本文給出基於彩色影象直方圖均衡化的OpenCV程式碼與結果示例！

具體內容包含：

1. 灰度影象直方圖均衡化

2. 對RGB三通道各自均衡化後，再組合輸出結果

3. RGB影象轉化為HSI，YUV，YCbCr顏色空間後，對亮度通道進行均衡化運算後再轉回RGB空間

1. /** 
2.  * @function EqualizeHist_Demo.cpp 
3.  * @brief Demo code for equalizeHist function 
4.  * @author OpenCV team 
5.  */
6. #include "opencv2/imgcodecs.hpp"
7. #include "opencv2/highgui/highgui.hpp"
8. #include "opencv2/imgproc/imgproc.hpp"
9. #include <iostream>
10. #include <stdio.h>
11. usingnamespace cv;  
12. usingnamespace std;  
13. // add by frank, 2014-09-25
14. Mat equalizeChannelHist(const Mat & inputImage)  
15. {  
16.     if( inputImage.channels() >= 3 )  
17.     {  
18.         vector<Mat> channels;  
19.         split(inputImage, channels);  
20.         Mat B,G,R;  
21.         equalizeHist( channels[0], B );  
22.         equalizeHist( channels[1], G );  
23.         equalizeHist( channels[2], R );  
24.         vector<Mat> combined;  
25.         combined.push_back(B);  
26.         combined.push_back(G);  
27.         combined.push_back(R);  
28.         Mat result;  
29.         merge(combined, result);  
30.         return result;  
31.     }  
32.     return Mat();  
33. }  
34. Mat equalizeIntensityHist(const Mat & inputImage)  
35. {  
36.     if(inputImage.channels() >= 3)  
37.     {  
38.         Mat ycrcb;  
39.         cvtColor(inputImage, ycrcb, COLOR_BGR2YCrCb);  
40.         vector<Mat> channels;  
41.         split(ycrcb, channels);  
42.         equalizeHist(channels[0], channels[0]);  
43.         Mat result;  
44.         merge(channels,ycrcb);  
45.         cvtColor(ycrcb, result, COLOR_YCrCb2BGR);  
46.         return result;  
47.     }  
48.     return Mat();  
49. }  
50. void getGrayImageHistImage(const Mat & src, Mat & histImage)  
51. {  
52.     Mat hist;  
53.     int histSize = 256;  
54.     calcHist(&src, 1, 0, Mat(), hist, 1, &histSize, 0);  
55.     normalize(hist, hist, 0, histImage.rows, NORM_MINMAX, CV_32F);  
56.     histImage = Scalar::all(255);  
57.     int binW = cvRound((double)histImage.cols/histSize);  
58.     for( int i = 0; i < histSize; i++ )  
59.         rectangle( histImage, Point(i*binW, histImage.rows),  
60.         Point((i+1)*binW, histImage.rows - cvRound(hist.at<float>(i))),  
61.         Scalar::all(0), -1, 8, 0 );  
62. }  
63. int main( int, char** argv )  
64. {  
65.     Mat src, dst;  
66.     Mat intensity_color_dst;  
67.     Mat channel_color_dst;  
68.     constchar* source_gray_window = "Source Gray Image";  
69.     constchar* equalized_gray_window = "Equalized Gray Image";  
70.     constchar* source_color_window = "Source Color Image";  
71.     constchar* equalized_intensity_color_window = "Equalized Intensity Color Image";  
72.     constchar* equalized_channels_color_window = "Equalized Channels Color Image";  
73.     /// Load image
74.     src = imread( argv[1], 1 );  
75.     if( src.empty() )  
76.     {   
77.         cout<<"Usage: ./Histogram_Demo <path_to_image>"<<endl;  
78.         return -1;  
79.     }  
80.     /// color image intensity equalization
81.     {  
82.         intensity_color_dst = equalizeIntensityHist(src);  
83.         namedWindow( source_color_window, WINDOW_AUTOSIZE );  
84.         namedWindow( equalized_intensity_color_window, WINDOW_AUTOSIZE );  
85.         imshow( source_color_window, src );  
86.         imshow( equalized_intensity_color_window, intensity_color_dst );  
87.     }  
88.     /// color image each channel equalization
89.     {  
90.         channel_color_dst = equalizeChannelHist(src);  
91.         namedWindow( equalized_channels_color_window, WINDOW_AUTOSIZE );  
92.         imshow( equalized_channels_color_window, channel_color_dst );  
93.     }  
94.     /// gray image equalization
95.     {  
96.         cvtColor( src, src, COLOR_BGR2GRAY );  
97.         equalizeHist( src, dst );  
98.         namedWindow( source_gray_window, WINDOW_AUTOSIZE );  
99.         namedWindow( equalized_gray_window, WINDOW_AUTOSIZE );  
100.         imshow( source_gray_window, src );  
101.         imshow( equalized_gray_window, dst );  
102.         /// get source gray image Histogram
103.         Mat graySrc_histImage = Mat::ones(200, 260, CV_8U)*255;  
104.         getGrayImageHistImage(src, graySrc_histImage);  
105.         imshow("source gray image histogram", graySrc_histImage);  
106.         /// get equalized gray image Histogram
107.         Mat grayDst_histImage = Mat::ones(200, 260, CV_8U)*255;  
108.         getGrayImageHistImage(dst, grayDst_histImage);  
109.         imshow("Equalized gray image histogram", grayDst_histImage);  
110.     }  
111.     /// Wait until user exits the program
112.     waitKey(0);  
113.     return 0;  
114. }  

