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

       /* This program reads in two pictures, and as it is doing so, it
          computes the difference between pixels at the same location,
          and write a new picture composed of these differences */

       /* char pic[256][256]; This statement would be useful for storing pic */

main(argc,argv)
int argc;
char **argv;
{ 
        int i,j,ci1,ci2,cdif;
        FILE *fo1, *fp2,*fp1, *fopen();
        char *foobar;

/* Open file 1 for reading */
    argc--; argv++;
    foobar = *argv;
    fp1=fopen(foobar,"rb");

/* Open file 1 for reading */
    argc--; argv++;
    foobar = *argv;
    fp2=fopen(foobar,"rb");

/* Open file 1 for writing */
    argc--; argv++;
    foobar = *argv;
    fo1=fopen(foobar,"wb");

    fprintf(fo1,"P5\n256 256\n255\n");  /* Print out the .pgm header */

        for (i=0;i<256;i++)             /* Pictures are 256 x 256 size */
        { for (j=0;j<256;j++)
                {  
                  ci1  =  getc (fp1);   /* read one pixel from Pic 1 */

                  ci2  =  getc (fp2);   /* read one pixel from Pic 2 */


                  cdif = abs(ci2 - ci1);     /* Take difference */
                  
                  if (cdif>255)cdif=255;
                   
                  fprintf(fo1,"%c",(char)cdif);  /* print the output Pic */

                }
        }
}