%%
%% The LaTeX Graphics Companion, 2ed (first printing May 2007)
%%
%% Example 5-15-7 on page 310.
%%
%% Copyright (C) 2007 Michel Goossens, Frank Mittelbach, Denis Roegel, Sebastian Rahtz, Herbert Vo\ss
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3
%% of this license or (at your option) any later version.
%%
%% See
http://www.latex-project.org/lppl.txt for details.
%%
\documentclass{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{135.83385pt}
\usepackage{pst-xkey,pstricks,pst-node}
\SpecialCoor
\makeatletter
\newif\ifPST@showFP % mark focal point?
\define@key[psset]{}{showFP}[true]{% "showFP" equals true
\@nameuse{PST@showFP#1}% use \ifPST@showFP
}
\psset{showFP=true} % default
%
\def\focalPoint{\pst@object{focalPoint}}
\def\focalPoint@i(#1)(#2)(#3)#4{{ % to keep everything local
\pst@killglue %
\begingroup
\use@par
\pst@getcoor{#1}\pst@tempa% point A % get coordinates as x y
\pst@getcoor{#2}\pst@tempb% point B % "
\pst@getcoor{#3}\pst@tempc% point C % "
\pnode(!% % set node
\pst@tempa /YA exch \pst@number\psyunit div def
/XA exch \pst@number\psxunit div def % x y in user coordinates
\pst@tempb /YB exch \pst@number\psyunit div def
/XB exch \pst@number\psxunit div def
\pst@tempc /YC exch \pst@number\psyunit div def
/XC exch \pst@number\psxunit div def
XA XB XC add add 3.0 div % xFP
YA YB YC add add 3.0 div % yFP
){#4} % #5 = node name
\ifPST@showFP\qdisk(#4){2pt}\fi
\endgroup
}\ignorespaces}
\makeatother
\StartShownPreambleCommands
\usepackage{pstricks,pst-node,pst-xkey}
% \focalPoint as defined above
\StopShownPreambleCommands
\begin{document}
\begin{pspicture}(4,4)
\psset{linewidth=2pt}
\pspolygon[linecolor=red](0,0)(2,4)(4,0)%
\focalPoint[showFP=true,linecolor=red](0,0)(2,4)(4,0){FP1}%
\pnode(0,0){A}\pnode(0,4){B}\pnode(4,2){C}%
\pspolygon[linecolor=blue](A)(B)(C)%
\focalPoint(A)(B)(C){FP2}%
{\psset{linecolor=blue}%
\qdisk(FP2){2pt}}%
\ncline[linewidth=1pt]{<->}{FP1}{FP2}%
\end{pspicture}
\end{document}
