Explain in detail why the number of pixels with a gray value of 255 calculated by the opencv histogram calculation function calchist() is 0

In the use of OpenCV Histogram calculation function calcHist() when , Gray value found to be 255 The number of pixels is always 0.
Even if the gray value in the image is 255 The number of pixels of is not 0, Use OpenCV Histogram calculation function calcHist() The calculated result is also 0.
An example is as follows ：

//OpenCV edition 3.0
// The author WeChat /QQ 2487872782
// If you have any questions, you can contact the author 
// Welcome to the image processing exchange group , Group number 271891601

#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"

#include <iostream>

int main( )
{
    
	//  Read the source image and convert it into gray image 
	cv::Mat srcImage = cv::imread("F:/material/images/P0027-coin-01.png");
	//  Judge whether the file is read correctly 
	if( !srcImage.data ) 
		return 1;

	cv::Mat ImageGray;
	cv::cvtColor(srcImage,ImageGray,CV_BGR2GRAY);


	//  Define histogram parameters 
    const int channels[1]={
    0};
    const int histSize[1]={
    256};
    float pranges[2]={
    0,255};
    const float* ranges[1]={
    pranges};

	cv::MatND hist;

	cv::calcHist(&ImageGray,1,channels,cv::Mat(),hist,1,histSize,ranges);

	std::cout<<hist<<std::endl;

	//  Find the maximum value of image pixels 
    double maxVal=0;
    cv::minMaxLoc(ImageGray, 0, &maxVal, 0, 0);
	std::cout<<" The maximum value of image pixels is ："<<maxVal<<std::endl;

	return 0;
}

Pictures used in the program “P0027-coin-01.png” Download link ：
link ：https://pan.baidu.com/s/1k13r2DdhEXuXWlxV-IbxNA Extraction code ：kwgo
The results of the program are shown in the following figure ：

As can be seen from the screenshot above , function calcHist() The calculated gray value is 255 The number of pixels is 0, But from “ The maximum value of image pixels is ：255” It can be seen that , The number is definitely not 0, At least greater than or equal to 1 Of .
How could this happen ？ Look up the function calcHist() Official documents of , The prototype is as follows ：

void cv::calcHist	(const Mat * images,
int 	nimages,
const int * 	channels,
InputArray 	mask,
OutputArray 	hist,
int 	dims,
const int * 	histSize,
const float ** 	ranges,
bool 	uniform = true,
bool 	accumulate = false 
)		

Notice the 8 Parameters “ranges” Explanation ：
“ranges Array of the dims arrays of the histogram bin boundaries in each dimension. When the histogram is uniform ( uniform =true), then for each dimension i it is enough to specify the lower (inclusive) boundary of the 0-th histogram bin and the upper (exclusive) boundary”
Notice the words I marked in red in the above paragraph . From this passage we can see , This ranges The determined gray statistical range interval is the left closed right open interval .
Why? OpenCV This function of does not put this ranges The determined gray statistical range interval is set to Left and right closed What about the interval ？
This starts with the drawing of histogram .
Let's start with a simple gray histogram .
Now let's count the gray histogram parameters drawn as follows ：
Number of statistical intervals ：2, In the code above histSize[1]={2};
Statistical range of gray value ：0~4, In the code above pranges[2]={0,4};
In the case of such parameters , Our histogram puts 0~4 The gray range of is divided into two statistical intervals , Respectively 0~2 and 2~4, As shown in the figure below ：

The key here is to understand the number of gray values in the two statistical intervals .
In the ① Within two intervals , The statistics shows that the gray value is 0 and 1 The number of pixels , Please note that , Excluding the right border 2 The number of pixels .
In the ② Within two intervals , The statistics shows that the gray value is 2 and 3 The number of pixels , Please note that , Excluding the right border 4 The number of pixels .
After understanding the contents marked in red above , You should understand , If my grayscale range is 0~4, So when I draw the histogram, I don't need the right boundary 4 Number of pixels , So I don't need to count the corresponding number of pixels .
Empathy , If my grayscale range is 0~255, So when I draw the histogram, I don't need the right boundary 255 Number of pixels , So I don't need to count the corresponding number of pixels . That's why functions calcHist() Calculated 255 The number of gray value pixels is 0 Why .
Sometimes we need to know that the gray value is 255 The number of pixels , Then what shall I do? ？
It's simple , Write the gray value as 0~266 That's it , In the code above

float pranges[2]={
    0,255};

Written as

float pranges[2]={
    0,256};

Let's change the above code to the following code ：

//OpenCV edition 3.0
// The author WeChat /QQ 2487872782
// If you have any questions, you can contact the author 
// Image processing development materials 、 Please add... For the exchange of image processing technology QQ Group , Group number  271891601

#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"

#include <iostream>

int main( )
{
    
	//  Read the source image and convert it into gray image 
	cv::Mat srcImage = cv::imread("F:/material/images/P0027-coin-01.png");
	//  Judge whether the file is read correctly 
	if( !srcImage.data ) 
		return 1;

	cv::Mat ImageGray;
	cv::cvtColor(srcImage,ImageGray,CV_BGR2GRAY);


	//  Define histogram parameters 
    const int channels[1]={
    0};
    const int histSize[1]={
    256};
    float pranges[2]={
    0,256};
    const float* ranges[1]={
    pranges};

	cv::MatND hist;

	cv::calcHist(&ImageGray,1,channels,cv::Mat(),hist,1,histSize,ranges);

	std::cout<<hist<<std::endl;

	//  Find the maximum value of image pixels 
    double maxVal=0;
    cv::minMaxLoc(ImageGray, 0, &maxVal, 0, 0);
	std::cout<<" The maximum value of image pixels is ："<<maxVal<<std::endl;

	return 0;
}

Here we are. , I believe you will understand “ Why? OpenCV Histogram calculation function calcHist() The calculated gray value is 255 The number of pixels is 0”. If you still don't understand , You can add the blogger's wechat /QQ 2487872782 communication .