Help With Mandelbrot Code (I'm in over my head)

I have some code that generates pixel values for a mandelbrot image. I haven't added the code to output to the file yet; right now I'm trying to figure out why it loops infinitely. I want it to generate a 512x512 pixel image, but as I mentioned, it just keeps on going. Any help would be appreciated. Forgive me for programming ignorance. Here's what I've got so far:

Code:

#include <iostream>


using namespace std;


const int maxiters = 255;
const int xpixels = 512;
const int ypixels = 512;


int getGray(int c) { return ((c)%8)*32; }
int getRed(int c)  { return c;          }
int getGreen(int c){ return 0;          }
int getBlue(int c) { return 255-((c)%16)*32; }


int getMbrotCount(double a0, double b0){
    int count=0;
    double a=a0;
    double b=b0;
    double anext,bnext;


    while(a*a+b*b<4.0 && count < maxiters ){
        anext = a*a - b*b + a0;
        bnext = 2.0*a*b +b0;
        a=anext;
        b=bnext;
        count++;
    }


    return count;
}


int main(){
    double x0=-2.0;
    double y0=1.5;
    double x1=1.0;
    double y1=-1.5;
    double x,y;
    int c;
    int i,j;


    cout << "P3\n" << xpixels << " " << ypixels << "\n255\n";


    for(i=0;i<ypixels;++i){
        y=y0+(y1-y0)/ypixels*i;


        for(j=0;j<xpixels;++j){
            x=x0+(x1-x0)/xpixels*j;


            c=getMbrotCount(x,y);
            if(c==maxiters) cout << "0 0 0 ";
            else{
                cout << getRed(c) << " ";
                cout << getGreen(c) << " ";
                cout << getBlue(c) << " ";
            }
        }
        cout << endl;
    }




    return 0;


}

Okay, I output it to a file and it's not infinite looping anymore.