Programmering cirkel.

Her er et udkast af min kode: Hvad skal jeg skrive i "r1", for at kunne danne en cirkel?

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

#define PI 3,1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253

int main(void) {

FILE *image_file;
int i, j;
int r1;

image_file = fopen("image-file-4.pnm", "wb");

fputs("P6\n", image_file);
fputs("1366 768\n", image_file);
fputs("255\n", image_file);

for(i = 0; i < 1366; i++)
for (j = 0; j < 768; j++){
r1 = 

fputc(r1, image_file);

}

fclose(image_file);
return 0;
}