path: root/Master/texmf-dist/tex/latex/sfg/sfg.sty

%@(#) sfg.sty: Signal flow graphs
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% This is the style file sfg.sty for drawing signal flow graphs in LaTeX2e.
%
%               Version 0.9, 1.7.1998, Hanspeter Schmid
%               Version 0.91, 22.08.2007 hv scalebox->\psscalebox
%
% If you find an error, or if you want me to implement some more 
% functions, want me to include more references to books on sfgs,
% or if you just want to say `hello thank you, I find you package
% useful', please don't hesitate and contact me:
%
%   Hanspeter Schmid, schmid@isi.ee.ethz.ch, 
%                     http://www.isi.ee.ethz.ch/~schmid/
%
%% This program can be redistributed and/or modified under the terms
%% of the LaTeX Project Public License Distributed from CTAN
%% archives in directory macros/latex/base/lppl.txt.
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% History: (Version numbers are going towards 3/pi.  
%           sfg.sty is not good enough yet to be called v1.0)
%
% created: 26.1.96, hps, (v0)
% modified for submission to CTAN: 1.7.98, hps, (v0.9) 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Plans:   - make a proper dtx-file out of this mess.  I'll do this if
%            and when I find the time to do it.
%          - include an sfg branch which has snake form, e.g. \sfgsnake
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% SEE DESCRIPTION AFTER THE \endinput
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\NeedsTeXFormat{LaTeX2e}
\RequirePackage[nomessages]{fp}
\RequirePackage{pstricks}
\ProvidesPackage{sfg}[2007/08/22]
\typeout{Style sfg, by Hanspeter Schmid, version 0.91}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Line thicknesses:
\newdimen\sfg@Li   \sfg@Li=0.40pt         % normal lines (LaTeX `thin')
\newdimen\sfg@Lii  \sfg@Lii=1.80\sfg@Li   % thick lines (`thick') (hacked)
\newdimen\sfg@Liii \sfg@Liii=3\sfg@Li     % thickness of heavy lines
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% internal functions
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\def\sfg@degtorad{0.01745329252}% conversion factor deg -> rad
\def\sfg@radtodeg{57.2957795132}% conversion factor rad -> deg

\def\sfg@curvescale{0.577350269}% standard sfgcurve angle, cot(60 deg)

\def\sfg@arrowloc{0.5}          % standard location of the arrow is in 
                                % the middle of an sfg branch

                                % calculate the angle of vector (#1,#2) 
                                % in degrees, given that #1 
                                % is positive, ...
                                % ... and finally add #3.
\def\sfg@angleadd#1#2#3{{%      % formula: arcsin(#3/sqrt(#1^2+#2^2))
  \FPmul\sfg@xx{#1}{#1}\FPmul\sfg@yy{#2}{#2}%
  \FPadd\sfg@rr\sfg@xx\sfg@yy\FProot\sfg@len\sfg@rr{2}%
                                % calculate length of vector
  \FPdiv\sfg@sin{#2}\sfg@len\FParcsin\sfg@result\sfg@sin% angle
  \FPmul\sfg@resultb\sfg@result\sfg@radtodeg%
  \FPadd\sfg@result\sfg@resultb{#3}%
  \FPtrunc\sfg@angle\sfg@result{2}\global\let\sfg@angle\sfg@angle}}%

                                % ... and finally subtract from #3.
\def\sfg@anglesub#1#2#3{{%
  \FPmul\sfg@xx{#1}{#1}\FPmul\sfg@yy{#2}{#2}%
  \FPadd\sfg@rr\sfg@xx\sfg@yy\FProot\sfg@len\sfg@rr{2}%
                                % calculate length of vector
  \FPdiv\sfg@sin{#2}\sfg@len\FParcsin\sfg@result\sfg@sin% angle
  \FPmul\sfg@resultb\sfg@result\sfg@radtodeg%
  \FPsub\sfg@result{#3}\sfg@resultb%
  \FPtrunc\sfg@angle\sfg@result{2}\global\let\sfg@angle\sfg@angle}}% 

                                
\def\sfg@arrow#1{%              % rotateable bezier-curve based arrow 
  \begin{picture}(0,0){%        % zero-dimensional
      \rput{#1}(0,0){%          % rotate counter-clockwise by #1 degrees
        \psscalebox{\sfg@arrowlen}{% scale by factor \sfg@arrowlen
          \pscustom*[linetype=1,linewidth=\sfg@Li]{% draw arrow
          \psbezier(0.5,0)(0.15,-0.0825)(-0.075,-0.1625)(-0.5,-0.375)%
          \psline(-0.5,0.375)%
          \psbezier(-0.075,0.1625)(0.15,0.0825)(0.5,0)}}}}%
  \end{picture}}%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% functions to configure the signal flow graphs
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
                                % set the package's numerical parameters

