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# Odesolv.ps

The odesolv.ps program illustrates the solving of systems of ordinary differential equations (ODEs) directly in the PostScript programming language [1], here specifically illustrated for the 3D trajectory of the Lorenz attractor [2], named after the meteorologist Edward Norton Lorenz [3]. The instructions defined in the supplied routines are typically interpreted by either a PostScript viewer (such as GhostView with GhostScript as the interpreting engine) or the PostScript interpretor of your printer (in which case no computation is performed by your local computer, putting the whole task into the hands of your printer).

## Current revision

odesolv.ps [7.2 kB] The PostScript [1] code constituting the program.

## Description

As the method of choice for solving the system of ODEs of the Lorenz attractor, I have here implemented the standard four-point Runge-Kutta algorithm [4], somewhat tweaked to operate directly on a stack.

For the parameters of the Lorenz equations,

they are in this specific case chosen as σ = 10 (the Prandtl number), ρ = 28 (the Rayleigh number), and β = 8/3, while the starting point is chosen as (x0, y0, z0) = (0, 1.0, 0.9). However, please feel free to play around with these parameters by altering the indicated block of user customizable parameters in the PostScript code.

Figure 1. Graphical output of the odesolv.ps PostScript program, for parameters σ = 10, ρ = 28, β = 8/3, with starting point at (x0, y0, z0) = (0, 1.0, 0.9). Notice that all output here was generated using the PostScript interpretor. Download PDF [458 kB]

## PostScript code

Listing of the odesolv.ps PostScript code, as used when creating the image in the above example:

```
```

Deprecated: Function split() is deprecated in /storage/content/45/2011745/jonsson.eu/public_html/php/htmlicise.php on line 46
%%Creator: Fredrik Jonsson <http://jonsson.eu>
%%Title: odesolv.ps  <http://jonsson.eu/programs/postscript/odesolv/>
%%Orientation: Portrait
%%CreationDate: 17-Feb-10

%
% The odesolv.ps program illustrates the solving of systems of ordinary
% differential equations (ODEs) directly in the PostScript programming
% language [1], here specifically illustrated for the 3D trajectory of the
% Lorenz attractor [2]. The instructions defined in the supplied routines
% are typically interpreted by either a PostScript viewer (such as GhostView
% with GhostScript as the interpreting engine) or the PostScript interpretor
% of your printer (in which case no computation is performed by your local
% computer, putting the whole task into the hands of your printer).
%
% As the method of choice for solving the system of ODEs of the Lorenz
% attractor, I have here implemented the standard four-point Runge-Kutta
% algorithm [3], somewhat tweaked to operate directly on a stack.
%
% For the parameters of the Lorenz equations,
%
%         d  / x(t) \   / fx(x,y,z) \   / sigma*(y-x) \
%        --- | y(t) | = | fy(x,y,z) | = | x*(rho-z)-y |,
%         dt \ z(t) /   \ fz(x,y,z) /   \ x*y-beta*z  /
%
% they are in this specific case chosen as sigma = 10 (the Prandtl number),
% rho = 28 (the Rayleigh number), and beta = 8/3, while the starting point
% is chosen as (x0, y0, z0) = (0, 1.0, 0.9). However, please feel free to
% play around with these parameters.
%
% [2] http://en.wikipedia.org/wiki/Lorenz_attractor
% [3] http://en.wikipedia.org/wiki/Runge%E2%80%93Kutta_methods
%
% #################################################
% #     BEGIN OF USER CUSTOMIZABLE PARAMETERS     #
% #################################################
/sigma {10.0} def          % Prandtl number
/rho {28.0} def            % Rayleigh number
/beta {8.0 3.0 div} bind def
/r0 {0.00 1.00 0.90} def   % (x0, y0, z0), starting point of trajectory
/h {0.001} def             % Step length in fourth-order Runge-Kutta method
/num_iter {70000} def      % Number of iterations in the Runge-Kutta method
% #################################################
% #      END OF USER CUSTOMIZABLE PARAMETERS      #
% #################################################

