How can I prevent this tikzpicture from running into my text?

Question

I'm using tikzmark to define some arrows to point to variables in an equation.

\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz}
\usetikzlibrary{tikzmark}

\begin{document}
A \emph{complex number} is an expression of the form
\[
  z = \tikzmark{b}b\cdot\tikzmark{i}i+\tikzmark{a}a
  \begin{tikzpicture}[
    , line join=round
    , line cap=round
    , remember picture
    , overlay
    ]

    \draw[<-, thick]
    ([shift={(3pt,-2pt)}]pic cs:b) |- ([shift={(-10pt, -10pt)}]pic cs:b)
    node[anchor=east] {``imaginary part'' $\Im(z)\in\mathbb{R}$};

    \draw[<-, thick]
    ([shift={(4pt,-2pt)}]pic cs:i) |- ([shift={(-15pt,-25pt)}]pic cs:i)
    node[anchor=east] {``imaginary unit'' $i^2=-1$};

    \draw[<-, thick]
    ([shift={(4pt,-2pt)}]pic cs:a) |- ([shift={(15pt,-25pt)}]pic cs:a)
    node[anchor=west] {``real part'' $\Re(z)\in\mathbb{R}$};
  \end{tikzpicture}
\]
%The collection of complex numbers is denoted by $\mathbb{C}$.
\end{document}

My problem is that uncommenting the text after the equation produces:

The picture runs into the text! Is this fixable?

Answer 1

AFAIK there is no general simple solution since you use (and have to use) an overlay picture, which, by definition, disables the bounding box. What you can do, however, is to measure the space the nodes take with calc and insert it.

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz}
\usetikzlibrary{tikzmark,calc,fit}

\begin{document}
A \emph{complex number} is an expression of the form
\[
  z = \tikzmark{b}b\cdot\tikzmark{i}i+\tikzmark{a}a
  \begin{tikzpicture}[
    , line join=round
    , line cap=round
    , remember picture
    , overlay
    ]

    \draw[<-, thick]
    ([shift={(3pt,-2pt)}]pic cs:b) |- ([shift={(-10pt, -10pt)}]pic cs:b)
    node[anchor=east](a) {``imaginary part'' $\Im(z)\in\mathbb{R}$};

    \draw[<-, thick]
    ([shift={(4pt,-2pt)}]pic cs:i) |- ([shift={(-15pt,-25pt)}]pic cs:i)
    node[anchor=east](b) {``imaginary unit'' $i^2=-1$};

    \draw[<-, thick]
    ([shift={(4pt,-2pt)}]pic cs:a) |- ([shift={(15pt,-25pt)}]pic cs:a)
    node[anchor=west](c) {``real part'' $\Re(z)\in\mathbb{R}$};
    \node[fit=(a)(b)(c),inner sep=0pt](f){};
    \path let \p1=($(pic cs:a)-(f.south)$) in
    \pgfextra{\xdef\myh{\y1}};
  \end{tikzpicture}
\vspace*{\myh}
\]
The collection of complex numbers is denoted by $\mathbb{C}$.
\end{document}

P.S. I is not completely inconceivable to let TikZ record the would-be bounding box, but this would require major surgery.

Answer 2

In order to avoid this overlap, it is possible to write the expression of the complex number in a node. And with the \subnode command of the tikzmark package to create subnodes within it. But since \subnode doesn't position the arrows perfectly, I prefer to use the \tikzmarknode command instead, which centers them perfectly.

I used the hv path style with the bold arrow defined on page 74 of the tikz manual to draw the arrows at right angles with an edge operation.

hv path/.style={-{>[sep]},thick,to path={-| (\tikztotarget)}}

For a reason that I don't understand, it is necessary to keep the remember picture option (but without the overlay option). If an expert can explain why I would be interested to know.

I commented without deleting your original code which is now useless.

\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz}
\usetikzlibrary{tikzmark,positioning}

\begin{document}
A \emph{complex number} is an expression of the form

  \begin{tikzpicture}[
    , line join=round
    , line cap=round
    ,hv path/.style={-{>[sep]},thick,to path={-| (\tikztotarget)}}
    ,remember picture
 %   , overlay
    ]
\node (z) at (5,0) {$  z = \tikzmarknode{b}{b}\cdot\tikzmarknode{i}{i}+\tikzmarknode{a}{a}$};

%    \draw[<-, thick]   
%    ([shift={(3pt,-2pt)}]pic cs:b) |- ([shift={(-10pt, -10pt)}]pic cs:b)
    \node[below left = 0mm and 0mm of z] (i-part) {``imaginary part'' $\Im(z)\in\mathbb{R}$} edge[hv path] (pic cs:b);

%    \draw[<-, thick]
%    ([shift={(4pt,-2pt)}]pic cs:i) |- ([shift={(-15pt,-25pt)}]pic cs:i)
    \node[below = 0mm of i-part] (i-unit) {``imaginary unit'' $i^2=-1$}edge[hv path] (pic cs:i);

%    \draw[<-, thick]
%    ([shift={(4pt,-2pt)}]pic cs:a) |- ([shift={(15pt,-25pt)}]pic cs:a)
    \node[right = 2.5cm of i-unit] {``real part'' $\Re(z)\in\mathbb{R}$}edge[hv path] (pic cs:a);
    \end{tikzpicture}

The collection of complex numbers is denoted by $\mathbb{C}$.
\end{document}

Answer 3

Alternatively, one can fake the overlay horizontally by saving the dimensions.