\def\sfgsetunit#1{%             % unitlength
  \setlength{\unitlength}{#1}%
  \psset{unit=#1, linewidth=\sfg@Li}}%

\def\sfgsetangle#1{%            % incidence angle of sfgcurves
  {\FPmul\sfg@tmp\sfg@degtorad{#1}%
    \FPcot\sfg@curvescale\sfg@tmp%
    \global\let\sfg@curvescale\sfg@curvescale}}%

\def\sfgsetarrowloc#1{%         % location of the arrow on sfgbranches
  \def\sfg@arrowloc{#1}}%

\def\sfgsetsize#1#2#3#4{%       % size of arrows and nodes ...
  \gdef\sfg@noderad{#1}\gdef\sfg@arrowlen{#2}%
  \gdef\sfg@nlabel{#3}\gdef\sfg@alabel{#4}%
                                % label distances for \NW, \NE, \SW, \SE
  \FPmul\sfg@nlabeld{#3}{0.707106781188}% multiply with 1/sqrt(2)
  \global\let\sfg@nlabeld\sfg@nlabeld%
  \FPmul\sfg@alabeld{#3}{0.707106781188}%
  \global\let\sfg@alabeld\sfg@alabeld%
  }%

\def\sfgsetlines#1#2#3{%        % line thicknesses
  \sfg@Li=#1\sfg@Lii=#2\sfg@Li\sfg@Liii=#3\sfg@Li%
  \psset{linewidth=\sfg@Li}}
                                