%
% Given a coordinate triplet (x,y,z) on the stack, the fx, fy, fz and f
% routines computes the right-hand side of the Lorenz equation
%
%         d  / x(t) \   / fx(x,y,z) \   / sigma*(y-x) \
%        --- | y(t) | = | fy(x,y,z) | = | x*(rho-z)-y |
%         dt \ z(t) /   \ fz(x,y,z) /   \ x*y-beta*z  /
%
/fx {         % x y z (as we enter)
3 1 roll   % z x y
dup        % z x y y
4 2 roll   % y y z x
dup        % y y z x x
5 4 roll   % y z x x y
exch       % y z x y x
sub        % y z x (y-x)
sigma      % y z x (y-x) sigma
mul        % y z x sigma*(y-x)
exch       % y z sigma*(y-x) x
4 2 roll   % sigma*(y-x) x y z == fx x y z
} def

/fy {         % fx x y z (as we enter)
dup        % fx x y z z
rho        % fx x y z z rho
sub        % fx x y z (z-rho)
5 3 roll   % y z (z-rho) fx x
dup        % y z (z-rho) fx x x
4 3 roll   % y z fx x x (z-rho)
mul        % y z fx x (x*(z-rho))
5 4 roll   % z fx x (x*(z-rho)) y
dup        % z fx x (x*(z-rho)) y y
3 1 roll   % z fx x y (x*(z-rho)) y
add        % z fx x y ((x*(z-rho))+y)
neg        % z fx x y ((x*(rho-z))-y)
3 1 roll   % z fx ((x*(rho-z))-y) x y
5 4 roll   % fx ((x*(rho-z))-y) x y z == fx fy x y z
} def

/fz {         % fx fy x y z (as we enter)
beta       % fx fy x y z beta
mul        % fx fy x y (beta*z)
3 1 roll   % fx fy (beta*z) x y
mul        % fx fy (beta*z) (x*y)
sub        % fx fy (beta*z-x*y)
neg        % fx fy (x*y-beta*z) == fx fy fz
} def

/f {          % x y z (as we enter)
fx         % fx x y z
fy         % fx fy x y z
fz         % fx fy fz == {sigma*(y-x), x*(rho-z)-y, x*y-beta*z}
} def

%
% The dupvec routine duplicates a 3D vector on the stack.
%
/dupvec {     % x y z (as we enter)
dup        % x y z z
4 1 roll   % z x y z
exch       % z x z y
dup        % z x z y y
5 1 roll   % y z x z y
3 2 roll   % y z z y x
dup        % y z z y x x
6 1 roll   % x y z z y x
exch       % x y z z x y
3 2 roll   % x y z x y z
} def

%
% The mulvec routine multiplies a 3D vector on the stack by a scalar.
%
/mulvec {     % x y z a (as we enter)
dup        % x y z a a
dup        % x y z a a a
6 2 roll   % a a x y z a
mul        % a a x y a*z
4 2 roll   % a y a*z a x
mul        % a y a*z a*x
4 2 roll   % a*z a*x a y
mul        % a*z a*x a*y
3 2 roll   % a*x a*y a*z == a*(x y z)
} def

%
%
/addvec {     % x y z a b c (as we enter)
3 1 roll   % x y z c a b
exch       % x y z c b a
4 2 roll   % x y b a z c
add        % x y b a (z+c)
5 2 roll   % a (z+c) x y b
add        % a (z+c) x (y+b)
3 2 roll   % a x (y+b) (z+c)
4 2 roll   % (y+b) (z+c) a x
3 1 roll   % (a+x) (y+b) (z+c) == (x y z) + (a b c)
} def

%
% The swapvec routine swaps two 3D vectors on the stack.
%
/swapvec {    % x y z a b c (as we enter)
6 3 roll   % a b c x y z
} def

