Fractals/Iterations in the complex plane/MandelbrotSetExterior

Colouring of exterior of Mandelbrot set can be :

• non-smooth :
  • Boolean/binary Escape Time Method ( bETM/M )
  • discrete = Level Set Method = LSM/M = iETM/M
• Smooth :
  • radial measures
    • Real Escape Time Method( rETM/M )
    • Distance Estimation Method( DEM/M )
    • radius of complex potential = Complex Potential Method ( CPM/M )
• angular measures
  • • argument of complex potential
  • SAC = stripe average coloring
  • other
    • "triangle inequality"
    • Orbit trap

One can also draw curves :

• external rays
• equipotential lines ( closed curves - quasi circles)

Similar projects:

• Mandelbrot Notebook by Claude Heiland-Allen
  • mandelbrot-book
  • mandelbrot-book/book : one file c programs , images and description
  • fork of the mandelbrot-book and book in pdf format
• different drawing techniques and algorithms by Arnaud Cheritat
  • Mandelbrot set and example C programs ( not by A Cheritat)
• Linas Vepstas' Art Gallery:
  • original site
  • fork at gitlab

Escape time or dwell

Here for given point c on parameter plane one checks how critical point $z=0.0$ behaves on dynamical plane under forward iteration. If you change initial point you will get different result

To draw given plane one needs to check/scan (all) its points. See here for more details ( optimisation) Read definitions first.

Boolean escape time

Here complex plane consists of 2 sets : Mandelbrot set $M\,$ and its complement $M^{c}\,$ :

$\mathbb {C} =M\cup M^{c}$

ASCI graphic ( on screen)

ASCI graphic : Boolean escape time in text mode

// http://mrl.nyu.edu/~perlin/
main(k){float i,j,r,x,y=-16;while(puts(""),y++<15)for(x
=0;x++<84;putchar(" .:-;!/>)|&IH%*#"[k&15]))for(i=k=r=0;
j=r*r-i*i-2+x/25,i=2*r*i+y/10,j*j+i*i<11&&k++<111;r=j);}

-- Haskell code by Ochronus
-- http://blog.mostof.it/mandelbrot-set-in-ruby-and-haskell/
import Data.Complex
mandelbrot a = iterate (\z -> z^2 + a) a !! 500
main = mapM_ putStrLn [[if magnitude (mandelbrot (x :+ y)) < 2 then '*' else ' '
                           | x <- [-2, -1.9685 .. 0.5]]
                           | y <- [1, 0.95 .. -1]]

; common lisp
(loop for y from -1.5 to 1.5 by 0.05 do
      (loop for x from -2.5 to 0.5 by 0.025 do
		(let* ((c (complex x y)) ; parameter
                   	(z (complex 0 0))
                   	(iMax 20) ; maximal number of iterations
			(i 0)) ; iteration number

		(loop  	while (< i iMax ) do 
			(setq z (+ (* z z) c)) ; iteration
			(incf i)
			(when (> (abs z) 2) (return i)))
	   ; color of pixel
           (if (= i iMax) (princ (code-char 42)) ; inside M
                          (princ (code-char 32))))) ; outside M
      (format t "~%")) ; new line

Comparison programs in various languages

Graphic file ( PPM )

Here are various programs for creating pbm file

• GLSL using shadertoy
• short c programs by ed Burke

C

This is complete code of C one file program.

• It makes a ppm file which consists an image. To see the file (image) use external application ( graphic viewer).
• Program consists of 3 loops:
  • iY and iX, which are used to scan rectangle area of parameter plane
  • iterations.

For each point of screen (iX,iY) it's complex value is computed c=cx+cy*i.

For each point c is computed iterations of critical point $z_{0}=z_{cr}=0\,$

It uses some speed_improvement. Instead of checking :

sqrt(Zx2+Zy2)<ER

it checks :

(Zx2+Zy2)<ER2 // ER2 = ER*ER

It gives the same result but is faster.

