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Implementation code sharing of html5 canvas image processing

黄舟

Mar 25, 2017 pm 03:38 PM

canvas html5 Image Processing

I accidentally read "pro html5 programming" two days ago and found that html5 can also support image processing. Let me explain it a little here.

Similar to matlab processing images, the form of image matrix is also used here.

Here is a simple example:

<!DOCTYPE html>   
<html>
<head>   
<title>canvas图像处理</title>  
</head>  
<body>  
<h1>canvas</h1>  
<canvas id="canvas1" width="200" height="150">是时候更换浏览器了<a href="http://firefox.com.cn/download/">点击下载firefox</a></canvas> 
<script>
    var canvas1=document.getElementById(&#39;canvas1&#39;);
    var context1=canvas1.getContext(&#39;2d&#39;);
    image=new Image();
    image.src="z.JPG";
    image.onload=function(){
	context1.drawImage(image,0,0);//绘制原始图像，（0,0）表示图像的左上角位与canvas画布的位置
    }
</script>
<br/>
<button onclick="draw()">图像的反转</button>
<br/>
<canvas id="canvas2" width="200" height="150"></canvas> 
<script>
	function draw(){
		var canvas2=document.getElementById(&#39;canvas2&#39;);
		var context2=canvas2.getContext(&#39;2d&#39;);
			var imagedata=context1.getImageData(0,0,image.width,image.height);
			var imagedata1=context2.createImageData(image.width,image.height);
			for(var j=0;j<image.height;j+=1)
			for(var i=0;i<image.width;i+=1){
			  k=4*(image.width*j+i);
			  imagedata1.data[k+0]=255-imagedata.data[k+0];
			  imagedata1.data[k+1]=255-imagedata.data[k+1];
			  imagedata1.data[k+2]=255-imagedata.data[k+2];
              imagedata1.data[k+3]=255;
			}
			context2.putImageData(imagedata1,0,0);
	}
</script> 
</body>  
</html>

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1) Canvas call of html5

var canvas1=document.getElementById(&#39;canvas1&#39;);//获取canvas元素
var context1=canvas.getContext(&#39;2d&#39;);//此时获取到canvas图像上下文

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2) Create an image and draw the original image

            image=new Image();//创建image对象
	    image.src="z.JPG";//image的地址
	    image.onload=function(){
		context1.drawImage(image,0,0);//绘制原始图像，（0,0）表示图像的左上角位与canvas画布的位置
        }

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3 ) Obtain the rgba matrix of the image and operate

var imagedata=context1.getImageData(0,0,image.width,image.height);
//getImageData(x1,y1,x2,y2)获取图像的rgba矩阵，其中截取图像的大小为(x1,y1)-(x2,y2) 的矩阵
var imagedata1=context2.createImageData(image.width,image.height);
//createImageData(x,y):创建宽高分别为x,y的图像矩阵
   for(var j=0;j<image.height;j+=1)
	for(var i=0;i<image.width;i+=1){
         	  k=4*(image.width*j+i);
		  imagedata1.data[k+0]=255-imagedata.data[k+0];
		  imagedata1.data[k+1]=255-imagedata.data[k+1];
		  imagedata1.data[k+2]=255-imagedata.data[k+2];
                  imagedata1.data[k+3]=255;
		}
	context2.putImageData(imagedata1,0,0);
//putImageData(image,0,0):将image矩阵的添加为context 原矩阵的一部分，起点为(0,0)
}

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The following is a detailed description of the storage form of html5 images:

There are four pixels in each pixel in the matrix The channels store the values of r/g/b/a respectively. (Four values are arranged continuously in order, which is a one-dimensional matrix)

So every two pixels are separated by 4 bits. When calculating,

k=4*(image.width*j+i);为像素点（i,j）的位置，

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imagedata1.data[k+0]表示R分量，依次类推，其中剩下的分别为G、B分量还有透明度。

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In this way, the above program realizes a simple image reversal function.
The effect is as follows:

In addition, it should be noted that the getImageData() function may involve cross-domain problems, so it is recommended to configure the apache environment and place the html Go to its root directory to operate.

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