/**
 * @function EqualizeHist_Demo.cpp
 * @brief Demo code for equalizeHist function
 * @author OpenCV team
 */

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>

using namespace cv;
using namespace std;

// add by frank, 2014-09-25
Mat equalizeChannelHist(const Mat & inputImage)
{
	if( inputImage.channels() >= 3 )
	{
		vector<Mat> channels;
		split(inputImage, channels);

		Mat B,G,R;

		equalizeHist( channels[0], B );
		equalizeHist( channels[1], G );
		equalizeHist( channels[2], R );

		vector<Mat> combined;
		combined.push_back(B);
		combined.push_back(G);
		combined.push_back(R);

		Mat result;
		merge(combined, result);

		return result;
	}

	return Mat();
}

Mat equalizeIntensityHist(const Mat & inputImage)
{
	if(inputImage.channels() >= 3)
	{
		Mat ycrcb;

		cvtColor(inputImage, ycrcb, COLOR_BGR2YCrCb);

		vector<Mat> channels;
		split(ycrcb, channels);

		equalizeHist(channels[0], channels[0]);

		Mat result;
		merge(channels,ycrcb);

		cvtColor(ycrcb, result, COLOR_YCrCb2BGR);

		return result;
	}

	return Mat();
}

void getGrayImageHistImage(const Mat & src, Mat & histImage)
{
	Mat hist;
	int histSize = 256;

	calcHist(&src, 1, 0, Mat(), hist, 1, &histSize, 0);
	normalize(hist, hist, 0, histImage.rows, NORM_MINMAX, CV_32F);

	histImage = Scalar::all(255);
	int binW = cvRound((double)histImage.cols/histSize);

	for( int i = 0; i < histSize; i++ )
		rectangle( histImage, Point(i*binW, histImage.rows),
		Point((i+1)*binW, histImage.rows - cvRound(hist.at<float>(i))),
		Scalar::all(0), -1, 8, 0 );
}


int main( int, char** argv )
{
	Mat src, dst;
	Mat intensity_color_dst;
	Mat channel_color_dst;

	const char* source_gray_window = "Source Gray Image";
	const char* equalized_gray_window = "Equalized Gray Image";
	const char* source_color_window = "Source Color Image";
	const char* equalized_intensity_color_window = "Equalized Intensity Color Image";
	const char* equalized_channels_color_window = "Equalized Channels Color Image";

	/// Load image
	src = imread( argv[1], 1 );

	if( src.empty() )
	{ 
		cout<<"Usage: ./Histogram_Demo <path_to_image>"<<endl;
		return -1;
	}

	/// color image intensity equalization
	{
		intensity_color_dst = equalizeIntensityHist(src);
		
		namedWindow( source_color_window, WINDOW_AUTOSIZE );
		namedWindow( equalized_intensity_color_window, WINDOW_AUTOSIZE );

		imshow( source_color_window, src );
		imshow( equalized_intensity_color_window, intensity_color_dst );
	}

	/// color image each channel equalization
	{
		channel_color_dst = equalizeChannelHist(src);
		namedWindow( equalized_channels_color_window, WINDOW_AUTOSIZE );
		imshow( equalized_channels_color_window, channel_color_dst );
	}

	/// gray image equalization
	{
		cvtColor( src, src, COLOR_BGR2GRAY );
		equalizeHist( src, dst );

		namedWindow( source_gray_window, WINDOW_AUTOSIZE );
		namedWindow( equalized_gray_window, WINDOW_AUTOSIZE );

		imshow( source_gray_window, src );
		imshow( equalized_gray_window, dst );

		/// get source gray image Histogram
		Mat graySrc_histImage = Mat::ones(200, 260, CV_8U)*255;
		getGrayImageHistImage(src, graySrc_histImage);
		imshow("source gray image histogram", graySrc_histImage);

		/// get equalized gray image Histogram
		Mat grayDst_histImage = Mat::ones(200, 260, CV_8U)*255;
		getGrayImageHistImage(dst, grayDst_histImage);
		imshow("Equalized gray image histogram", grayDst_histImage);
	}

	/// Wait until user exits the program
	waitKey(0);

	return 0;
}

執行結果，如下圖所示：

           原灰度影象                                                          直方圖均衡化增強後圖像

             原彩色影象               RGB各通道直方圖均衡化後圖像           YCbCr 亮度通道Y直方圖均衡化後圖像

分RGB通道均衡化後圖像顏色有失真情況，而亮度通道均衡化結果不會，主要原因：

Histogram equalization is a non-linear process. Channel splitting and equalizing each channel separately is not the proper way for equalization of contrast. Equalization involves Intensity values of the image not the color components. So for a simple RGB color image, HE should not be applied individually on each channel. Rather, it should be applied such that intensity values are equalized without disturbing the color balance of the image. So, the first step is to convert the color space of the image from RGB into one of the color space which separates intensity values from color components. Some of these are:
  HSV/HLS
  YUV
  YCbCr
Convert the image from RGB to one of the above mentioned color spaces. YCbCr is preferred as it is designed for digital images.Perform HE of the intensity plane Y. Convert the image back to RGB.