 /* 
 c program:
 --------------------------------
  1. draws Mandelbrot set for Fc(z)=z*z +c
  using Mandelbrot algorithm ( boolean escape time )
 -------------------------------         
 2. technique 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)
  */
 #include <stdio.h>
 #include <math.h>
 int main()
 {
          /* screen ( integer) coordinate */
        int iX,iY;
        const int iXmax = 800; 
        const int iYmax = 800;
        /* world ( double) coordinate = parameter plane*/
        double Cx,Cy;
        const double CxMin=-2.5;
        const double CxMax=1.5;
        const double CyMin=-2.0;
        const double CyMax=2.0;
        /* */
        double PixelWidth=(CxMax-CxMin)/iXmax;
        double PixelHeight=(CyMax-CyMin)/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="new1.ppm";
        char *comment="# ";/* comment should start with # */
        static unsigned char color[3];
        /* Z=Zx+Zy*i  ;   Z0 = 0 */
        double Zx, Zy;
        double Zx2, Zy2; /* Zx2=Zx*Zx;  Zy2=Zy*Zy  */
        /*  */
        int Iteration;
        const int IterationMax=200;
        /* bail-out value , radius of circle ;  */
        const double EscapeRadius=2;
        double ER2=EscapeRadius*EscapeRadius;
        /*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++)
        {
             Cy=CyMin + iY*PixelHeight;
             if (fabs(Cy)< PixelHeight/2) Cy=0.0; /* Main antenna */
             for(iX=0;iX<iXmax;iX++)
             {         
                        Cx=CxMin + iX*PixelWidth;
                        /* initial value of orbit = critical point Z= 0 */
                        Zx=0.0;
                        Zy=0.0;
                        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 Mandelbrot set = black */
                           color[0]=0;
                           color[1]=0;
                           color[2]=0;                           
                        }
                     else 
                        { /* exterior of Mandelbrot set = white */
                             color[0]=255; /* Red*/
                             color[1]=255;  /* Green */ 
                             color[2]=255;/* Blue */
                        };
                        /*write color to the file*/
                        fwrite(color,1,3,fp);
                }
        }
        fclose(fp);
        return 0;
 }

Integer escape time = LSM/M = dwell bands

• 

  Number of details is proportional to maximal number of iterations

• 

  Mandelbrot animation based on a static number of iterations per pixel. Here you can see why offset is sometimes used ( because - color gradient changes : for high MaxIteration disapears.

Here color is proportional to last iteration ( of final_n, final iteration).

This is also called Level Set Method ( LSM )

$L_{n}=\{c:z_{n}\in T~~{\mbox{and}}~~z_{k}\notin T~~{\mbox{where}}~~k<n\}\,$

C

LSM/M image with full code in C

Difference between Mandelbrot algorithm and LSM/M is in only in part instruction, which computes pixel color of exterior of Mandelbrot set. In LSM/M is :

 if (Iteration==IterationMax)
   { /* interior of Mandelbrot set = black */
     color[0]=0;
     color[1]=0;
     color[2]=0;                           
   }
   /* exterior of Mandelbrot set = LSM */
   else if ((Iteration%2)==0) 
             { /* even number = black */
             color[0]=0; /* Red */
             color[1]=0; /* Green */ 
             color[2]=0; /* Blue */
             }
           else 
             {/* odd number =  white */
             color[0]=255; /* Red */
             color[1]=255; /* Green */ 
             color[2]=255; /* Blue */    
             };

Here is C function whithout explicit complex numbers, only doubles:

int GiveEscapeTime(double C_x, double C_y, int iMax, double _ER2)
{ 
    int i;
    double Zx, Zy;
    double Zx2, Zy2; /* Zx2=Zx*Zx;  Zy2=Zy*Zy  */
 
    Zx=0.0; /* initial value of orbit = critical point Z= 0 */
    Zy=0.0;
    Zx2=Zx*Zx;
    Zy2=Zy*Zy;
 
    for (i=0;i<iMax && ((Zx2+Zy2)<_ER2);i++)
    {
      Zy=2*Zx*Zy + C_y;
      Zx=Zx2-Zy2 +C_x;
      Zx2=Zx*Zx;
      Zy2=Zy*Zy;
    };
 return i;
}

here a short code with complex numbers:

// https://gitlab.com/adammajewski/mandelbrot_wiki_ACh/blob/master/betm.c
int iterate(double complex C , int iMax)
  {
   int i;
   double complex Z= 0.0; // initial value for iteration Z0
   
   for(i=0;i<iMax;i++)
    {
      Z=Z*Z+C; // https://stackoverflow.com/questions/6418807/how-to-work-with-complex-numbers-in-c
      if(cabs(Z)>EscapeRadius) break;
    }
   return i; 
 }

C++

Here is C++ function which can be used to draw LSM/M :

 int iterate_mandel(complex C , int imax, int bailout)
  {
   int i;
   std::complex Z(0,0); // initial value for iteration Z0
   
   for(i=0;i<=imax-1;i++)
    {
      Z=Z*Z+C; // overloading of operators
      if(abs(Z)>bailout)break;
    }
   return i;
 }

I think that it can't be coded simpler (it looks better than pseudocode), but it can be coded in other way which can be executed faster .

Here is faster code :

// based on cpp code by Geek3
inline int fractal(double cx, double cy, int max_iters)
// gives last iteration 
{
	double zx = 0, zy = 0;	
	if (zx * zx + zy * zy > 4) return(0); // it=0
	for (int it = 1; it < max_iters; it++)
	{	double zx_old = zx;
		zx = zx * zx - zy * zy;
		zy = 2 * zx_old * zy;
		zx += cx;
		zy += cy;
		if (zx * zx + zy * zy > 4.0) return(it);
	}
	return(max_iters);
}

A touch more optimised :

// optimised from cpp code by Geek3
inline int fractal(double cReal, double cImg, int max_iters)
// gives last iteration 
{
	double zReal = 0, zImg = 0, zReal2 = 0, zImg2 = 0;
	//iteration zero is always 0^2+0^2, it will never escape
	for (int it = 1; it < max_iters; it++)
	{	//because we have zReal^2 and zImg^2 pre-calculated
		//we can caclulate zImg first
		//then we don't need to calculate/store the "old" zReal
		zImg = (2 * zReal * zImg ) + cImg;
		zReal = zReal2 - zImg2 + cReal;

		// calculate next iteration: zReal^2 and zImg^2
		// they are used twice so calculate them once
		zReal2 = zReal * zReal;
		zImg2 = zImg * zImg;
		if (zReal2 + zImg2 > 4.0) return(it);
	}
	return(max_iters);
}

See also :

• Rendering The Mandelbrot Set In C++: Part 1 - Saving Images , Video by Superdavo0001

GLSL

• shadertoy

Java

//Java code by Josef Jelinek 
// http://java.rubikscube.info/
 
 int mandel(double cx, double cy) {
    double zx = 0.0, zy = 0.0;
    double zx2 = 0.0, zy2 = 0.0;
    int iter = 0;
    while (iter < iterMax && zx2 + zy2 < 4.0) {
      zy = 2.0 * zx * zy + cy;
      zx = zx2 - zy2 + cx;
      zx2 = zx * zx;
      zy2 = zy * zy;
      iter++;
    }
    return iter;
  }

Java Script

Here is JavaScript function which does not give last iteration but LastIteration modulo maxCol. It makes colour cycling ( if maxCol < maxIt ).

function iterate(Cr,Ci) {
// JavaScript function by Evgeny Demidov
// http://www.ibiblio.org/e-notes/html5/fractals/mandelbrot.htm
  var I=0, R=0,  I2=0, R2=0,   n=0;
  if (R2+I2 > max) return 0;
  do  {  I=(R+R)*I+Ci;  R=R2-I2+Cr;  R2=R*R;  I2=I*I;  n++;
  } while ((R2+I2 < max) && (n < maxIt) );
  if (n == maxIt) return maxCol;  else return n % maxCol;
}

Above functions do not use explicit definition of complex number.