\def\sfgsetcompass{%            % define macros for label placement    
  \gdef\E{0}\gdef\NE{1}\gdef\N{2}\gdef\NW{3}%
  \gdef\W{4}\gdef\SW{5}\gdef\S{6}\gdef\SE{7}}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% drawing signal flow graphs
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\def\sfgtermnode#1#2{%          % terminal node (white with black line}
  \begin{picture}(0,0){%        % zero-size pictures make life easier
      \put(0,0){%
                                % circle, radius \sfg@noderad
        \pscircle*[linecolor=white,dimen=outer](0,0){\sfg@noderad}% 
        \pscircle[linewidth=\sfg@Lii](0,0){\sfg@noderad}}%
                                % label placement
      \ifcase#1                 % E (east)
                                % text #2 in a distance of \sfg@nlabel
        \put(\sfg@nlabel,0){\makebox(0,0)[l]{#2}}% 
      \or                       % NE (north-east)
        \put(\sfg@nlabeld,\sfg@nlabeld){\makebox(0,0)[bl]{#2}}%
      \or                       % N (...)
        \put(0,\sfg@nlabel){\makebox(0,0)[b]{#2}}%
      \or                       % NW (\sfg@tmp := -\sfg@nlabel)
        \FPneg\sfg@tmp{\sfg@nlabeld}% 
        \put(\sfg@tmp,\sfg@nlabeld){\makebox(0,0)[br]{#2}}%
      \or                       % W
        \FPneg\sfg@tmp{\sfg@nlabel}%
        \put(\sfg@tmp,0){\makebox(0,0)[r]{#2}}%
      \or                       % SW
        \FPneg\sfg@tmp{\sfg@nlabeld}%
        \put(\sfg@tmp,\sfg@tmp){\makebox(0,0)[tr]{#2}}%
      \or                       % S
        \FPneg\sfg@tmp{\sfg@nlabel}%
        \put(0,\sfg@tmp){\makebox(0,0)[t]{#2}}%
      \or                       % SE
        \FPneg\sfg@tmp{\sfg@nlabeld}%
        \put(\sfg@nlabeld,\sfg@tmp){\makebox(0,0)[tl]{#2}}%
      \else                     % no label
        \PackageWarning{sfg}%
        {sfgtermnode: label placement option #1 unknown}%
      \fi%
  }\end{picture}}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\def\sfgnode#1#2{%              % normal node (just black)
  \begin{picture}(0,0){%
      \put(0,0){%
        \pscircle*[linewidth=0pt](0,0){\sfg@noderad}}% filled circle
                                % label placement, comments -> sfgtermnode
      \ifcase#1 \put(\sfg@nlabel,0){\makebox(0,0)[l]{#2}}%
      \or\put(\sfg@nlabeld,\sfg@nlabeld){\makebox(0,0)[bl]{#2}}%
      \or\put(0,\sfg@nlabel){\makebox(0,0)[b]{#2}}%
      \or\FPneg\sfg@tmp{\sfg@nlabeld}%
        \put(\sfg@tmp,\sfg@nlabeld){\makebox(0,0)[br]{#2}}%
      \or\FPneg\sfg@tmp{\sfg@nlabel}%
        \put(\sfg@tmp,0){\makebox(0,0)[r]{#2}}%
      \or\FPneg\sfg@tmp{\sfg@nlabeld}%
        \put(\sfg@tmp,\sfg@tmp){\makebox(0,0)[tr]{#2}}%
      \or\FPneg\sfg@tmp{\sfg@nlabel}%
        \put(0,\sfg@tmp){\makebox(0,0)[t]{#2}}%
      \or\FPneg\sfg@tmp{\sfg@nlabeld}%
        \put(\sfg@nlabeld,\sfg@tmp){\makebox(0,0)[tl]{#2}}%
      \else\PackageWarning{sfg}%
        {sfgnode: label placement option #1 unknown}%
      \fi%
  }\end{picture}}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\def\sfgbranch#1#2#3#4{%         % straight sfg branch
  \begin{picture}(0,0){%        
                                % determine in which quadrant the vector's
                                % end point (x,y) lies (x=#1, y=#2)
      \FPset\sfg@flag{0}%       % use a bit-flag
      \FPifzero{#1}\FPadd\sfg@flag\sfg@flag{1}\fi%   % x=0 -> bit 0 
      \FPifneg{#1}\FPadd\sfg@flag\sfg@flag{2}\fi%    % x<0 -> bit 1 
      \FPifzero{#2}\FPadd\sfg@flag\sfg@flag{4}\fi%   % y=0 -> bit 2 
      \FPifneg{#2}\FPadd\sfg@flag\sfg@flag{8}\fi%    % y<0 -> bit 3
                                % case-distinction: correct the main values
                                % of arcsin 
                                % #2#1
                                % -0-0
      \ifcase\sfg@flag          % 0000 first quadrant
        \sfg@angleadd{#1}{#2}{0}%
      \or                       % 0001 positive y-axis
        \FPset\sfg@angle{90}%
      \or                       % 0010 second quadrant
        \sfg@anglesub{#1}{#2}{180}%
      \or                       % 0011 impossible
      \or                       % 0100 positive x-axis
        \FPset\sfg@angle{0}%
      \or                       % 0101 zero vector
        \FPset\sfg@angle{0}%
        \PackageWarning{sfg}{sfgbranch: zero-length branch drawn.}%
      \or                       % 0110 negative x-axis
        \FPset\sfg@angle{180}%
      \or                       % 0111 impossible
      \or                       % 1000 third quadrant
        \sfg@angleadd{#1}{#2}{0}%
      \or                       % 1001 negative y-axis
        \FPset\sfg@angle{-90}%
      \or                       % 1010 fourth quadrant
        \sfg@anglesub{#1}{#2}{180}%
      \else                     % every other bit pattern is impossible.
      \fi %                     % now draw the line
      \put(0,0){\psline[linewidth=\sfg@Li,arrows=C-C](0,0)(#1,#2)}%
                                % calculate centre of arrow and label