Note that [remember picture] works by recording the origin location in the aux file. Overlay places the origin at the current baseline/tikzpicture location.

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz}
\usetikzlibrary{tikzmark,calc,fit}

\newsavebox{\mybox}% global
\newlength{\myleft}
\newlength{\myright}

\begin{document}
A \emph{complex number} is an expression of the form
\savebox{\mybox}{\begin{tikzpicture}[
    , line join=round
    , line cap=round
    , remember picture
    , baseline=(origin)
    ]
    \coordinate (origin) at (0,0);

    \draw[<-, thick]
    ([shift={(3pt,-2pt)}]pic cs:b) |- ([shift={(-10pt, -10pt)}]pic cs:b)
    node[anchor=east] {``imaginary part'' $\Im(z)\in\mathbb{R}$};

    \draw[<-, thick]
    ([shift={(4pt,-2pt)}]pic cs:i) |- ([shift={(-15pt,-25pt)}]pic cs:i)
    node[anchor=east] {``imaginary unit'' $i^2=-1$};

    \draw[<-, thick]
    ([shift={(4pt,-2pt)}]pic cs:a) |- ([shift={(15pt,-25pt)}]pic cs:a)
    node[anchor=west] {``real part'' $\Re(z)\in\mathbb{R}$};

    \pgfextractx{\myleft}{\pgfpointanchor{current bounding box}{west}}
    \global\myleft=\myleft
    \pgfextractx{\myright}{\pgfpointanchor{current bounding box}{east}}
    \global\myright=\myright
\end{tikzpicture}}
\[
  z = \tikzmark{b}b\cdot\tikzmark{i}i+\tikzmark{a}a
  \hspace{\myleft}\usebox\mybox\hspace{-\myright}
\]
The collection of complex numbers is denoted by $\mathbb{C}$.
\end{document}

Answer 4

Here are two other solutions.

In the first solution, I remove the overlay option from your tikzpicture and compute the depth of yours annotations (the distance between the top of first node and the bottom of the last node).

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz}
\usetikzlibrary{calc,tikzmark}
\begin{document}
A \emph{complex number} is an expression of the form
\[
  z = \tikzmark{b}b\cdot\tikzmark{i}i+\tikzmark{a}a
\begin{tikzpicture}[
  , line join=round
  , line cap=round
  , remember picture
  , baseline=0 % to compute depth...
  ]
  \begin{scope}[overlay]
    \draw[<-, thick] ([shift={(3pt,-2pt)}]pic cs:b) |- ([shift={(-10pt, -10pt)}]pic cs:b)
    node[anchor=mid east] (a) {``imaginary part'' $\Im(z)\in\mathbb{R}$};

    \draw[<-, thick] ([shift={(4pt,-2pt)}]pic cs:i) |- ([shift={(-15pt,-25pt)}]pic cs:i)
    node[anchor=mid east] {``imaginary unit'' $i^2=-1$};

    \draw[<-, thick] ([shift={(4pt,-2pt)}]pic cs:a) |- ([shift={(15pt,-25pt)}]pic cs:a)
    node[anchor=mid west] (b) {``real part'' $\Re(z)\in\mathbb{R}$};\
  \end{scope}
  \path let \p1=(a.north west), \p2=(b.south east) in  (0,0) -- (0,\y2-\y1);
\end{tikzpicture}
\]

The collection of complex numbers is denoted by $\mathbb{C}$.
\end{document}

The second solution uses \tikzmarknode and fit to compute the math targets with margins then positions the commentary nodes with the same margins and finally draws the arrows. The syntax is more regular: no shift and just two distances (the 2pt margin and the 1em horizontal shifting).

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz}
\usetikzlibrary{calc,tikzmark,fit}
\begin{document}
A \emph{complex number} is an expression of the form
\begin{equation}
  z = \tikzmarknode{b}{b}\cdot\tikzmarknode{i}{i}+\tikzmarknode{a}{a}
\begin{tikzpicture}[
  , line join=round
  , line cap=round
  , remember picture
  , baseline=0 % to compute depth
  ]
  \begin{scope}[overlay]
    \tikzset{
      text node/.style={inner sep=2pt},
      fitmath/.style={node contents={},text node,fit=#1},
    }
    % math target nodes
    \node[fitmath=(b),name=bt];
    \node[fitmath=(i),name=it];
    \node[fitmath=(a),name=at];
    % commentary nodes
    \path (bt.south) ++ (-1em,0)
    node[text node,anchor=north east] (bc) {``imaginary part'' $\Im(z)\in\mathbb{R}$};
    \path (bc.south east)
    node[text node,anchor=north east] (ic) {``imaginary unit'' $i^2=-1$};
    \path (ic.mid -| at) ++ (1em,0)
    node[text node,anchor=mid west] (ac)   {``real part'' $\Re(z)\in\mathbb{R}$};
    % arrows between nodes
    \draw[<-, thick] (bt.south) |- (bc.mid east);
    \draw[<-, thick] (it.south) |- (ic.mid east);
    \draw[<-, thick] (at.south) |- (ac.mid west);
  \end{scope}
  \path let \p1=(a.south), \p2=(ac.south) in  (0,0) -- (0,\y2-\y1);
\end{tikzpicture}
\end{equation}

The collection of complex numbers is denoted by $\mathbb{C}$.
\end{document}