Khan Academy

• khan academy mandelbrot-set

Lisp program

Whole Lisp program making ASCII graphic based on code by Frank Buss

; common lisp
(loop for y from -1.5 to 1.5 by 0.1 do
      (loop for x from -2.5 to 0.5 by 0.04 do
            (let* ((i 0)
                   (z (complex x y))
                   (c z))
              (loop while (< (abs
                              (setq z (+ (* z z) c)))
                             2)
                    while (< (incf i) 32))
              (princ (code-char (+ i 32))))) ; ASCII chars <= 32 contains non-printing characters
      (format t "~%"))

MathMap plugin for Gimp

filter mandelbrot (gradient coloration)
    c=ri:(xy/xy:[X,X]*1.5-xy:[0.5,0]);
    z=ri:[0,0]; # initial value z0 = 0 
    # iteration of z
    iter=0;
    while abs(z)<2 && iter<31
    do
        z=z*z+c;  # z(n+1) = fc(zn)
        iter=iter+1
    end;
    coloration(iter/32) # color of pixel
end

Pov-Ray

Pov-Ray has a built-in function mandel

Matemathica

Here is code by Paul Nylander

Level Curves of escape time Method = LCM/M

edge detection of Level sets

Lemniscates of Mandelbrot set

Lemniscates are boundaries of Level Sets of escape time ( LSM/M ). They can be drawn using :

• edge detection of Level sets.
  • Algorithm described in paper by M. Romera et al. This method is fast and allows looking for high iterations.
• boundary trace
• drawing curves $L_{n}(T)=\{c:abs(z_{n})=ER\}\,$, see explanation and source code. This method is very complicated for iterations > 5.

Decomposition of exterior of Mandelbrot set

Decomposition is modification of escape time algorithm.

The target set is divided into parts (2 or more). Very large escape radius is used, for example ER = 12.

Binary decomposition of LSM/M

binary decomposition: image with full code in C

Here target set $T\,$ on dynamic plane is divided into 2 parts (binary decomposition = 2-decomposition ):

• upper half ( white) $T_{u}=\{z:|z|>ER~~{\mbox{and}}~~Im(z)>0\}\,$
• lower half (black) $T_{l}=\{z:|z|>ER~~{\mbox{and}}~~Im(z)\leq 0\}\,$

Division of target set induces decomposition of level sets $L_{n}\,$ into $2^{n+1}\,$ parts:

• $L_{n,u}=\{c:|z_{n}|>ER~~{\mbox{and}}~~Im(z_{n})>0\}\,$ which is colored white,
• $L_{n,l}=\{c:|z_{n}|>ER~~{\mbox{and}}~~Im(z_{n})\leq 0\}\,$ which is colored black.

External rays of angles (measured in turns):

$angle=(k/2^{n})~~{\mbox{mod }}~1\,$

can be seen.

Difference between binary decomposition algorithm and Mandel or LSM/M is in only in part of instruction , which computes pixel color of exterior of Mandelbrot set. In binary decomposition is :

 if (Iteration==IterationMax)
  { /* interior of Mandelbrot set = black */
   color[0]=0;
   color[1]=0;
   color[2]=0;           
  }
  /* exterior of Mandelbrot set = LSM */
  else if (Zy>0) 
         { 
          color[0]=0; /* Red */
          color[1]=0; /* Green */ 
          color[2]=0; /* Blue */
         }
        else 
          {
           color[0]=255; /* Red */
           color[1]=255; /* Green */ 
           color[2]=255; /* Blue */    
          };

also GLSL code from Fragmentarium :

#include "2D.frag"
#group Simple Mandelbrot

// maximal number of iterations
uniform int iMax; slider[1,100,1000] // increase iMax
// er2= er^2 wher er= escape radius = bailout
uniform float er2; slider[4.0,1000,10000] // increase er2

// compute color of pixel
 vec3 color(vec2 c) {
   vec2 z = vec2(0.0);  // initial value

 // iteration
  for (int i = 0; i < iMax; i++) {
    z = vec2(z.x*z.x-z.y*z.y,2*z.x*z.y) +  c; // z= z^2+c
    if (dot(z,z)> er2)   // escape test 
      // exterior
      if (z.x>0){ return vec3( 1.0);} // upper part of the target set 
      else return vec3(0.0); //lower part of the target set 
  }
  return vec3(0.0); //interior
}

Point c is plotting white or black if imaginary value of last iteration ( Zy) is positive or negative.

nth-decomposition

This method is extension of binary decomposition.