      \FPmul\sfg@dx{#1}\sfg@arrowloc\FPmul\sfg@dy{#2}\sfg@arrowloc%
      \put(\sfg@dx,\sfg@dy){%   % place arrow and label
        \sfg@arrow{\sfg@angle}%
                                % label placement (see sfgtermnode)
        \ifcase#3\put(\sfg@alabel,0){\makebox(0,0)[l]{#4}}% 
        \or\put(\sfg@alabeld,\sfg@alabeld){\makebox(0,0)[bl]{#4}}%
        \or\put(0,\sfg@alabel){\makebox(0,0)[b]{#4}}%
        \or\FPneg\sfg@tmp{\sfg@alabeld}%
        \put(\sfg@tmp,\sfg@alabeld){\makebox(0,0)[br]{#4}}%
        \or\FPneg\sfg@tmp{\sfg@alabel}%
        \put(\sfg@tmp,0){\makebox(0,0)[r]{#4}}%
        \or\FPneg\sfg@tmp{\sfg@alabeld}%
        \put(\sfg@tmp,\sfg@tmp){\makebox(0,0)[tr]{#4}}%
        \or\FPneg\sfg@tmp{\sfg@alabel}%
        \put(0,\sfg@tmp){\makebox(0,0)[t]{#4}}%
        \or\FPneg\sfg@tmp{\sfg@alabeld}%
        \put(\sfg@alabeld,\sfg@tmp){\makebox(0,0)[tl]{#4}}%
        \else\PackageWarning{sfg}%
        {sfgbranch: label placement option #3 unknown}%
        \fi %
        } %
      }\end{picture}}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\def\sfgcurve#1#2#3#4#5{%       % curved sfg branch
  \begin{picture}(0,0){%        % qadrant ... (see sfgbranch)
      \FPset\sfg@flag{0}%
      \FPifzero{#1}\FPadd\sfg@flag\sfg@flag{1}\fi%
      \FPifneg{#1}\FPadd\sfg@flag\sfg@flag{2}\fi% 
      \FPifzero{#2}\FPadd\sfg@flag\sfg@flag{4}\fi%
      \FPifneg{#2}\FPadd\sfg@flag\sfg@flag{8}\fi% 
      \ifcase\sfg@flag\sfg@angleadd{#1}{#2}{0}%
      \or\FPset\sfg@angle{90}%
      \or\sfg@anglesub{#1}{#2}{180}%
      \or\or\FPset\sfg@angle{0}%
      \or\FPset\sfg@angle{0}% 
      \or\FPset\sfg@angle{180}%
      \or\or\sfg@angleadd{#1}{#2}{0}%
      \or\FPset\sfg@angle{-90}%
      \or\sfg@anglesub{#1}{#2}{180}%
      \else\fi %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      {%                        % calculate path shift and new 
                                % arrow location:
                                % mid: vector to the straight 
                                % path's arrow midpoint
        \FPmul\sfg@midx{#1}{0.5}\FPmul\sfg@midy{#2}{0.5}%
                                % p: rotate mid 90 degrees counterclockwise
        \FPneg\sfg@px\sfg@midy\FPset\sfg@py\sfg@midx%
                                % len: length of p (and therefore mid)
        \FPmul\sfg@xx\sfg@px\sfg@px\FPmul\sfg@yy\sfg@py\sfg@py%
        \FPadd\sfg@rr\sfg@xx\sfg@yy\FProot\sfg@len\sfg@rr{2}%
                                % scale: #3/len
        \FPdiv\sfg@scale{#3}\sfg@len%
                                % a: p scaled to the absolute length #3
        \FPmul\sfg@ax\sfg@px\sfg@scale\FPmul\sfg@ay\sfg@py\sfg@scale%
                                % d: mid+a; vector to the arrow centre
        \FPadd\sfg@dx\sfg@ax\sfg@midx\FPadd\sfg@dy\sfg@ay\sfg@midy%
                                % export this value
        \global\let\sfg@dx\sfg@dx\global\let\sfg@dy\sfg@dy% exported
                                % now calculate the four additional 
                                % bezier points. Two of them are at the
                                % intersections of the tangents and 
                                % the line parallel to mid through 
                                % the point (dx,dy).
                                % Their distance from arrow centre:
                                % dist = len-(|#3|*cot(alpha))
        \FPabs\sfg@three{#3}\FPmul\sfg@tmp{\sfg@three}\sfg@curvescale%
        \FPsub\sfg@dist\sfg@len\sfg@tmp%
                                % now compare this to 2*arrowlength,
                                % which is the minimum allowed value if
                                % sfg paths shouldn't look crippled
        \FPmul\sfg@limit\sfg@arrowlen{2}%
        \FPiflt\sfg@dist\sfg@limit%
          \PackageWarning{sfg}%
          {sfgcurve: incidence angle has been made steeper}%
l          \FPset\sfg@dist\sfg@limit%
        \fi%
                                % calculate the new scalefactor:
        \FPdiv\sfg@scale\sfg@dist\sfg@len%
                                % a: mid scaled to length dist
        \FPmul\sfg@ax\sfg@midx\sfg@scale\FPmul\sfg@ay\sfg@midy\sfg@scale%
                                % d.b = mid+a
        \FPadd\sfg@dxb\sfg@ax\sfg@dx\FPadd\sfg@dyb\sfg@ay\sfg@dy%
                                % d.a = mid-a
        \FPsub\sfg@dxa\sfg@dx\sfg@ax\FPsub\sfg@dya\sfg@dy\sfg@ay%
                                % Two more bezier points lie in between
                                % these points and the curves end points
                                % .i = (d.a)/2
        \FPdiv\sfg@xi\sfg@dxa{2}\FPdiv\sfg@yi\sfg@dya{2}%
                                % .iv = (d.b + (#1,#2) )/2
        \FPadd\sfg@tmpx\sfg@dxb{#1}\FPadd\sfg@tmpy\sfg@dyb{#2}%
        \FPdiv\sfg@xiv\sfg@tmpx{2}\FPdiv\sfg@yiv\sfg@tmpy{2}%
                                % and now draw the path with bezier lines
        \put(0,0){\psbezier[linewidth=\sfg@Li]{-}%
