HTML5 組件Canvas實現圖像灰度化 · HTML5 Canvas編程

HTML5發布已經有很長一段時間了，一直以來從來沒有仔細的看過，過年剛來隨便看看發現HTML5中的Canvas組件功能是如此的強大，不怪很多牛人預言Flash已死，死不死不是我要關心的，我關心的Canvas可以很輕松在網頁中實現簡單相框和圖像灰度化。一起來看看HTML5 Canvas是怎么做到的吧！ **1. 新建一個html頁面，在body tag之間加入** ~~~ <canvas id="myCanvas" >Gray Filter</canvas> ~~~ **2. 添加一段最簡單的JavaScript代碼：** ~~~ window.onload = function() { var canvas = document.getElementById("myCanvas"); // do something here!! } ~~~ **從Canvas對象獲取繪制對象上下文Context的代碼如下：** ~~~ var context = canvas.getContext("2d"); ~~~ **在html頁面中加入一幅圖像的html代碼如下:** ~~~ <img id="imageSource" src="hanjiaren.jpg" alt="Canvas Source" /> ~~~ **從html?img對象中獲取image?對象的javascript代碼如下：** ~~~ var image = document.getElementById("imageSource"); ~~~ **將得到的圖像繪制在Canvas對象中的代碼如下:** ~~~ context.drawImage(image, 0, 0); ~~~ **從Canvas對象中獲取圖像像素數據的代碼如下：** ~~~ var canvasData = context.getImageData(0, 0, canvas.width, canvas.height); ~~~ **讀取像素值與實現灰度計算的代碼如下：** ~~~ 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為RGBA?分別代表red?color,?green?color,?blue?color,?alpha?channel 處理完成的數據要重新載入到Canvas中。代碼如下： ~~~ context.putImageData(canvasData, 0, 0); ~~~ **程序最終的效果如下：** ![](https://box.kancloud.cn/2016-05-17_573adb34b5694.gif) 完全源代碼如下： ~~~ <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" /> <br /> <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瀏覽器中測試通過千萬不要在本地嘗試運行上面的代碼，google 瀏覽器的安全檢查會自動阻止從瀏覽器中讀寫非domain的文件最好在tomcat或者任意個web?container的server上發布以后從google瀏覽器查看效果即可。