%%
%% A DANTE-Edition example
%%
%% Example 14-04-1 on page 161.
%%
%% Copyright (C) 2011 Herbert Voss
%%
%% 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.
%%
%%
%% ====
% Show page(s) 1
%%
\documentclass[]{article}
\pagestyle{empty}
\setlength\textwidth{201.70511pt}
\setlength\parindent{0pt}
\usepackage{pstricks,pst-node} \SpecialCoor
\makeatletter
\define@boolkey[psset]{}[Pst@]{showFP}[true]{}% mark focal point?
\psset{showFP} % true as default
\def\focalPoint{\@ifnextchar[{\focalPoint@i}{\focalPoint@i[]}}
\def\focalPoint@i[#1](#2)(#3)(#4)#5{{ % everything stays local
\psset{#1} % set parameters
\pst@getcoor{#2}\pst@tempA% point A % get coordinates as x y
\pst@getcoor{#3}\pst@tempB% point B %
\pst@getcoor{#4}\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 % xSP
YA YB YC add add 3.0 div % ySP
){#5} % #5 = node name
\ifPst@showFP\qdisk(#5){2pt}\fi}\ignorespaces}
\makeatother
\begin{document}
\begin{pspicture}[showgrid](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){SP1}%
\pnode(0,0){A}\pnode(0,4){B}\pnode(4,2){C}%
\pspolygon[linecolor=blue](A)(B)(C)\focalPoint(A)(B)(C){SP2}%
{\psset{linecolor=blue}\qdisk(SP2){2pt}}
\ncline[linewidth=0.2pt]{<->}{SP1}{SP2}%
\end{pspicture}
\end{document}