The target set T = { z : |zn| > R } with a very large escape radius ( for example R = 12 ) is divided into more then 2 parts ( for example 8).

Real Escape Time

Other names of this method/algorithm are :

• the fully-renormalized fractional iteration count ( by Linas Vepstas in 1997)
• smooth iteration count for generalized Mandelbrot sets ( by Inigo Quilez in 2016)
• continuous iteration count for the Mandelbrot set
• Normalized Iteration Count Algorithm
• Continuous coloring
• smooth colour gradient
• fractional iterations
• fractional escape time

Here color of exterior of Mandelbrot set is proportional not to Last Iteration ( which is integer number) but to real number :

$\nu (z)=\lim _{i\to \infty }(i-\log _{2}\log _{2}|z_{i}|)\,$

Other methods and speedups

Colouring formula in Ultrafractal :

smooth iter = iter + 1 + ( log(log(bailout)-log(log(cabs(z))) )/log(2)

where :

• log(log(bailout) can be precalculated

C

To use log2 function add :

#include <math.h>

at the beginning of program.

if (Iteration==IterationMax)
 { /*  interior of Mandelbrot set = black */
  color[0]=0;
  color[1]=0;
  color[2]=0;                           
 }
 /* exterior of Mandelbrot set  */
 else GiveRainbowColor((double)(Iteration- log2(log2(sqrt(Zx2+Zy2))))/IterationMax,color);

where :

• Zx2 = Zx*Zx
• Zy2 = Zy*Zy

Here is another version by Tony Finch

while (n++ < max &&
 x2+y2 < inf) {
y = 2*x*y + b;
x = x2-y2 + a;
y2 = y*y;
x2 = x*x;
}
nu = n - log(log(x2+y2)/2)/ log(2);

based on equation

$\nu (z)=n-\log _{2}\log(z_{n})\,$

C++

// based on cpp code by Geek3 from http://en.wikibooks.org/wiki/File:Mandelbrot_set_rainbow_colors.png
sqrxy = x * x + y * y;
double m = LastIteration + 1.5 - log2(log2(sqrxy));

java

 /**
Smooth coloring algorithm
https://gitlab.com/shreyas.siravara/mandelbrot-with-smooth-coloring/blob/master/Mandelbrot.java
Mandelbrot with Smooth Coloring by Shreyas Siravara

*/
                double nsmooth = (iterations + 1 - Math.log(Math.log(Zn.getMagnitude())) / Math.log(ESCAPE_RADIUS));

                double smoothcolor = nsmooth / MAX_ITERATIONS;

                if (iterations < MAX_ITERATIONS) {
                    int rgb = Color.HSBtoRGB((float) (0.99f + 1.9 * smoothcolor), 0.9f, 0.9f);
                    g2d.setColor(new Color(rgb));
                } else {
                    g2d.setColor(Color.black.darker());
                }

Matemathica

Here is code by Paul Nylander. It uses different formula :

$cet=n+log_{2}ln(R)-log_{2}ln|z|$

Python

Python code using mpmath library

def mandelbrot(z):
    c = z
    for i in xrange(ITERATIONS):
        zprev = z
        z = z*z + c
        if abs(z) > ESCAPE_RADIUS:
            return ctx.exp(1j*(i + 1 - ctx.log(ctx.log(abs(z)))/ctx.log(2)))
    return 0

Distance estimation DEM/M

• 

  Exterior DEM/M

• 

  simple boundary with DEM/M

• 

  Boundary with DEM/M and Sobel filter

Variants :

• exterior DEM/M
• interior DEM/M

Description

Complex potential

Uniformization of complement of Mandelbrot set

Complex potential is a complex number, so it has 2 parts:

• a real part = real potential = absolute value
• an imaginary part = external angle

One can take also its:

• curl
• divergence

So on one image one can use more than one variable to color image.

