file.cpp

#include <stdio.h>
  #include <stdlib.h>
  #include <malloc.h>
  
  #define STB_IMAGE_IMPLEMENTATION
  #include "stb_image.h"
  #define STB_IMAGE_WRITE_IMPLEMENTATION
  #include "stb_image_write.h"
  
  int main(void) {
  	int width, height, channels;
  	unsigned char *img = stbi_load("image.jpeg", &width, 		&height,&channels, 0);
  	
      if(img == NULL) {
          printf("Error in loading the image\n");
          exit(1);
      }
     printf("Loaded image with a width of %dpx, a height of %dpx and %d channels\n", width, height, channels); 
     
      stbi_write_png("1.png", width, height, channels, img, width * channels);
      
     size_t img_size = width * height * channels;
     int gray_channels = channels == 4 ? 2 : 1;
     size_t gray_img_size = width * height * gray_channels;
 
     unsigned char *gray_img = (unsigned char*) malloc(gray_img_size);
     if(gray_img == NULL) {
         printf("Unable to allocate memory for the gray image.\n");
         exit(1);
     }
 

	
     for(unsigned char *p = img, *pg = gray_img; p != img + img_size; p += channels, pg += gray_channels) {
         *pg = (uint8_t)((*p + *(p + 100) + *(p + 35))/3.0);
         if(channels == 4) {
             *(pg + 1) = *(p + 3);
         }
     }
     
        stbi_write_jpg("sky_gray.jpg", width, height, gray_channels, gray_img, 100);

 }