#include <stdio.h>
#include <gd.h>
#include <cmath>
#include <cstdlib>
int main()
{
    const unsigned txW = 256, txH = 256;
    gdImagePtr im;
    im = gdImageCreateTrueColor(txW, txH);
    for(unsigned y=0; y<txH; ++y)
        for(unsigned x=0; x<txW; ++x)
        {
            double Y = 
             0.7 - ( (1.0-std::sqrt( int(x-txW/2)*int(x-txW/2)/double(txW/2)/(txW/2)
                                   + int(y-txH/2)*int(y-txH/2)/double(txH/2)/(txH/2))) *0.6
                    -!(x<8 || y<8 || (x+8)>=txW || (y+8)>=txH))
                 * (0.1 + 0.3*std::pow((std::rand()%100)/100.0, 2.0));
             ;
            int yy = Y*255; if(yy < 0) yy = 0; if(yy > 255) yy = 255;
        
            gdImageSetPixel(im, x,y, gdTrueColor(yy, yy, yy));
        }
    gdImagePng(im, stdout);
}