Create colour images using only standard C

In this tutorial we will see how to work with PPM images. For those of you who have missed the previous post, PPM refers to one of the image formats implemented in the Netpbm package.

In the previous post, we saw how to create a very small PGM image using a text file. We could have been doing it with any other programming language, as PGM is in text format. We will use C to create a PPM image, but before we must understand the structure of this format.

In a PGM image, each pixel is represented with one numerical value. With PPM , things change a little. Because PPM is an RGB format, we need three numerical value to represent the three channels of each pixel. If we write the data directly to a file, for three red pixels, after the header, we need to write:

  

255 0 0 255 0 0 255 0 0 ... 

The magic number for a PPM image is P3, if the file is written in ascii mode, and P6 if it's written directly in binary. The following is a program that create a 5 by 3 PPM image containing random colours:

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

int main()
{
  FILE * file = fopen("image.ppm", "wb");
 
  if(!file)
    {
      printf("Error! Cannot open file image.ppm");
      exit(EXIT_FAILURE);
    }

  int width = 5;
  int height = 3;
  int i, j;
  
  //header
  fprintf(file, "P6\n%d %d\n255\n", width, height);
 
  for(i=0; i<height; i++)
    for(j=0; j<width; j++)
      {
        fputc((random()%255), file);//red
        fputc((random()%255), file);//green     
        fputc((random()%255), file);//blue
      }
  fclose(file);
  return 0;
}

The image below is a scaled version of the one generated by the code above on my machine:

In the next post we will do some image processing, using only a PPM image and our best friend language, C.