          (0,0)(\sfg@xi,\sfg@yi)(\sfg@dxa,\sfg@dya)(\sfg@dx,\sfg@dy)}%
        \put(0,0){\psbezier[linewidth=\sfg@Li]{-}%
          (#1,#2)(\sfg@xiv,\sfg@yiv)(\sfg@dxb,\sfg@dyb)(\sfg@dx,\sfg@dy)}%
        }%
      \put(\sfg@dx,\sfg@dy){%   
        \sfg@arrow{\sfg@angle}%
                                % label placement (see sfgtermnode)
        \ifcase#4\put(\sfg@alabel,0){\makebox(0,0)[l]{#5}}% 
        \or\put(\sfg@alabeld,\sfg@alabeld){\makebox(0,0)[bl]{#5}}%
        \or\put(0,\sfg@alabel){\makebox(0,0)[b]{#5}}%
        \or\FPneg\sfg@tmp{\sfg@alabeld}%
        \put(\sfg@tmp,\sfg@alabeld){\makebox(0,0)[br]{#5}}%
        \or\FPneg\sfg@tmp{\sfg@alabel}%
        \put(\sfg@tmp,0){\makebox(0,0)[r]{#5}}%
        \or\FPneg\sfg@tmp{\sfg@alabeld}%
        \put(\sfg@tmp,\sfg@tmp){\makebox(0,0)[tr]{#5}}%
        \or\FPneg\sfg@tmp{\sfg@alabel}%
        \put(0,\sfg@tmp){\makebox(0,0)[t]{#5}}%
        \or\FPneg\sfg@tmp{\sfg@alabeld}%
        \put(\sfg@alabeld,\sfg@tmp){\makebox(0,0)[tl]{#5}}%
        \else\PackageWarning{sfg}%
        {sfgcurve: label placement option #4 unknown}%
        \fi %
        }}\end{picture}}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\endinput % That's it.  I Hope you have a fast computer.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Documentation
~~~~~~~~~~~~~
This style file defines some commands to draw signal flow graphs using
LaTeX2e.  Signal flow graphs (sfg) are used by electrical and
electronics engineers and graph theorists.  If you haven't heard of
sfgs before, then sfg.sty is probably not useful for you.

If you really want to know what a signal flow graph is, you might read
one of the references given below.

The package sfg requires the packages `pstricks' and `fp'.  It
therefore requires LaTeX2e and a reasonably fast machine.

To draw a signal flow graph, you should proceed as follows:

1. Insert \usepackage{pstricks,sfg} in your LaTeX2e-file.
2. Draw your sfg on graph paper (*) and determine the size of the 
   picture in units (squares).
   (*) I mean the paper with a 4mm, 5mm or 1/8in grid on it ...
3. Set the unitlength using \sfgsetunit.
4. Set the arrowlength, node radius and label offsets using \sfgsetsize.
5. In a LaTeX picture environment of the size you determined above,
   - first draw all paths using \sfgbranch and \sfgcurve,
   - then draw all nodes using \sfgnode and \sfgtermnode.


Setting the Controlling Parameters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\sfgsetunit{length} sets \unitlength and the PS unit to `length', and
        defines the default pstricks linewidth as 0.4pt.  This has an
        influence on ALL ps-tricks commands that are used afterwards.

\sfgsetsize{nr}{al}{no}{ao} sets the node radius to `nr' units, the
        arrowlength to `al' units, the offset of the nodes' labels to
        `no' units and the offset of the arrows' labels to `ao' units.
        All four parameters are numbers.

\sfgsetangle{an} sets the incidence angle of sfgcurves `an' degrees.
        Default value: 60 degrees.

\sfgsetarrowloc{mid} sets the location of the sfgbranch's arrow.  0.5
        means in the middle, 0.1 means 10% of the branch length from
        the first node.
        Default value: 0.5

\sfgsetlines{li_len}{lii}{liii} sets the line thickness \sfg@Li (used
        for all lines) to `li_len', the thickness \sfg@Lii (used for
        terminal nodes) to `lii'*`li_len', and the thickness \sfg@Liii
        to `liii'*`li_len'.  It also resets the default pstricks
        linewidth to the new \sfg@Li.

        Default values:  \sfg@Li=0.40pt    
                         \sfg@Lii=1.80\sfg@Li
                         \sfg@Liii=3\sfg@Li 


Drawing signal flow graph (sfg) branches
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\sfgbranch{dx}{dy}{pos}{text} draws a straight sfg branch from (0,0)
        to (dx,dy). An arrow is placed in the middle of the branch,
        and a label `text' is typeset near the arrow. The parameter
        `pos' is explained below (see \sfgnode).

        The distance of the label from the arrow's centre as well as
        the arrow's length can be set using \sfgsetsize.

        The location of the arrow can be changed using
        \sfgsetarrowloc.

\sfgcurve{dx}{dy}{off}{pos}{text} draws a curved sfg path from (0,0)
        to (dx,dy). The arrow's position is `off' units left of the
        middle of the path (`off' can be negative to put the arrow
        right of the path). The curve leaves the endpoints in an angle
        of 60 degrees (this value can be changed using \sfgsetangle)
        if possible. If the arrow is too far away from the straight
        path, the angle is increased to guarantee a ``nice'' curve,
        and a warning message is issued.


Drawing Nodes
~~~~~~~~~~~~~
\sfgnode{pos}{text} draws a node with label `text'. `pos' decides
        where the label is typeset. It is a number between 0 and 7.
        After the command

