【模式识别】K-近邻分类算法KNN

一时失言乱红尘 2022-08-26 14:22 152阅读 0赞

K-近邻（K-Nearest Neighbors, KNN）是一种很好理解的分类算法，简单说来就是从训练样本中找出K个与其最相近的样本，然后看这K个样本中哪个类别的样本多，则待判定的值（或说抽样）就属于这个类别。

## KNN算法的步骤 ##

*  计算已知类别数据集中每个点与当前点的距离；
 *  选取与当前点距离最小的K个点；
 *  统计前K个点中每个类别的样本出现的频率；
 *  返回前K个点出现频率最高的类别作为当前点的预测分类。

## OpenCV中使用CvKNearest ##

OpenCV中实现CvKNearest类可以实现简单的KNN训练和预测。

int main()
    {
    	float labels[10] = {0,0,0,0,0,1,1,1,1,1};
    	Mat labelsMat(10, 1, CV_32FC1, labels);
    	cout<<labelsMat<<endl;
    	float trainingData[10][2];
    	srand(time(0)); 
    	for(int i=0;i<5;i++){
    		trainingData[i][0] = rand()%255+1;
    		trainingData[i][1] = rand()%255+1;
    		trainingData[i+5][0] = rand()%255+255;
    		trainingData[i+5][1] = rand()%255+255;
    	}
    	Mat trainingDataMat(10, 2, CV_32FC1, trainingData);
    	cout<<trainingDataMat<<endl;
    	CvKNearest knn;
    	knn.train(trainingDataMat,labelsMat,Mat(), false, 2 );
    	// Data for visual representation
    	int width = 512, height = 512;
    	Mat image = Mat::zeros(height, width, CV_8UC3);
    	Vec3b green(0,255,0), blue (255,0,0);
    
    	for (int i = 0; i < image.rows; ++i){
    		for (int j = 0; j < image.cols; ++j){
    			const Mat sampleMat = (Mat_<float>(1,2) << i,j);
    			Mat response;
    			float result = knn.find_nearest(sampleMat,1);
    			if (result !=0){
    				image.at<Vec3b>(j, i)  = green;
    			}
    			else  
    				image.at<Vec3b>(j, i)  = blue;
    		}
    	}
    
    		// Show the training data
    		for(int i=0;i<5;i++){
    			circle(	image, Point(trainingData[i][0],  trainingData[i][1]), 
    				5, Scalar(  0,   0,   0), -1, 8);
    			circle(	image, Point(trainingData[i+5][0],  trainingData[i+5][1]), 
    				5, Scalar(255, 255, 255), -1, 8);
    		}
    		imshow("KNN Simple Example", image); // show it to the user
    		waitKey(10000);
    
    }

使用的是之前[BP神经网络][BP]中的例子，分类结果如下：

![20140415201435109][]

预测函数find\_nearest()除了输入sample参数外还有些其他的参数：

float CvKNearest::find_nearest(const Mat& samples, int k, Mat* results=0, 
    const float** neighbors=0, Mat* neighborResponses=0, Mat* dist=0 )

![20140415201848234][]

即，samples为样本数\*特征数的浮点矩阵；K为寻找最近点的个数；results与预测结果；neibhbors为k\*样本数的指针数组（输入为const，实在不知为何如此设计）；neighborResponse为样本数\*k的每个样本K个近邻的输出值；dist为样本数\*k的每个样本K个近邻的距离。

