What's wrong with my Mandelbrot set code?

Question

I'm trying to implement the Mandelbrot set in C, but I'm having a weird 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 looks like this:

Thanks for your help!

Answer 1

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