%
% The k1 routine computes the first Runge-Kutta term k1 = k1x k1y k1z, where
%    k1x = fx((x,y,z)),
%    k1y = fy((x,y,z)),
%    k1z = fz((x,y,z)).
%
/k1 {         % x y z (as we enter)
dupvec     % x y z x y z
f          % x y z fx fy fz == x y z k1x k1y k1z
} def

%
% The k2 routine computes the second Runge-Kutta term k2 = k2x k2y k2z, where
%    k2x = fx((x,y,z)+(h/2)*(k1x,k1y,k1z)),
%    k2y = fy((x,y,z)+(h/2)*(k1x,k1y,k1z)),
%    k2z = fz((x,y,z)+(h/2)*(k1x,k1y,k1z)).
%
/k2 {         % x y z k1x k1y k1z (as we enter)
dupvec     % x y z k1x k1y k1z k1x k1y k1z
h          % x y z k1x k1y k1z k1x k1y k1z h
2          % x y z k1x k1y k1z k1x k1y k1z h 2
div        % x y z k1x k1y k1z k1x k1y k1z (h/2)
mulvec     % x y z k1x k1y k1z (h/2)*k1x (h/2)*k1y (h/2)*k1z
9 6 roll   % k1x k1y k1z (h/2)*k1x (h/2)*k1y (h/2)*k1z x y z
dupvec     % k1x k1y k1z (h/2)*k1x (h/2)*k1y (h/2)*k1z x y z x y z
12 3 roll  % x y z k1x k1y k1z (h/2)*k1x (h/2)*k1y (h/2)*k1z x y z
addvec     % x y z k1x k1y k1z (x+(h/2)*k1x) (y+(h/2)*k1y) (z+(h/2)*k1z)
f          % x y z k1x k1y k1z k2x k2y k2z
} def

%
% The k3 routine computes the third Runge-Kutta term k3 = k3x k3y k3z, where
%    k3x = fx((x,y,z)+(h/2)*(k2x,k2y,k2z)),
%    k3y = fy((x,y,z)+(h/2)*(k2x,k2y,k2z)),
%    k3z = fz((x,y,z)+(h/2)*(k2x,k2y,k2z)).
%
/k3 {         % {x} {k1} {k2} (as we enter)
dupvec     % {x} {k1} {k2} {k2}
h          % {x} {k1} {k2} {k2} h
2          % {x} {k1} {k2} {k2} h 2
div        % {x} {k1} {k2} {k2} (h/2)
mulvec     % {x} {k1} {k2} (h/2)*{k2}
12 9 roll  % {k1} {k2} (h/2)*{k2} {x}
dupvec     % {k1} {k2} (h/2)*{k2} {x} {x}
15 3 roll  % {x} {k1} {k2} (h/2)*{k2} {x}
addvec     % {x} {k1} {k2} {x}+(h/2)*{k2}
f          % {x} {k1} {k2} {k3}
} def

%
% The k4 routine computes the fourth Runge-Kutta term k4 = k4x k4y k4z, where
%    k4x = fx((x,y,z)+h*(k3x,k3y,k3z)),
%    k4y = fy((x,y,z)+h*(k3x,k3y,k3z)),
%    k4z = fz((x,y,z)+h*(k3x,k3y,k3z)).
%
/k4 {         % {x} {k1} {k2} {k3} (as we enter)
dupvec     % {x} {k1} {k2} {k3} {k3}
h          % {x} {k1} {k2} {k3} {k3} h
mulvec     % {x} {k1} {k2} {k3} h*{k3}
15 12 roll % {k1} {k2} {k3} h*{k3} {x}
dupvec     % {k1} {k2} {k3} h*{k3} {x} {x}
18 3 roll  % {x} {k1} {k2} {k3} h*{k3} {x}
addvec     % {x} {k1} {k2} {k3} {x}+h*{k3}
f          % {x} {k1} {k2} {k3} {k4}
} def

