 程序最終的效果如下：  完全源代碼如下： 復(fù)制代碼 代碼如下:<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> 代碼中的文件可以替換任意你想要看到的圖片文件 HTML5, 原來如此神奇。程序在google瀏覽器中測試通過， 最后的忠告，千萬不要在本地嘗試運(yùn)行上面的代碼，google瀏覽器的安全檢查會(huì)自動(dòng)阻止從瀏覽器中讀寫非domain的文件 最好在tomcat或者任意個(gè)web container的server上發(fā)布以后從google瀏覽器查看效果即可。
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