Implementations:

• Matemathica

Names:

• Boettcher coordinate

Real potential = CPM/M

Names:

• eponimes:
  • Douady-Hubbard Potential
  • Hubbard-Douady potential
• electric:
  • The electric potential
  • the voltage
• Green function = G(c)

$G(c)=\lim _{n\to \infty }{\frac {1}{2^{n}}}ln|z_{n}|$

$V(c)\approx V_{n}(c)={\frac {log|z_{n}|}{2^{n}}}$

In Fractint :

potential =  log(modulus)/2^iterations

One can use real potential to:

• smooth (continuous) coloring
• discrete coloring ( level sets of potential)
• 3D view

Code:

• c code from mandel by Wolf Jung

Here is Delphi function which gives level of potential :

 Function GiveLevelOfPotential(potential:extended):integer;
  var r:extended;
  begin
     r:= log2(abs(potential));
     result:=ceil(r);
  end;

/******************************************************************/
// /fractint/common/calcfrac.c
/*
CALCFRAC.C contains the high level ("engine") code for calculating the
fractal images (well, SOMEBODY had to do it!).
Original author Tim Wegner, but just about ALL the authors have contributed
SOME code to this routine at one time or another, or contributed to one of
the many massive restructurings.
The following modules work very closely with CALCFRAC.C:
  FRACTALS.C    the fractal-specific code for escape-time fractals.
  FRACSUBR.C    assorted subroutines belonging mainly to calcfrac.
  CALCMAND.ASM  fast Mandelbrot/Julia integer implementation
Additional fractal-specific modules are also invoked from CALCFRAC:
  LORENZ.C      engine level and fractal specific code for attractors.
  JB.C          julibrot logic
  PARSER.C      formula fractals
  and more
 -------------------------------------------------------------------- */
/* Continuous potential calculation for Mandelbrot and Julia      */
/* Reference: Science of Fractal Images p. 190.                   */
/* Special thanks to Mark Peterson for his "MtMand" program that  */
/* beautifully approximates plate 25 (same reference) and spurred */
/* on the inclusion of similar capabilities in FRACTINT.          */
/*                                                                */
/* The purpose of this function is to calculate a color value     */
/* for a fractal that varies continuously with the screen pixels  */
/* locations for better rendering in 3D.                          */
/*                                                                */
/* Here "magnitude" is the modulus of the orbit value at          */
/* "iterations". The potparms[] are user-entered paramters        */
/* controlling the level and slope of the continuous potential    */
/* surface. Returns color.  - Tim Wegner 6/25/89                  */
/*                                                                */
/*                     -- Change history --                       */
/*                                                                */
/* 09/12/89   - added floatflag support and fixed float underflow */
/*                                                                */
/******************************************************************/

static int _fastcall potential(double mag, long iterations)
{
   float f_mag,f_tmp,pot;
   double d_tmp;
   int i_pot;
   long l_pot;

   if(iterations < maxit)
   {
      pot = (float)(l_pot = iterations+2);
      if(l_pot <= 0 || mag <= 1.0)
         pot = (float)0.0;
      else /* pot = log(mag) / pow(2.0, (double)pot); */
      {
         if(l_pot < 120 && !floatflag) /* empirically determined limit of fShift */
         {
            f_mag = (float)mag;
            fLog14(f_mag,f_tmp); /* this SHOULD be non-negative */
            fShift(f_tmp,(char)-l_pot,pot);
         }
         else
         {
            d_tmp = log(mag)/(double)pow(2.0,(double)pot);
            if(d_tmp > FLT_MIN) /* prevent float type underflow */
               pot = (float)d_tmp;
            else
               pot = (float)0.0;
         }
      }
      /* following transformation strictly for aesthetic reasons */
      /* meaning of parameters:
            potparam[0] -- zero potential level - highest color -
            potparam[1] -- slope multiplier -- higher is steeper
            potparam[2] -- rqlim value if changeable (bailout for modulus) */

      if(pot > 0.0)
      {
         if(floatflag)
            pot = (float)sqrt((double)pot);
         else
         {
            fSqrt14(pot,f_tmp);
            pot = f_tmp;
         }
         pot = (float)(potparam[0] - pot*potparam[1] - 1.0);
      }
      else
         pot = (float)(potparam[0] - 1.0);
      if(pot < 1.0)
         pot = (float)1.0; /* avoid color 0 */
   }
   else if(inside >= 0)
      pot = inside;
   else /* inside < 0 implies inside=maxit, so use 1st pot param instead */
      pot = (float)potparam[0];

