# How can I put labels at centre of all polygons?

I want to input label like this picture In the figure, polygons A, E, and F are isosceles right triangles; B, C, and D are squares with sides of length 1; and G is an equilateral triangle. The figure can be folded along its edges to form a polyhedron having the polygons as faces.

I tried

\documentclass[border=1.5mm,12pt]{standalone}
%\usepackage[utf8]{inputenc}
\usepackage{fouriernc}
\usepackage{tkz-euclide,amsmath}
\usetkzobj{all}
\tikzset{hidden/.style = {thick, dashed}}
\begin{document}
\begin{tikzpicture}
\tkzDefPoint(0,0){O}
\tkzDefPoint(1,0){A}
\tkzDefPoint(2,0){B}
\tkzDefPoint(3,0){E}
\tkzDefPoint(1,-1){C}
\tkzDefPoint(0,-1){M}
\tkzDefPoint(2,-1){N}
\tkzDefPoint(1,-2){F}
\tkzDefPoint(1,1){G}
\tkzDefPoint(2,1){H}
\tkzDefPoint(1,2){I}
\tkzDefPointsBy[rotation=center N angle 60](F){D}
%\tkzDrawPoints(A,B,C,D,O,N,E)
%\tkzLabelPoints[above](A,G,H,I)
%\tkzLabelPoints[right](E,D,N,B)
%\tkzLabelPoints[left](A,C,G,F)
\tkzDrawSegments[thick](O,E O,M F,E A,C C,F)
\tkzDrawPolygon[thick](O,M,N,B)
\tkzDrawPolygon[thick](N,F,D)
\tkzDrawPolygon[thick](G,H,I)
\tkzDrawPolygon[thick](A,B,H,G)
\end{tikzpicture}
\end{document}


How can I put labels A, B, ...,F, G at centre of all polygons?

• You could use TikZ' barycentric coordinate system, as in e.g. tex.stackexchange.com/a/200731/586 – Torbjørn T. Mar 3 '17 at 9:08
• @TorbjørnT. I tried \node[fill,circle,inner sep=0 pt,label={right:$B$}] at (barycentric cs:A=0.5,B=0.5,H=0.5,G=0.5) {}; label B was not put at centre, but a circle at centre. – minhthien_2016 Mar 3 '17 at 9:51
• Of course, you told TikZ to place a filled, circular node at the given coordinate, with the label B to the right of it. I meant e.g. \node at (barycentric cs:N=1,F=1,D=1) {aa};. – Torbjørn T. Mar 3 '17 at 10:38
• @TorbjørnT. Nice it works. Add it as an answer. – Bobyandbob Mar 3 '17 at 13:21

## 3 Answers

If you want to stick to tkz-euclide, you can use \tkzDefBarycentricPoint to obtain the midpoints of triangles and rectangles. There are other possibilities as well though.

For rectangles you can use \tkzDefMidPoint(A,B), where A and B are two opposing corners.

For triangles there are different "center points", which can be found with \tkzCentroid(A,B,C) (same as \tkzDefBarycentricPoint(A=1,B=1,C=1) I believe), \tkzCircumCenter(A,B,C) (center of circumscribed circle) and \tkzInCenter(A,B,C) (center of inscribed circle). As with the barycentric point, the macro must be followed by \tkzGetPoint{<name of point>}.

You could also mix in pure TikZ syntax as I mention in a comment, and you in your answer. I.e. use \node at (barycentric cs:A=1,B=1,C=1) {a};

Note also that due to the way you draw the polygons, some of the vertices have ugly line joins:

But this can be easily remedied by drawing it more carefully, as in the code below.

\documentclass[border=1.5mm,12pt]{standalone}
\usepackage[utf8]{inputenc}
\usepackage{fouriernc}
\usepackage{tkz-euclide,amsmath}
\usetkzobj{all}
\tikzset{hidden/.style = {thick, dashed}}
\begin{document}
\begin{tikzpicture}
\tkzDefPoint(0,0){o}
\tkzDefPoint(1,0){a}
\tkzDefPoint(2,0){b}
\tkzDefPoint(3,0){e}
\tkzDefPoint(1,-1){c}
\tkzDefPoint(0,-1){m}
\tkzDefPoint(2,-1){n}
\tkzDefPoint(1,-2){f}
\tkzDefPoint(1,1){g}
\tkzDefPoint(2,1){h}
\tkzDefPoint(1,2){i}
\tkzDefPointsBy[rotation=center n angle 60](f){d}

%\tkzLabelPoints[left](o,a,b,c,d,e,f,g,h,i,m,n)
\tkzDrawPolygon[thick](i,h,b,e,n,d,f)
\tkzDrawSegments[thick](n,f g,h)
\tkzDrawPolygon[thick](o,m,n,b)

