Image:Demj.jpg

/* 
 c console  program:
  1. draws Julia setfor Fc(z)=z*z +c
  using DEM/J algorithm ( Distance Esthimation Method for Julia set )
 -------------------------------         
 2. technic of creating ppm file is  based on the code of Claudio Rocchini
 http://en.wikipedia.org/wiki/Image:Color_complex_plot.jpg
 create 24 bit color graphic file ,  portable pixmap file = PPM 
 see http://en.wikipedia.org/wiki/Portable_pixmap
 to see the file use external application ( graphic viewer)
 ---------------------------------
 I think that creating graphic can't be simpler
 comments : Adam Majewski 
 */
 #include <stdio.h>
 /*
 estimates distance from point c to nearest point in Julia  set 
 for Fc(z)= z*z + c
 z(n+1) = Fc(zn)  
 this function is based on function jdist from mbrot.cpp 
 from program mandel by Wolf Jung (GNU GPL )
 http://www.mndynamics.com/indexp.html 
 */
 double jdist(double Zx, double Zy, double Cx, double Cy ,  int iter_max)
 { 
 int i;
 double x = Zx, /* Z = x+y*i */
         y = Zy, 
         /* Zp = xp+yp*1 = 1  */
         xp = 1, 
         yp = 0, 
         /* temporary */
         nz,  
         nzp,
         /* a = abs(z) */
         a; 
 for (i = 1; i <= iter_max; i++)
  { /* first derivative   zp = 2*z*zp + 1 = xp + yp*i; */
    nz = 2*(x*xp - y*yp) + 1; 
    yp = 2*(x*yp + y*xp); 
    xp = nz;
    /* z = z*z + c = x+y*i */
    nz = x*x - y*y + Cx; 
    y = 2*x*y + Cy; 
    x = nz; 
    /* */
    nz = x*x + y*y; 
    nzp = xp*xp + yp*yp;
    if (nzp > 1e60 || nz > 1e60) break;
  }
 a=sqrt(nz);
 /* distance = 2 * |Zn| * log|Zn| / |dZn| */
 return 2* a*log(a)/sqrt(nzp); 
 }
 /* ------------------------------------------------------*/
 int main()
 {
  const double Cx=-0.74543;
        const double Cy=0.11301;
 /* screen ( integer) coordinate */
        int iX,iY;
        const int iXmax = 10000; 
        const int iYmax = 10000;
        /* world ( double) coordinate = parameter plane*/
        const double ZxMin=-2.;
        const double ZxMax=2.0;
        const double ZyMin=-2.0;
        const double ZyMax=2.0;
        /* */
        double PixelWidth=(ZxMax-ZxMin)/iXmax;
        double PixelHeight=(ZyMax-ZyMin)/iYmax;
         /* color component ( R or G or B) is coded from 0 to 255 */
        /* it is 24 bit color RGB file */
        const int MaxColorComponentValue=255; 
        FILE * fp;
        char *filename="demj-.ppm";
        char *comment="# ";/* comment should start with # */
        static unsigned char color[3];
        double Zx, Zy,    /* Z=Zx+Zy*i   */
               Z0x, Z0y,  /* Z0 = Z0x + Z0y*i */
               Zx2, Zy2; /* Zx2=Zx*Zx;  Zy2=Zy*Zy  */
        /*  */
        int Iteration;
        const int IterationMax=2000;
        /* bail-out value , radius of circle ;  */
        const int EscapeRadius=400;
        int ER2=EscapeRadius*EscapeRadius;
         double distanceMax=PixelWidth/15; /*jdist( 0,0,Cx,Cy, IterationMax);*/
    /*create new file,give it a name and open it in binary mode  */
        fp= fopen(filename,"wb"); /* b -  binary mode */
        /*write ASCII header to the file*/
        fprintf(fp,"P6\n %s\n %d\n %d\n %d\n",comment,iXmax,iYmax,MaxColorComponentValue);
     /* compute and write image data bytes to the file*/
        for(iY=0;iY<iYmax;++iY)
        {
              Z0y=ZyMax - iY*PixelHeight; /* reverse Y  axis */
                 if (fabs(Z0y)<PixelHeight/2) Z0y=0.0; /*  */    
             for(iX=0;iX<iXmax;++iX)
             {    /* initial value of orbit Z0 */
                Z0x=ZxMin + iX*PixelWidth;
                  /* Z = Z0 */
                  Zx=Z0x;
                  Zy=Z0y;
                 Zx2=Zx*Zx;
                  Zy2=Zy*Zy;
                 /* */
                 for (Iteration=0;Iteration<IterationMax && ((Zx2+Zy2)<ER2);Iteration++)
                        {
                            Zy=2*Zx*Zy + Cy;
                            Zx=Zx2-Zy2 +Cx;
                            Zx2=Zx*Zx;
                            Zy2=Zy*Zy;
                        };
               /* compute  pixel color (24 bit = 3 bytes) */
               if (Iteration==IterationMax)
                        { /*  interior of Julia set  = black */
                           color[0]=0;
                           color[1]=0;
                           color[2]=0;                           
                        }
               else /* exterior of Filled-in Julia set  =  */
                       {  double distance=jdist(Z0x,Z0y,Cx,Cy,IterationMax);
                          if (distance<distanceMax)
                          { /*  Julia set  = white */
                             color[0]=255; /* Red*/
                             color[1]=255;  /* Green */ 
                             color[2]=255;/* Blue */
                          }
                          else 
                          { /*  exterior of Julia set  = black */
                           color[0]=0;
                           color[1]=0;
                           color[2]=0;                           
                        };
                       }
                   /* check the orientation of Z-plane */
                   /* mark first quadrant of cartesian plane*/     
              /*     if (Z0x>0 && Z0y>0) color[0]=255-color[0];  */
            /*write color to the file*/
                  fwrite(color,1,3,fp);
                }
        }
        fclose(fp);
        printf("file saved ");
        getchar();
        return 0;
  }