   i_pot = (int)((l_pot = (long)(pot * 256)) >> 8);
   if(i_pot >= colors)
   {
      i_pot = colors - 1;
      l_pot = 255;
   }

   if(pot16bit)
   {
      if (dotmode != 11) /* if putcolor won't be doing it for us */
         writedisk(col+sxoffs,row+syoffs,i_pot);
      writedisk(col+sxoffs,row+sydots+syoffs,(int)l_pot);
   }

   return(i_pot);
}

External angle and external ( parameter) ray

Example code ( Mathematica)

Image[Table[If[ColorQ@#,#,Black]&@Hue[(Arg[MandelbrotSetBoettcher[x+I y]]+Pi)/(2Pi)],{y,-2,2,.005},{x,-2,2,.005}]]

Methods

First find angle of last iteration. It is easy to compute and shows some external rays as a borders of level sets.

• • binary decomposition of LSM/M
  • nth-decomposition : color of exterior is proportional to quadrant in which last iteration lands. ( see below )
  • stripe average coloring

Then one go futher.

• automatically calculate external angles from nucleus and period by Claude Heiland-Allen

Methods:

• newton method
  • Tomoki Kawahira paper
  • Wolf Jung explanation
  • speeding up external ray tracing by Mandelbrot set Newton basins - Claude Heiland-Allen
• lines perpendicular to equipotential lines
• field line calculations, images and videos by Michael T Everest
• field lines by Chris M. Thomasson
• using Runge-Kutta integration by
  • images
  • mandelbrot-phase-angle-gif-loop

Tests

The Wolf Jung test

Part of parameter plane with external rays 1/7, 321685687669320/2251799813685247 and 321685687669322/2251799813685247 landing on the Mandelbrot set

The external parameter rays for angles (in turns)

• 321685687669320/2251799813685247 (period 51, lands on c1 = -0.088891642419446 +0.650955631292636i )
• 321685687669322/2251799813685247 ( period 51 lands on c2 = -0.090588078906990 +0.655983860334813i )
• 1/7 ( period 3, lands on c3 = -0.125000000000000 +0.649519052838329i )

Angles differ by about $10^{-15}$, but the landing points of the corresponding parameter rays are about 0.035 apart. It can be computed with Maxima CAS :

(%i1) c1: -0.088891642419446  +0.650955631292636*%i;
(%o1) 0.650955631292636*%i−0.088891642419446
(%i2) c2:-0.090588078906990  +0.655983860334813*%i;
(%o2) 0.655983860334813*%i−0.09058807890699
(%i3) abs(c2-c1);
(%o3) .005306692383854863
(%i4) c3: -0.125000000000000  +0.649519052838329*%i$
(%i5) abs(c3-c1);
(%o5) .03613692356607755
(%i6) a3:1/7$
(%i7) float(abs(a3-a1));
(%o7) 4.440892098500628*10^−16

Informations from W Jung program :

The angle  1/7  or  p001 has  preperiod = 0  and  period = 3.
The conjugate angle is  2/7  or  p010 .
The kneading sequence is  AA*  and the internal address is  1-3 .
The corresponding parameter rays are landing at the root of a satellite component of period 3.
It is bifurcating from period 1.

The angle  321685687669320/2251799813685247  or  p001001001001001001001001001001001001001001001001000 has  preperiod = 0  and  period = 51.
The conjugate angle is  321685687669319/2251799813685247  or  p001001001001001001001001001001001001001001001000111 .
The kneading sequence is  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABB*  and the internal address is  1-49-50-51 .
The corresponding parameter rays are landing at the root of a primitive component of period 51.

The angle  321685687669322/2251799813685247  or  p001001001001001001001001001001001001001001001001010 has  preperiod = 0  and  period = 51.
The conjugate angle is  321685687669329/2251799813685247  or  p001001001001001001001001001001001001001001001010001 .
The kneading sequence is  AABAABAABAABAABAABAABAABAABAABAABAABAABAABAABAABAA*  and the internal address is  1-3-51 .
The corresponding parameter rays are landing at the root of a satellite component of period 51.
It is bifurcating from period 3.