\tkzDefBarycentricPoint(i=1,h=1,g=1) \tkzGetPoint{A}
\tkzDefBarycentricPoint(g=1,h=1,a=1,b=1) \tkzGetPoint{B}
\tkzDefBarycentricPoint(o=1,a=1,m=1,c=1) \tkzGetPoint{C}
\tkzDefBarycentricPoint(a=1,b=1,c=1,n=1) \tkzGetPoint{D}
\tkzDefBarycentricPoint(b=1,e=1,n=1) \tkzGetPoint{E}
\tkzDefBarycentricPoint(c=1,n=1,f=1) \tkzGetPoint{F}
\tkzDefBarycentricPoint(d=1,n=1,f=1) \tkzGetPoint{G}

\tkzLabelPoints[anchor=center,font=\scriptsize](A,B,C,D,E,F,G)
\end{tikzpicture}
\end{document}


• Can you show in your code "by drawing it more carefully"? – minthao_2011 Mar 14 '17 at 1:13
• @minthao_2011 There are three lines of code that draws the polygon, you can compare those to the original. The images I showed of bad corners are (left to right) the lower right corner of triangle A, the upper right corner of triangle E, and the left corner of triangle G. It basically a question of how the path goes around those corners. – Torbjørn T. Mar 14 '17 at 7:26

While you are waiting for a tkz-euclide answer to be written up, here's a way to solve the same problem in Metapost. Plain MP provides a center macro that find the centre of the bounding box of any given closed path, which is usually the wrong place if your path is a triangle. Here I've defined a simple centroid macro that will find the centroid of a triangular path, that gives a better place for a label.

\RequirePackage{luatex85}
\documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\mplibtextextlabel{enable}
\begin{mplibcode}
vardef centroid primary trig_path =
save a,b,c,m;
pair a,b,c,m;
a = point 0 of trig_path;
b = point 1 of trig_path;
c = point 2 of trig_path;
m = whatever [a, 1/2[b,c]] = whatever [b, 1/2[a,c]]; m
enddef;
beginfig(1);
path A,B,C,D,E,F,G, t[];

C = unitsquare scaled 100 rotated 15;
D = C shifted point 1 of C;
B = C shifted point 3 of D;

t0 = (point 0 of C -- point 1 of C -- point 3 of C -- cycle);
t1 = t0 rotated -90;
A = t0 shifted point 3 of B;
E = t1 shifted point 2 of D;
F = t1 shifted point 0 of D;

t2 = origin -- point 2 of C -- point 2 of C rotated -60 -- cycle;
G = t2 shifted point 1 of F;

forsuffixes @=A,B,C,D,E,F,G:
draw @;
label(str @, if length @=3: centroid else: center fi @);
endfor

endfig;
\end{mplibcode}
\end{document}


Compile this with lualatex to get the luamplib support.

Base on comment of TorbjørnT, this is the code

\documentclass[border=1.5mm,12pt]{standalone}
%\usepackage[utf8]{inputenc}
\usepackage{fouriernc}
\usepackage{tkz-euclide,amsmath}
\usetkzobj{all}
\tikzset{hidden/.style = {thick, dashed}}
\begin{document}
\begin{tikzpicture}
\tkzDefPoint(0,0){O}
\tkzDefPoint(1,0){A}
\tkzDefPoint(2,0){B}
\tkzDefPoint(3,0){E}
\tkzDefPoint(1,-1){C}
\tkzDefPoint(0,-1){M}
\tkzDefPoint(2,-1){N}
\tkzDefPoint(1,-2){F}
\tkzDefPoint(1,1){G}
\tkzDefPoint(2,1){H}
\tkzDefPoint(1,2){I}
\tkzDefPointsBy[rotation=center N angle 60](F){D}
\tkzDrawSegments[thick](O,E O,M F,E A,C C,F)
\tkzDrawPolygon[thick](O,M,N,B)
\tkzDrawPolygon[thick](N,F,D)
\tkzDrawPolygon[thick](G,H,I)
\tkzDrawPolygon[thick](C,F,N)
\tkzDrawPolygon[thick](A,B,H,G)
\node at (barycentric cs:N=1,F=1,D=1) {$G$};
\node at (barycentric cs:N=1,B=1,E=1) {$E$};
\node at (barycentric cs:A=1,B=1,N=1,C=1) {$D$};
\node at (barycentric cs:G=1,H=1,I=1) {$A$};
\node at (barycentric cs:A=1,B=1,H=1,G=1) {$B$};
\node at (barycentric cs:O=1,A=1,M=1,C=1) {$C$};
\node at (barycentric cs:C=1,F=1,N=1) {$G$};
\end{tikzpicture}
\end{document}