Unexpected result for child nodes: why they seem to be typeset in bold?

Question

I was trying to draw something similar to:

without the border around nodes and where arrows should have a line (with a given width to be visible) connecting the tail and the tip.

My code is:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.arrows,calc} 

% knowing the angle starting from two coordinates
% by Altermundus:
% http://tex.stackexchange.com/questions/39293/coordinates-a-b-compute-b-a-and-angle-between-x-and-b-a
\newcommand{\pgfextractangle}[3]{%
    \pgfmathanglebetweenpoints{\pgfpointanchor{#2}{center}}
                              {\pgfpointanchor{#3}{center}}
    \global\let#1\pgfmathresult  
}

% style of the arrow
\tikzset{stylefreccia/.style={single arrow, draw=none,fill=gray!80!black,minimum height=#1,minimum width=0.5cm}}

\tikzset{freccia/.code={
\coordinate (A) at (\tikzparentnode.south);
\coordinate (B) at (\tikzchildnode.north);
\pgfextractangle{\angle}{A}{B} 
\node [stylefreccia=1cm,single arrow head indent=1ex,rotate=\angle] (x) at ($(\tikzparentnode.south)!0.5!(\tikzchildnode.north)$){}; % <= insert the node exactly in between parent and child node with the angle computed before
\draw[gray!20,draw,line width=2pt](x.tail)--(x.tip); % <= draw the line connecting tail and tip
}
}

\begin{document}
\tikzset{edge from parent/.style={freccia}}
\begin{center}
\begin{tikzpicture}[level distance=2cm,
level 1/.style={sibling distance=30mm},
level 2/.style={sibling distance=20mm},
level 3/.style={sibling distance=10mm}
]
\node {root}
child {node {left}
child {node{child}}}
child {node {right}
child {node {child}}
child {node {child}}
};
\end{tikzpicture}
\end{center}
\end{document}

which actually gets the point because the output is:

My question is: why leaves seem to be typeset in bold?

Answer 1

It might be the case that the child nodes are printed twice. I think there's something going on when you assign a code to a style but I can't see it. For some reason the following doesn't exhibit the same behavior. I have tried to make the construction in one go except the gray line. Also added an arrow head for a better join at the tip of the arrows.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.arrows,arrows,calc} 
\tikzset{mycomplicatededge/.style={edge from parent path={
[draw=none]                   %It's a nondrawn path
let \p1=(\tikzparentnode.south), 
    \p2=(\tikzchildnode.north),
        \p3=($(\p2)-(\p1)$),  %point diff
        \n3={atan2(\x3,\y3)}  %angle
        in (\p1) -- (\p2)     %path spec
        node [midway,         %put the node in the middle
        single arrow,
        single arrow head indent=1ex,
        minimum height=1cm,
        rotate=\n3,           %use angle var
        draw=none,
        fill=gray!80!black,
        minimum width=0.5cm](x){};
        \draw[gray!20,
            line width=2pt,
            shorten <=.1\pgflinewidth,
            shorten >=.15\pgflinewidth,
            -triangle 90 cap
            ](x.tail)--(x.tip);
        }
    }
}
\begin{document}
\begin{tikzpicture}[level distance=2cm,
level 1/.style={sibling distance=30mm},
level 2/.style={sibling distance=20mm},
level 3/.style={sibling distance=10mm},
mycomplicatededge
]
\node {root}
child {node {left}
child {node{child}}}
child {node {right}
child {node {child}}
child {node {child}}
};
\end{tikzpicture}
\end{document}

I would consider drawing a genuine (meta)arrow and using it as an edge instead of placing a node on the edges though.

Answer 2

Why do you want use the tree syntax from TikZ for a very simple tree. You can make it with simple commands or use something like tkz-graph. The problem is different for a more complex structure.

\documentclass[11pt]{scrartcl}
\usepackage{tkz-graph}

\begin{document}
\begin{tikzpicture}
\Huge
\GraphInit[unit=4,vstyle=Empty]
 \tikzset{EdgeStyle/.style = {gray!20,
            line width=12pt,
            shorten <=.1\pgflinewidth,
            shorten >=.15\pgflinewidth,
            -triangle 90 cap,->}} 

\Vertex[L=Root]{root}
\SOWE[L=Left](root){lev l1}
\SOEA[L=Right](root){lev r1}
\SO[L=Child](lev l1){lev 2}
\SOWE[L=Child](lev r1){lev l2}
\SOEA[L=Child](lev r1){lev r2}
 \Edges(root,lev l1,lev 2) \Edges(root,lev r1,lev l2) \Edges(lev r1,lev r2)   
\end{tikzpicture}
\end{document}