Ecualización de histograma en C | Procesamiento de imágenes

// C program to perform histogram equalisation to adjust contrast levels
  
// All the needed library functions for this program
#include <fcntl.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
  
// Function to perform histogram equalisation on an image
// Function takes total rows, columns, input file name and output
// file name as parameters
void histogramEqualisation(int cols, int rows,
                           char* input_file_name, char* output_file_name)
{
    // creating image pointer
    unsigned char* image;
  
    // Declaring 2 arrays for storing histogram values (frequencies) and
    // new gray level values (newly mapped pixel values as per algorithm)
    int hist[256] = { 0 };
    int new_gray_level[256] = { 0 };
  
    // Declaring other important variables
    int input_file, output_file, col, row, total, curr, i;
  
    // allocating image array the size equivalent to number of columns
    // of the image to read one row of an image at a time
    image = (unsigned char*)calloc(cols, sizeof(unsigned char));
  
    // opening input file in Read Only Mode
    input_file = open(input_file_name, O_RDONLY);
    if (input_file < 0) {
        printf("Error opening input file\n");
        exit(1);
    }
  
    // creating output file that has write and read access
    output_file = creat(output_file_name, 0666);
    if (output_file < 0) {
        printf("Error creating output file\n");
        exit(1);
    }
  
    // Calculating frequency of occurrence for all pixel values
    for (row = 0; row < rows; row++) {
        // reading a row of image
        read(input_file, &image[0], cols * sizeof(unsigned char));
  
        // logic for calculating histogram
        for (col = 0; col < cols; col++)
            hist[(int)image[col]]++;
    }
  
    // calculating total number of pixels
    total = cols * rows;
  
    curr = 0;
  
    // calculating cumulative frequency and new gray levels
    for (i = 0; i < 256; i++) {
        // cumulative frequency
        curr += hist[i];
  
        // calculating new gray level after multiplying by
        // maximum gray count which is 255 and dividing by
        // total number of pixels
        new_gray_level[i] = round((((float)curr) * 255) / total);
    }
  
    // closing file
    close(input_file);
  
    // reopening file in Read Only Mode
    input_file = open(input_file_name, O_RDONLY);
  
    // performing histogram equalisation by mapping new gray levels
    for (row = 0; row < rows; row++) {
        // reading a row of image
        read(input_file, &image[0], cols * sizeof(unsigned char));
  
        // mapping to new gray level values
        for (col = 0; col < cols; col++)
            image[col] = (unsigned char)new_gray_level[image[col]];
  
        // reading new gray level mapped row of image
        write(output_file, &image[0], cols * sizeof(unsigned char));
    }
  
    // freeing dynamically allocated memory
    free(image);
  
    // closing input and output files
    close(input_file);
    close(output_file);
}
  
// driver code
int main()
{
    // declaring variables
    char* input_file_name;
    char* output_file_name;
    int cols, rows;
  
    // defining number of rows and columns in an image
    // here, image size is 512*512
    cols = 512;
    rows = 512;
  
    // defining input file name (input image name)
    // this boat_512_512 is a raw grayscale image
    input_file_name = "boat_512_512";
  
    // defining output file name (output image name)
    output_file_name = "boat_512_512_histogram_equalised";
  
    // calling function to do histogram equalisation
    histogramEqualisation(cols, rows, input_file_name, output_file_name);
  
    return 0;
}