Draw the Mandelbrot Set

blog2.c

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


#include <complex.h>

#define MAX_X 384
#define MAX_Y 216

#define X_LOWER -2.5
#define X_UPPER 1.0555555
#define Y_LOWER -1
#define Y_UPPER 1

#define MAX_ITERATIONS 255


/* Calculate the number of iterations a point takes to leave a bound. */
int iterations(double cr, double ci, int max_it) {

  // Create our initial numbers, c and z, along with a counter for iterations, i.
  double complex z = 0 + 0*I;
  double complex c = cr + ci*I;
  int i = 0;

  // Keep iterating z until:
  // (a) z gets higher than our upper bound, or
  // (b) we hit the maximum number of iterations
  for (; i<max_it; i++) {
    z = z*z + c;
    if (cabs(z) > 4)
      break;
  }
  return i;
}


/* Linearly map a value from one range into another. */
double linmap(double val, double lower1, double upper1, double lower2, double upper2) {
  return ((val - lower1) / (upper1 - lower1)) * (upper2 - lower2) + lower2;
}


/* Calculate the colour for coordinates (x, y) and write into color[]. */
void get_color(int x, int y, int maxX, int maxY, char color[]) {

  // Map the screen coordinates to the Mandelbrot coordinates.
  double x1 = linmap(x, 0, maxX, X_LOWER, X_UPPER);
  double y1 = linmap(y, 0, maxY, Y_LOWER, Y_UPPER);
  int its = iterations(x1, y1, MAX_ITERATIONS);

  // Map the number of iterations to a colour between 0 and 255.
  char r = linmap(its, 0, MAX_ITERATIONS, 0, 255);
  char g = r;
  char b = g;

  // Write the colours.
  color[0] = r;
  color[1] = g;
  color[2] = b;
}


/* Write the header for a PPM file. */
void write_header(FILE *fp, int dimx, int dimy) {
  fprintf(fp, "P6\n");  // Write magic chars.
  fprintf(fp, "%d %d\n", dimx, dimy);  // Write width and height.
  fprintf(fp, "255\n");  // Write max colour value.
}


/* Generate a Mandelbrot image. */
void generate_file(int dimx, int dimy) {

  // Create a file and write to it.
  FILE *fp = fopen("mandelbrot.ppm", "wb");
  write_header(fp, dimx, dimy);

  // For each coordinate, calculate its color then write to file.
  char color[3];
  for (int j=0; j<dimy; ++j)
    for (int i=0; i<dimx; ++i)
      {
	get_color(i, j, dimx, dimy, color);
	fwrite(color, 1, 3, fp);
      }

  // Close the file pointer to clean up.
  fclose(fp);
}


int main(int argc, char *argv[]) {
  // Grab our parameters from the system args.
  int dimx = atoi(argv[1]);
  int dimy = atoi(argv[2]);

  // Write the file with the given resolution.
  generate_file(dimx, dimy);
  return EXIT_SUCCESS;
}