1 #include <stdio.h>
    2 
    3 #define WIDTH 800
    4 #define HEIGHT 600
    5 #define ITERATIONS 30
    6 #define LEFT -2.0
    7 #define RIGHT 1.0
    8 #define TOP 1.0
    9 #define BOTTOM -1.0
   10 
   11 /*
   12   Transform pixel coordinates to "display field" coordinates.
   13 
   14   Input range are integer coordinates from (0,0) for top left
   15   to (width - 1, height - 1) for bottom right.
   16 
   17   Dividing x by WIDTH and y by HEIGHT, brings them into a
   18   normalized [0,1] range. A simple linear transform then
   19   maps them into the "display field" that we are interested in.
   20  */
   21 void transform(unsigned int x, unsigned int y, double *x1, double *y1)
   22 {
   23         *x1 = LEFT + (RIGHT - LEFT) * ((double)x) / ((double)WIDTH);
   24         *y1 = TOP  - (TOP - BOTTOM) * ((double)y) / ((double)HEIGHT);
   25 }
   26 
   27 /*
   28     Returns non-zero if point (pX,pY) is in the mandelbrot set, zero if not.
   29     "ratio" returns the ratio of iterations needed to determine if a point was
   30     outside the mandelbrot set to the maximum number of iterations.
   31 
   32     In essence, we apply the iteration formula until the maximum is reached, or
   33     the point (x,y) is further away from the origin than 2.
   34  */
   35 int is_in_mandelbrot(double pX, double pY, double *ratio)
   36 {
   37         double x = 0, y = 0, temp;
   38         int i;
   39 
   40         for (i = 0; ((x * x + y * y) <= 4) && i < ITERATIONS; ++i) {
   41                 temp = x * x - y * y + pX;
   42                 y = 2.0 * x * y + pY;
   43                 x = temp;
   44         }
   45 
   46         *ratio = ((double)i) / ((double)ITERATIONS);
   47 
   48         return i == ITERATIONS;
   49 }
   50 
   51 int main(void)
   52 {
   53         unsigned char image[WIDTH * HEIGHT * 3], *ptr;
   54         unsigned int x, y;
   55         double fx, fy, r;
   56         FILE *file;
   57 
   58         /* for each pixel in the image */
   59         for (ptr = image, y = 0; y < HEIGHT; ++y) {
   60                 for (x = 0; x < WIDTH; ++x, ptr += 3) {
   61                         /* transform pixel coordinates */
   62                         transform(x, y, &fx, &fy);
   63 
   64                         /* Inside set -> white, outside -> based on ratio */
   65                         if (is_in_mandelbrot(fx, fy, &r)) {
   66                                 ptr[0] = 0xFF;
   67                                 ptr[1] = 0xFF;
   68                                 ptr[2] = 0xFF;
   69                         } else {
   70                                 ptr[0] = (9*(1-r)*r*r*r) * 255.0;
   71                                 ptr[1] = (15*(1-r)*(1-r)*r*r) * 255.0;
   72                                 ptr[2] = (8.5*(1-r)*(1-r)*(1-r)*r) * 255.0;
   73                         }
   74                 }
   75         }
   76 
   77         /* write to a ppm file */
   78         file = fopen("out.ppm", "wb");
   79         fprintf(file, "P6\n%d %d\n255\n", WIDTH, HEIGHT);
   80         fwrite(image, 1, WIDTH * HEIGHT * 3, file);
   81         fclose(file);
   82         return 0;
   83 }