## 另一个例子 ##

OpenCV refman也提供了一个类似的示例，使用CvMat格式的输入参数：

int main( int argc, char** argv )
    {
    	const int K = 10;
    	int i, j, k, accuracy;
    	float response;
    	int train_sample_count = 100;
    	CvRNG rng_state = cvRNG(-1);
    	CvMat* trainData = cvCreateMat( train_sample_count, 2, CV_32FC1 );
    	CvMat* trainClasses = cvCreateMat( train_sample_count, 1, CV_32FC1 );
    	IplImage* img = cvCreateImage( cvSize( 500, 500 ), 8, 3 );
    	float _sample[2];
    	CvMat sample = cvMat( 1, 2, CV_32FC1, _sample );
    	cvZero( img );
    	CvMat trainData1, trainData2, trainClasses1, trainClasses2;
    	// form the training samples
    	cvGetRows( trainData, &trainData1, 0, train_sample_count/2 );
    	cvRandArr( &rng_state, &trainData1, CV_RAND_NORMAL, cvScalar(200,200), cvScalar(50,50) );
    	cvGetRows( trainData, &trainData2, train_sample_count/2, train_sample_count );
    	cvRandArr( &rng_state, &trainData2, CV_RAND_NORMAL, cvScalar(300,300), cvScalar(50,50) );
    	cvGetRows( trainClasses, &trainClasses1, 0, train_sample_count/2 );
    	cvSet( &trainClasses1, cvScalar(1) );
    	cvGetRows( trainClasses, &trainClasses2, train_sample_count/2, train_sample_count );
    	cvSet( &trainClasses2, cvScalar(2) );
    	// learn classifier
    	CvKNearest knn( trainData, trainClasses, 0, false, K );
    	CvMat* nearests = cvCreateMat( 1, K, CV_32FC1);
    	for( i = 0; i < img->height; i++ )
    	{
    		for( j = 0; j < img->width; j++ )
    		{
    			sample.data.fl[0] = (float)j;
    			sample.data.fl[1] = (float)i;
    			// estimate the response and get the neighbors’ labels
    			response = knn.find_nearest(&sample,K,0,0,nearests,0);
    			// compute the number of neighbors representing the majority
    			for( k = 0, accuracy = 0; k < K; k++ )
    			{
    				if( nearests->data.fl[k] == response)
    					accuracy++;
    			}
    			// highlight the pixel depending on the accuracy (or confidence)
    			cvSet2D( img, i, j, response == 1 ?
    				(accuracy > 5 ? CV_RGB(180,0,0) : CV_RGB(180,120,0)) :
    				(accuracy > 5 ? CV_RGB(0,180,0) : CV_RGB(120,120,0)) );
    		}
    	}
    	// display the original training samples
    	for( i = 0; i < train_sample_count/2; i++ )
    	{
    		CvPoint pt;
    		pt.x = cvRound(trainData1.data.fl[i*2]);
    		pt.y = cvRound(trainData1.data.fl[i*2+1]);
    		cvCircle( img, pt, 2, CV_RGB(255,0,0), CV_FILLED );
    		pt.x = cvRound(trainData2.data.fl[i*2]);
    		pt.y = cvRound(trainData2.data.fl[i*2+1]);
    		cvCircle( img, pt, 2, CV_RGB(0,255,0), CV_FILLED );
    	}
    	cvNamedWindow( "classifier result", 1 );
    	cvShowImage( "classifier result", img );
    	cvWaitKey(0);
    	cvReleaseMat( &trainClasses );
    	cvReleaseMat( &trainData );
    	return 0;
    }

分类结果：

![20140415201457109][]

KNN的思想很好理解，也非常容易实现，同时分类结果较高，对异常值不敏感。但计算复杂度较高，不适于大数据的分类问题。

### （转载请注明作者和出处：[http://blog.csdn.net/xiaowei\_cqu][http_blog.csdn.net_xiaowei_cqu] 未经允许请勿用于商业用途） ###

[BP]: http://blog.csdn.net/xiaowei_cqu/article/details/9027617
[20140415201435109]: /images/20220824/19b4ee4c8e744aa78fabbd67e761fda3.png
[20140415201848234]: /images/20220824/086c5d5a3c8a4d7fa709947d467367b6.png
[20140415201457109]: /images/20220824/a94a258f6aae4a4c84b68030cf245704.png
[http_blog.csdn.net_xiaowei_cqu]: http://blog.csdn.net/xiaowei_cqu