用HTML5 Canvas组件实现图像灰度化
web前端开发
共 4322字,需浏览 9分钟
·
2021-03-14 14:11
来源 | 看云
1、新建一个html页面,在body标签之间加入
<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对象获取重新定位对象某些上下文的代码如下:
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" />
var image = document.getElementById("imageSource");
将得到的图像绘制在画布对象中的代码如下:
context.drawImage(image, 0, 0);
从Canvas对象中获取图像编码数据的代码如下:
var canvasData = context.getImageData(0, 0, canvas.width, canvas.height);
读取数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灰色= 0.299×红色+ 0.578×绿色+ 0.114 *蓝色
读取出来的像素值顺序为RGBA分别代表红色,绿色,蓝色,alpha通道
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" />
<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。
程序在谷歌浏览器中测试通过千万不要在本地尝试运行上面的代码,谷歌
浏览器的安全检查会自动阻止从浏览器中识别非域的文件最好在tomcat或
者任意一个Web容器的服务器上发布以后从谷歌浏览器查看效果即可。
评论