%
% Given a 3D coordinate triplet on the stack, the rn routine computes the
% next point of the Lorenz trajectory by the four-point Runge-Kutta method,
% given the step size h and right-hand side of the Lorenz system of ODEs
% as defined by the f routine (wrapping up the fx, fy and fz routines).
%
/rn {         % {x} == x y z (as we enter)
k1         % {x} {k1}
k2         % {x} {k1} {k2}
k3         % {x} {k1} {k2} {k3}
k4         % {x} {k1} {k2} {k3} {k4}
9 3 roll   % {x} {k1} {k4} {k2} {k3}
addvec     % {x} {k1} {k4} {k2}+{k3}
2          % {x} {k1} {k4} {k2}+{k3} 2
mulvec     % {x} {k1} {k4} 2*({k2}+{k3})
h          % {x} {k1}+2*({k2}+{k3})+{k4} h
6          % {x} {k1}+2*({k2}+{k3})+{k4} h 6
div        % {x} {k1}+2*({k2}+{k3})+{k4} (h/6)
mulvec     % {x} (h/6)*({k1}+2*({k2}+{k3})+{k4})
} def

%
% Define a set of parameters to be used for the actual mapping of the
% numerical solution of the ODE to page coordinates. (This may be read
% as being the "Page Setup" section.)
%
/xc 140 def   % Page x-coordinate of origo in points (1/72 inch)
/yc 500 def   % Page y-coordinate of origo in points (1/72 inch)
/scale 7 def  % Scale in points per dimensionless unit in plot
/origo {0 0 0} def
/xaxislength {20} def
/yaxislength {20} def
/zaxislength {20} def
/xaxis {1 0 0 xaxislength mulvec} def
/yaxis {0 1 0 yaxislength mulvec} def
/zaxis {0 0 1 zaxislength mulvec} def

%
% Compute the 2D page coordinates (xp,yp) in terms of coordinates expressed
% in the 3D reference frame (x,y,z) in which the differential equation is
% solved. For an informal description of the 3D projection matrix, see
% http://en.wikipedia.org/wiki/3D_projection
%
% Notice how the PostScript operator 'exch' throughout is used for the parsing
% of input parameters to this routine, in the form "/{myregister} exch def".
% Example: Suppose we wish to assign the value '1' to the register '/radius'.
% If our input argument is '1', the PostScript stack then starts with:
%      1
% After performing the 'exch' we then have the stack
%      1
% after which the def command finally performs the actual assignment of the
% value into the register '/radius'.
%
/screencoord {  % Stack: x y z       (as we enter)
dupvec       % Stack: x y z x y z
3 dict begin % Begin dictionary; Active until next 'end' statement
/az exch def % Stack: x y z x y   (with z read into az register)
/ay exch def % Stack: x y z x     (with y read into ay register)
/ax exch def % Stack: x y z       (with x read into ax register)
/cx {10} def
/cy {10} def
/cz {10} def
/axcx {ax cx sub} def % ax-cx
/aycy {ay cy sub} def % ay-cy
/azcz {az cz sub} def % az-cz
/theta_x {0} def % Angle expressed in degrees
/theta_y {20} def % Angle expressed in degrees
/theta_z {20} def % Angle expressed in degrees
/ctx {theta_x cos} bind def
/stx {theta_x sin} bind def
/cty {theta_y cos} bind def
/sty {theta_y sin} bind def
/ctz {theta_z cos} bind def
/stz {theta_z sin} bind def
/szyy {stz aycy mul} bind def
/czxx {ctz axcx mul} bind def
/syzz {sty azcz mul} bind def
/cyzz {cty azcz mul} bind def
/czyy {ctz aycy mul} bind def
/szxx {stz axcx mul} bind def
/dx {szyy czxx add cty mul syzz sub} bind def
%   /dy {Still to be written for the general Euler angles}
%   /dz {Still to be written for the general Euler angles}
end          % End of dictionary
100 100
} bind def

