Fractals/Iterations in the complex plane/Mandelbrot set/lavaurs

Topological models of Mandelbrot set
 * Thurston model ( abstract Mandelbrot set )
 * Lavaurs algorithm
 * Abstract Mandelbrot tree
 * Cactus model ( go to the image, where the src code and description of algorithm is )
 * Fake Mandelbrot Set = M-set without hairs, filaments and primitive hyberbolic components

=Lavaurs Algorithm=



This algorithm shows :
 * how to compute angles of external rays landing of root points of Mandelbrot set
 * how to draw diagram ( lamination on a circle) showing structure of Mandelbrot set

External angles (measured in turns modulo 1) landing on root points of period p components of Mandelbrot set are:

$$\frac{n}{2^p - 1}$$

Arc (chord) consists of:
 * two external angles landing on the same root point
 * root point.

Names

 * Quadratic Minor Lamination = QML.
 * It is the collection of Minor Leaves of any quadratic invariant lamination. The minor leaf is the image of the longest leaf in the lamination. An approximation of the Quadratic Minor Lamination contains only periodic minor leaves.
 * The quadratic minor lamination QML contains minor tags of dendritic quadratic polynomials.
 * lamination on the circle associated with Mandelbrot set
 * abstract Mandelbrot set ( Thurston)
 * pinching disk model of Mandelbrot set : after pinching ( glue) all points of circle, which are connected by arcs. It is space homeomorphic to Mandelbrot set

Mathematical description

 * draw unit circle $$S^1$$
 * connect angles of period two : 1/3 and 2/3 with arc in unit circle perpendicular to the boundary $$S^1$$
 * for next period connect 2 angles of the same period (first and second) with arc, such that :
 * no arc crosses any other ( previous) arc,
 * first angle is the smallest angles not yet connected, and second angle is the next smallest angle not yet connected

Programmer's description

 * draw main circle with center (x0, y0) and radius r0 in the middle of your image
 * choose period, for example p=3
 * compute pairs of angles ( in turns)
 * for each pair (alpha,beta) draw an arc using part of new circle, which is orthogonal to main circle in 2 points : c1,c2

Computing pair of angles
(to do )

Converting units of angles
First units are in turns ( list of pairs of angles )

Using ttr function:

convert them to radians:

Computing points of intersection
Main circle (x0,y0,r0) and new orthogonal circle ( x,y,r) have 2 common points :
 * c1 = (a,b) = (x0 + r0 * cos(alpha), y0 + r0 * sin(alpha))
 * c2 = (x0 + r0 * cos(beta), y0 + r0 * sin(beta))

Radius and center
At points of intersection radius r0 and new radius or orthogonal circle are also orthogonal. Center of new circle ( x,y) is on the line through point (x0,y0) and slope defined by angle gamma :

gamma = alpha + (balpha - alpha)/2

Using this information one can compute new circle : (x,y,r)

new angles
One has:
 * main circle (x0, y0m r0)
 * new circle (x, y, r)
 * point of intersections c1 and c2 and its angles : alpha and beta

Because arc will be drawn using new circle one has to compute (convert) new angles ( angles measured in new circle) :

angle of point c1 = (a,b) measured in new circle units.

Drawing arc
It depends of available procedures.

From point to point
Easiest case is drawing arc from point c1 to c2.

Postscript
In postscript there is arct procedure :

x1 y1 x2 y2 r arct

So in Lisp one can directly create ps file and use this procedure :

SVG
In SVG there is the elliptical arc curve command.

It is a version of path command that draws an elliptical arc from the current point to (x, y).

The size and orientation of the ellipse are defined by two radii (rx, ry) and an x-axis-rotation, which indicates how the ellipse as a whole is rotated relative to the current coordinate system.

The center (cx, cy) of the ellipse is calculated automatically to satisfy the constraints imposed by the other parameters.

large-arc-flag and sweep-flag contribute to the automatic calculations and help determine how the arc is drawn.

rx ry x-axis-rotation large-arc-flag sweep-flag x y

SVG path elements :
 * M100 100 specifies an absolute start point (100,100) for the circular arc
 * A 100 100 means that both the major axis and the minor axis of this circular arc have the same length = 100
 * 0 0 0 are x-axis-rotation large-arc-flag sweep-flag
 * 162.55 162.45 is an end point of arc

So one can draw arc by writing path command to svg file like that :

Remember that initial coordinate system in SVG has the origin at the top/left with the x-axis pointing to the right and the y-axis pointing down. For drawing all arcs it may be not important, but labels with angels will be wrong.

From angle to angle
This case is harder, because one must convert angles from main circle to new, orthogonal circle. When angles are converted then :
 * move current point to first point of arc ( intersection). Here c1 = (a,b)
 * draw an arc. It is easier to draw arc clockwise instead of counterclockwise

In Vecto Common Lisp package there is arcn procedure

Examples of code

 * Common Lisp code :
 * for drawing poscript files by Ruben Berenguel
 * for drawing png files using Vecto package
 * for drawing svg files
 * Haskell code with SVG output by Claude Heiland-Allen
 * c++ code from demo 4 page 7 from program Mandel by Wolf Jung