Check if images are bright or dark (university homework - libgd)

are_images_bright.c

// author: Maciej Ciemborowicz
// Jagiellonian University
// ZGK 2011/2012

#include <stdio.h>
#include <math.h>
#include <gd.h>
#include "gd_wewy.h"

#define DARK 0
#define BRIGHT 1

int main(int argc, char * argv[])
{

  // check if RGB pixel is BRIGHT or DARK
  int is_pixel_bright(int r, int g, int b) {

    // http://www.nbdtech.com/Blog/archive/2008/04/27/Calculating-the-Perceived-Brightness-of-a-Color.aspx
    float brightness = sqrt(.241*r*r + .691*g*g + .068*b*b);
    return (127.5 <= brightness) ? BRIGHT : DARK;
  
  }
  
  //////////////////////////////////////////////////////////////////////
  
  // index 0 - dark pixels
  // index 1 - bright pixels
  int pixels_number[2];
  int x, y;
  int i, j, k;
  gdImagePtr img;
  int color;

  //////////////////////////////////////////////////////////////////////

  if (argc < 2) {
    fprintf(stderr, "Error: No argument!\n");
    return 1;
  }
  
  //////////////////////////////////////////////////////////////////////

  for (k=1; k<argc; ++k) {

    img = wczytajPng(argv[k]);
    
    if (img == NULL) {
      fprintf(stderr, "%s: Can't read this file!\n", argv[k]);
    }
    else {
      
      pixels_number[DARK] = 0;
      pixels_number[BRIGHT] = 0;
      x = gdImageSX(img);
      y = gdImageSY(img);
        
      for (i=0; i<x; ++i) {
        for (j=0; j<y; ++j) {
          color = gdImageGetPixel(img, i, j);
          // like a true geek:)
          ++pixels_number[is_pixel_bright(gdImageRed(img, color), gdImageGreen(img, color), gdImageBlue(img, color))];
        }
      }
        
      printf("%s: ", argv[k]);
        
      if (pixels_number[DARK] > pixels_number[BRIGHT]) {
        printf("This image is dark.\n");
      }
      else if (pixels_number[DARK] < pixels_number[BRIGHT]) {
        printf("This image is bright.\n");
      }
      else {
        printf("This image is neutral.\n");
      }
        
      gdImageDestroy(img);
      
    }
  
  }

  return 0;
}