The test by G. Pastor and Miguel Romera

The external parameter rays for angles (in turns)

• 6871947673/34359738367 ( period 35 )
• 9162596898/34359738367 ( period 35 )

the central babies Mandelbrot sets of the cauliflowers located at -0.153756141 + 1.030383223i

(not that 34359738367 = 2^35 - 1)

test by M. Romera,1 G. Pastor, A. B. Orue,1 A. Martin, M.-F. Danca,and F. Montoya

Part of parameter plane with external 5 rays landing on the Mandelbrot set.png

G Pastor gave an example of external rays for which the resolution of the IEEE 754 is not sufficient:

• $\theta _{267}^{-}=0.((001)^{88}010)_{2}={\frac {33877456965431938318210482471113262183356704085033125021829876006886584214655562}{237142198758023568227473377297792835283496928595231875152809132048206089502588927}}$
• $\theta _{267}^{+}=0.((001)^{87}010001)_{2}={\frac {33877456965431938318210482471113262183356704085033125021829876006886584214655569}{237142198758023568227473377297792835283496928595231875152809132048206089502588927}}$
• $\theta _{3}^{-}=0.(001)_{2}={\frac {1}{7}}=0.(142857)_{10}$ ( period 3, lands on root point of period 3 component c3 = -0.125000000000000 +0.649519052838329i )
• $\theta _{268}^{-}=0.((001)^{88}0001)_{2}={\frac {67754913930863876636420964942226524366713408170066250043659752013773168429311121}{474284397516047136454946754595585670566993857190463750305618264096412179005177855}}$
• $\theta _{268}^{+}=0.((001)^{88}0010)_{2}={\frac {67754913930863876636420964942226524366713408170066250043659752013773168429311122}{474284397516047136454946754595585670566993857190463750305618264096412179005177855}}$

One can analyze these angles using program by Claude Heiland-Allen :

./bin/mandelbrot_describe_external_angle ".(001)"
binary: .(001)
decimal: 1/7
preperiod: 0
period: 3

 
./bin/mandelbrot_describe_external_angle 
".(001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001010)"
binary: 
.(001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001010)
decimal: 
33877456965431938318210482471113262183356704085033125021829876006886584214655562/237142198758023568227473377297792835283496928595231875152809132048206089502588927
preperiod: 0
period: 267

./bin/mandelbrot_describe_external_angle 
".(001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001010001)"
binary: 
.(001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001010001)
decimal: 
33877456965431938318210482471113262183356704085033125021829876006886584214655569/237142198758023568227473377297792835283496928595231875152809132048206089502588927
preperiod: 0
period: 267

./bin/mandelbrot_describe_external_angle 
".(0010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010001)"
binary: 
.(0010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010001)
decimal: 
67754913930863876636420964942226524366713408170066250043659752013773168429311121/474284397516047136454946754595585670566993857190463750305618264096412179005177855
preperiod: 0
period: 268

 
./bin/mandelbrot_describe_external_angle 
".(0010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010)"
binary: 
.(0010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010)
decimal: 
67754913930863876636420964942226524366713408170066250043659752013773168429311122/474284397516047136454946754595585670566993857190463750305618264096412179005177855
preperiod: 0
period: 268

Landing points of above rays are roots with angled internal addresses ( description by Claude Heiland-Allen) :

• the upper one will be 1 -> 1/3 -> 3 -> 1/(period/3) -> period because it's the nearest bulb to the lower root cusp of 1/3 bulb and child bulbs of 1/3 bulb have periods 3 * denominator(internal angle) ie, 1 -> 1/3 -> 3 -> 1/89 -> 267
• the lower one will be 1 -> floor(period/3)/period -> period because it's the nearest bulb below the 1/3 cusp ie, 1 -> 89/268 -> 268
• the middle ray .(001) lands at the root of 1 -> 1/3 -> 3, from the cusp on the lower side (which is on the right in a standard unrotated view)

See also

• https://web.archive.org/web/20071008112609/http://rgba.scenesp.org/iq/trastero/fieldlines/
• http://fraktal.republika.pl/mset_bottcher.html