# 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  :
• 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

One can also draw curves :

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

Similar projects:

## 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

This algorithm answers the question: “For which values of c will the Julia fractal, J(c), be line-like and for which dust-like?”

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)

// 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
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 

##### 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";
static unsigned char color;
/* 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 ;  */
/*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;
color=0;
color=0;
}
else
{ /* exterior of Mandelbrot set = white */
color=255; /* Red*/
color=255;  /* Green */
color=255;/* Blue */
};
/*write color to the file*/
fwrite(color,1,3,fp);
}
}
fclose(fp);
return 0;
}


### Integer escape time = LSM/M = dwell bands

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 #### 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;
color=0;
color=0;
}
/* exterior of Mandelbrot set = LSM */
else if ((Iteration%2)==0)
{ /* even number = black */
color=0; /* Red */
color=0; /* Green */
color=0; /* Blue */
}
else
{/* odd number =  white */
color=255; /* Red */
color=255; /* Green */
color=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
}
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++)
{
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 :


#### 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.

#### 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

#### Wolfram Mathematica

Here is code by Paul Nylander

### Level Curves of escape time Method = LCM/M

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

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;
color=0;
color=0;
}
/* exterior of Mandelbrot set = LSM */
else if (Zy>0)
{
color=0; /* Red */
color=0; /* Green */
color=0; /* Blue */
}
else
{
color=255; /* Red */
color=255; /* Green */
color=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
• 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;
color=0;
color=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
return ctx.exp(1j*(i + 1 - ctx.log(ctx.log(abs(z)))/ctx.log(2)))
return 0


## Distance estimation DEM/M

Variants :

• exterior DEM/M
• interior DEM/M