        \sfgsetcompass

        eight macros named after compass directions can be used.

        
              3    2    1                 \NW   \N    \NE
                                                       
              4    .    0         or       \W    .    \E 
                                                       
              5    6    7                 \SW   \S    \SE

        The distance of the label from the node's centre as well as the 
        node's radius can be set using \sfgsetsize.

\sfgtermnode{pos}{text} draws a terminal node (black circle filled white).


Example:
~~~~~~~~
\documentclass[11pt,a4paper]{article}
\usepackage{pstricks,sfg}
\pagestyle{empty}
\setlength{\parindent}{0pt}

\begin{document}
\sfgsetunit{0.5cm}
\sfgsetsize{0.12}{0.4}{0.5}{0.3}
\sfgsetcompass
\begin{picture}(27,4)
                                % branches related to node 2
  \put(6,2){\sfgbranch{3}{0}\S{$\frac{1}{R_1}$}}
  \put(9,2){\sfgbranch{3}{0}\N{\boldmath $Z_2$}}
  \put(18,2){\sfgcurve{-9}{0}{2}\S{$\frac{1}{R_2}$}}
  \put(24,2){\sfgcurve{-15}{0}{-2}\N{$sC_1$}}
                                % branches related to node 3
  \put(12,2){\sfgbranch{3}{0}\N{$\frac{1}{R_2}$}}
  \put(15,2){\sfgbranch{3}{0}\N{\boldmath $Z_3$}}
                                % input, voltage gain, output
  \put(0,2){\sfgcurve{6}{0}{2}\N{$1$}}
  \put(18,2){\sfgcurve{6}{0}{-2}\S{$\alpha_{\mathrm{V}}$}}
  \put(24,2){\sfgbranch{3}{0}\S{$1$}}
                                % nodes
  \put(0,2){\sfgtermnode\S{$V_{\mathrm{in}}$}}
  \put(3,2){\sfgnode\S{$I_1$}}
  \put(6,2){\sfgnode\S{$V_1$}}
  \put(9,2){\sfgnode\S{$I_2$}}
  \put(12,2){\sfgnode\S{$V_2$}}
  \put(15,2){\sfgnode\S{$I_3$}}
  \put(18,2){\sfgnode\S{$V_3$}}
  \put(21,2){\sfgnode\S{$I_4$}}
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References
~~~~~~~~~~
   [Chen, 1995, Chapters 7, 8 and 18]
          Wai-Kai Chen. The Circuits and Filters Handbook. CRC Press,
          Inc., 1995.

or have a look at the original papers by Samuel Mason:

   [119]  
          Samuel J. Mason. Feedback theory --- some properties of signal 
          flow graphs. Proceedings of the Institute of Radio Engineers,  
          41(9):1144-1156, September 1953. 
   [120]  
          Samuel J. Mason. Feedback theory --- further properties of
          signal flow graphs. Proceedings of the Institute of Radio
          Engineers, 44(7):920-926, July 1956.