How to get Intensity Histogram of an Image

 

This blog shows how to get intensity histogram of an image.

Language: C++
Libraries: OpenCV

I use Eclipse to develop the code. If you are new to OpenCV and Eclipse, Please follow my previous blogs.

How to install OpenCV

https://sltechgeekx.blogspot.com/2022/06/how-to-installing-opencv-from-source.html

How to Create a Project in Eclipse with OpenCV Libraries

https://sltechgeekx.blogspot.com/2022/06/how-to-convert-rgb-image-to-gray.html

Program Source Code

#include <opencv2/opencv.hpp>

using namespace cv;

int main(int argc, char** argv)

{

char* ImageFile = argv[1];

Mat input_image;  /* mat object for storing original_image */

input_image = imread( ImageFile, IMREAD_COLOR ); /* read ImageFile */

if( argc != 2 || !input_image.data )

{

   printf( " No image data \n " );

   return -1;

}

Mat gray_image; /* mat object for storing gray_image */

cvtColor( input_image, gray_image, COLOR_BGR2GRAY ); /* convert image from color to gray */

    int histogram[256]; /* allcoate memory for no of pixels for each intensity value */

    /* initialize all intensity values to 0 */

    for(int i = 0; i < 255; i++)

    {

        histogram[i] = 0;

    }

    /* calculate the no of pixels for each intensity values */

    for(int y = 0; y < gray_image.rows; y++)

    {

    for(int x = 0; x < gray_image.cols; x++)

    {

            histogram[(int)gray_image.at<uchar>(y,x)]++;

    }

    }

    /* draw the histograms */

    int hist_w = 512; int hist_h = 400;

    int bin_w = cvRound((double) hist_w/256);

    Mat histImage(hist_h, hist_w, CV_8UC1, Scalar(255, 255, 255));

    /* find the maximum intensity element from histogram */

    int max = histogram[0];

    for(int i = 1; i < 256; i++)

    {

        if(max < histogram[i])

        {

            max = histogram[i];

        }

    }

    /* normalize the histogram between 0 and histImage.rows */

    for(int i = 0; i < 255; i++)

    {

        histogram[i] = floor(((double)histogram[i]/max)*histImage.rows);

    }

    /* draw the intensity line for histogram */

    for(int i = 0; i < 255; i++)

    {

        line(histImage, Point(bin_w*(i), hist_h),

                              Point(bin_w*(i), hist_h - histogram[i]),

             Scalar(0,0,0), 1, 8, 0);

    }

namedWindow( "Input Image in Gray", WINDOW_AUTOSIZE );   /* set window name Gray Image */

imshow( "Input Image in Gray", gray_image );   /* show window containing gray_image */

namedWindow("Intensity Histogram", WINDOW_AUTOSIZE);  /* set window name Intensity Histogram */

imshow("Intensity Histogram", histImage);   /* show window containing Intensity Histogram */

waitKey(0);      /* to exit */

return 0;

}

Program Output