/* Diagonal stripe pattern generator.

   ./stripes <width> <height> > output.pgm

   Doing this with a transformation matrix would be more straightforward
   (rotate and scale screen coordinates, then color odd/even rows).
   This file does it the less obvious way, without using transcendental
   functions.
                                                                         */
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#ifdef _WIN32
   #include<fcntl.h>
   #include<io.h>
#endif

int main(int argc, char **argv)
{
   int width, height, x, y;
   int a, b, offset, direction, parity;

   if( argc != 3 ||
       (width = atoi(argv[1])) <= 0 ||
       (height = atoi(argv[2])) <= 0 )
   {
      return printf("%s <width> <height>\n", *argv);
   }
   if( width >= 0x8000 || height >= 0x8000 )
      return !puts("Output size too large.");

   #ifdef _WIN32
      setmode(STDOUT_FILENO, O_BINARY);
   #endif

   srand(time(NULL));

   /* Generate stripe slopes. */
   a = (int)((double)rand() / RAND_MAX * (width / 10 + 1)) + 5;
   b = (int)((double)rand() / RAND_MAX * (height / 10 + 1)) + 5;
   direction = rand() > RAND_MAX / 2;

   /* Generate shift offset.  Since stripe lines extend infinitely,
      we only need to shift in one direction.                       */
   offset = (int)((double)rand() / RAND_MAX * a * 2);

   printf("P5\n%d %d\n255\n", width, height);
   for(y = 0; y < height; y++)
   {
      for(x = offset; x < width + offset; x++)
      {
         /* Divide screen into rectangles of sizes (a,b), then find points
            within those rectangles that are below the line of slope b/a.  */
         if( direction )
            parity = x % a * b > (b - 1 - y % b) * a;
         else
            parity = x % a * b > y % b * a;

         /* Previous step would have generated lots of triangles.  If
            we flip half of them, the triangles would align, creating
            the stripe pattern.                                       */
         parity ^= ((x / a) + (y / b)) & 1;

         fputc(parity * 255, stdout);
      }
   }
   return 0;
}