Example: Perpendicular bissector

Published 2009-10-26 | Author: Hugues Vermeiren

The construction of points on the perpendicular bissector of [AB]. It is achieved in an artisanal way, the purpose being to show how to achieve computations with pgf. A much simpler construction can certainly be done with other pgf commands but the idea here is to emphasize the analytical process.

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% Perpendicular bissector
% Author:  Hugues Vermeiren
\documentclass{minimal}
\usepackage{tikz,xifthen}
\begin{document}

\newcounter{index}
\setcounter{index}{0}

\begin{tikzpicture}[
scale=1.0,
MyPoints/.style={draw=blue,fill=white,thick},
Segments/.style={draw=blue!50!red!70,thick},
MyCircles/.style={green!50!blue!50,thin}
]
% Warning : all this is an artisanal way of computing points
% on the perpendicular bissector of [AB]
% It could very well be achieved with more powerfull tools...
% (package tkz-2d, for example)
\clip (-2.5,-2.5) rectangle (7,7.5);
\draw[color=gray,step=1.0,dotted] (-2.1,-2.1) grid (6.1,7.1);
\draw[->] (-2,0)--(6.5,0) node[right]{$x$};
\draw[->] (0,-2)--(0,7) node[above]{$y$};

% Feel free to change here coordinates of points A and B
\pgfmathparse{-sqrt(2)}		\let\Xa\pgfmathresult
\pgfmathparse{2}		\let\Ya\pgfmathresult
\coordinate (A) at (\Xa,\Ya);
\pgfmathparse{5}		\let\Xb\pgfmathresult
\pgfmathparse{13/3}		\let\Yb\pgfmathresult
\coordinate (B) at (\Xb,\Yb);

% Let I be the midpoint of [AB]
\pgfmathparse{(\Xb+\Xa)/2} \let\XI\pgfmathresult
\pgfmathparse{(\Yb+\Ya)/2} \let\YI\pgfmathresult
\coordinate (I) at (\XI,\YI);

\draw[red,thick] (A)--(B);

% deltaX and deltaY are coordinates of vector AB
\pgfmathparse{\Yb-\Ya} \let\deltaY\pgfmathresult
\pgfmathparse{\Xb-\Xa} \let\deltaX\pgfmathresult

% NormeddeltaX and NormeddeltaY are the normalized values of these coordinates
\pgfmathparse{sqrt(\deltaX*\deltaX+\deltaY*\deltaY)} \let\r\pgfmathresult
\pgfmathparse{\deltaX/\r} \let\NormeddeltaX\pgfmathresult
\pgfmathparse{\deltaY/\r} \let\NormeddeltaY\pgfmathresult

% R is a point on the perpendicular bissector of [AB],
% far away from the midpoint...
\pgfmathparse{\YI-10.0*\NormeddeltaX} \let\YR\pgfmathresult
\pgfmathparse{\XI+10.0*\NormeddeltaY} \let\XR\pgfmathresult

% S is the image of R by the symmetry of axis AB
\pgfmathparse{2*\YI-\YR} \let\YS\pgfmathresult
\pgfmathparse{2*\XI-\XR} \let\XS\pgfmathresult
\coordinate (R) at (\XR,\YR);
\coordinate (S) at (\XS,\YS);
\draw (R)--(S);

\foreach \i in {-3,-2,...,5}{
\ifthenelse{\equal{\i}{0}}% Do not redraw the segment [AB]
{}%
{%
\stepcounter{index}
% P(i) is a variable point on the perpendicular bissector.
% The distance between P(i) and P(i+1) is equal to 1
\pgfmathparse{\YI-\i*\NormeddeltaX} \let\YP\pgfmathresult
\pgfmathparse{\XI+\i*\NormeddeltaY} \let\XP\pgfmathresult
\coordinate (P) at (\XP,\YP);
\pgfmathparse{sqrt((\XP-\Xa)*(\XP-\Xa)+(\YP-\Ya)*(\YP-\Ya))}

\draw[Segments] (P)--(A);
\draw[Segments] (P)--(B);
\fill[MyPoints] (P) circle (0.8mm) node[right]{$P_{\theindex}$};
}%
};

\fill[MyPoints] (A) circle (0.8mm) node[left]{$A$};
\fill[MyPoints] (B) circle (0.8mm) node[right]{$B$};
\fill[MyPoints] (I) circle (0.8mm) node[right]{$I$};
\end{tikzpicture}

\end{document} 