What happened to my Mandelbrot code?

Question

I am trying to implement a Mandelbrot set in C, but I have a strange problem. My code is as follows:

#include <stdio.h> #include <math.h> #include <complex.h> int iterate_pt(complex c); int main() { FILE *fp; fp = fopen("mand.ppm", "w+"); double crmin = -.75; double crmax = -.74; double cimin = -.138; double cimax = -.75; //Changing this value to -.127 fixed my problem. int ncols = 256; int nrows = 256; int mand[ncols][nrows]; int x, y, color; double complex c; double dx = (crmax-crmin)/ncols; double dy = (cimax-cimin)/nrows; for (x = 0; x < ncols; x++){ for (y = 0; y < nrows; y++){ double complex imaginary = 0+1.0i; c = crmin+(x*dx) + (cimin+(y*dy)) * imaginary; mand[x][y] = iterate_pt(c); } } printf("Printing ppm header."); fprintf(fp, "P3\n"); fprintf(fp, "%d %d\n255\n\n", ncols, nrows); for (x = 0; x < ncols; x++) { for (y = 0; y < nrows; y++){ color = mand[x][y]; fprintf(fp, "%d\n", color); fprintf(fp, "%d\n", color); fprintf(fp, "%d\n\n", color); //Extra new line added, telling the ppm to go to next pixel. } } fclose(fp); return 0; } int iterate_pt(double complex c){ double complex z = 0+0.0i; int iterations = 0; int k; for (k = 1; k <= 255; k++) { z = z*z + c; if (sqrt( z*conj(z) ) > 50){ break; } else ++iterations; } return iterations; } 

However, the output of this program, which is stored as a ppm file, is as follows:

Thanks for your help!

Answer 1

Try setting cimax to -0.127, I am also working on this project and it seems to do the trick;)

Answer 2

The code looks good. But your starting rectangle doesn't look right!

you're using

 Real ranage [ -.75 , -.74 ] Imag range [ -.138 , -.75 ] 

Are you sure this is what you intended? It seems to me that this is a terribly stretched y-scale.

In addition, standard mandelbrot algorithms tend to use

 magnitude > 2 

not 50. as an evacuation check. Although this should not affect the actual form of the set.

Answer 3

By the way, it makes no sense to calculate sqrt z * conj (z). Just collect the expressions on either side of the inequality, giving if (z*conj(z) > 2500) , and you have improved performance.