/pagecoord_xy {  % x y z (as we enter)
3 1 roll   % z x y
dup        % z x y y
4 2 roll   % y z x y
exch       % y z y x
dup        % y z y x x
5 1 roll   % x y z y x
scale      % x y z y x scale
mul        % x y z y scale*x
xc         % x y z y scale*x xc
add        % x y z y xc+scale*x
exch       % x y z xc+scale*x y
scale      % x y z xc+scale*x y scale
mul        % x y z xc+scale*x scale*y
yc         % x y z xc+scale*x scale*y yc
add        % x y z xc+scale*x yc+scale*y
} def

/pagecoord_yz {  % x y z (as we enter)
dup        % x y z z
4 2 roll   % z z x y
dup        % z z x y y
scale      % z z x y y scale
mul        % z z x y scale*y
yc         % z z x y scale*y yc
add        % z z x y yc+scale*y
5 4 roll   % z x y yc+scale*y z
scale      % z x y yc+scale*y z scale
mul        % z x y yc+scale*y scale*z
xc         % z x y yc+scale*y scale*z xc
add        % z x y yc+scale*y xc+scale*z
exch       % z x y xc+scale*z yc+scale*y
5 2 roll   % xc+scale*z yc+scale*y z x y
3 2 roll   % xc+scale*z yc+scale*y x y z
5 3 roll   % x y z xc+scale*z yc+scale*y
} def

/drawcoordsys {
/Helvetica-Italic findfont 10 scalefont setfont
% Draw the X-axis and its label
origo pagecoord_yz moveto
xaxis pagecoord_yz lineto
(X) show % The previous 'lineto' has already put us in place for the label
stroke
% Draw the Y-axis and its label
origo pagecoord_yz moveto
yaxis pagecoord_yz lineto
(Y) show % The previous 'lineto' has already put us in place for the label
stroke
% Draw the Z-axis and its label
origo pagecoord_yz moveto
zaxis pagecoord_yz lineto
(Z) show % The previous 'lineto' has already put us in place for the label
stroke
} def

/title {
/Helvetica-Bold findfont 12 scalefont setfont
100 762 moveto (Lorenz attractor) show
/Helvetica findfont 10 scalefont setfont
100 748 moveto (Trajectory solved directly in PostScript by odesolv.ps) show
/Helvetica findfont 10 scalefont setfont
100 736 moveto (http://jonsson.eu/programs/postscript/odesolv/) show
} def

%
%  Main program begins here.
%
title            % Title and caption
drawcoordsys     % Draw coordinate axes
clear            % Clear stack
r0               % Stack: x0 y0 z0      (enter starting point)
pagecoord_yz     % Stack: x0 y0 z0 x_page y_page (map to page)
% screencoord
moveto           % Stack: x0 y0 z0      (position pen to starting point)
num_iter {       % Main loop
rn            % Stack: x_{n+1} y_{n+1} z_{n+1}    (next point solved for)
pagecoord_yz  % Stack: x_{n+1} y_{n+1} z_{n+1} x_page y_page (map to page)
% screencoord
lineto        % Stack: x_{n+1} y_{n+1} z_{n+1}    (draw line to next point)
} repeat         % Repeat main loop num_iter Runge-Kutta iterations
stroke
showpage         % Display everything drawn on the current page
```

```

## References

[1] For information on the PostScript programming language, see for example the PostScript area on the website of Adobe Systems Inc., at http://www.adobe.com/products/postscript/ or the reference book "PostScript Language - Tutorial and Cookbook" (Adison-Wesley, Reading, Massachusetts, 1985), ISBN 0-201-10179-3.

[2] http://en.wikipedia.org/wiki/Lorenz_attractor. The original paper on the transformation: Edward N. Lorenz, Deterministic Nonperiodic Flow, Journal of the Atmospheric Sciences 20, 130 (1963) PDF [1.02 MB]

[3] Edward Norton Lorenz (May 23, 1917 — April 16, 2008); not to be confused with the Dutch physicist and Nobel laureate Hendrik Antoon Lorentz (18 July 1853 — 4 February 1928) of the classical Lorentz transformation..

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