Highlight and use arrows in an equation In a robust manner

Question

I am trying to give a very simple analogy to what I am trying to do with LaTeX. I simply want to write two vectors, one below or next the other, and then, use nodes to link the last two elements of the first vector and the last element from the second vector. Here is the code I am using

\documentclass{article}
\usepackage{amsmath, amssymb}
\usepackage{tikz}
\usetikzlibrary{arrows}% For nice arrow tips
% One style for all TikZ pictures for working with overlays:
\tikzset{every picture/.style=remember picture}
% Define a TikZ node for math content:
\newcommand{\mathnode}[1]{%
  \mathord{\tikz[baseline=(#1.base), inner sep = 0pt]{\node (#1) {$#1$};}}}



\begin{document}

$\begin{bmatrix}
     1 \\ \mathnode{2\rho \pi \\ 3}
\end{bmatrix}$
\\
$\begin{bmatrix}
     1 \\ 2\rho \pi \\ \mathnode{3}
\end{bmatrix}$

% Now we draw connections between defined nodes:
\begin{tikzpicture}[overlay]
  \path [>=stealth, <->, shorten <= 3pt, shorten >=3 pt]
    (N1) edge [bend left=60] (N2);
\end{tikzpicture}


\end{document}

How can I modify the mathnode declaration, to give a name to the node, and to be able to fill each node separately ?

Please, any suggestions?

Thanks.

Answer 1

I don't quite understand the output you have in mind, but I definitely would go through the tikz matrix way, by putting two matrices of nodes in the same tikzpicture, without using remember picture nor overlay.

For example:

\documentclass{article}
\usepackage{amsmath, amssymb}
\usepackage{tikz}
\usetikzlibrary{arrows,matrix,positioning}% For nice arrow tips
\begin{document}

\tikzset{
    !/.style = {
        fill=yellow!30,
    },
    mymatrix/.style = {
        matrix of math nodes,
        left delimiter  = (,
        right delimiter = ),
        nodes={minimum width=6ex},
    }
}

\begin{tikzpicture}
    \matrix[mymatrix, name=M1]{
                1 \\
        |[!]|   2\rho\pi \\
        |[!]|   3\\
    };
    \matrix[mymatrix, name=M2, below=of M1] {
                 1 \\
                 2\rho\pi\\
        |[!]|    3\\
    };
   \draw [red, >=stealth, <->, shorten <= 3pt, shorten >=3 pt]
     (M1-2-1.south east) to[bend left=60] (M2-3-1.east);
\end{tikzpicture}

\end{document}

Produces:

Answer 2

By means of hf-tikz, one isn't forced to exploit the TikZ-matrix. This is an example showing how to perform such a task:

\documentclass[11pt,a4paper]{article}
\usepackage{amsmath, amssymb}
\usepackage[customcolors,markings]{hf-tikz}
\usetikzlibrary{calc}

\tikzset{aspect/.style={
    set fill color=green!50!lime!60,
    set border color=white,
    disable rounded corners=true
  },
  vert offsets/.style={
    above offset=0.32,
    below offset=-0.08
  },
  expr/.style={
    left offset=-0.05,
    right offset=0.25,
    vert offsets,
    aspect,
  },
  number/.style={
    left offset=-0.1,
    right offset=0.1,
    vert offsets,
    aspect
  }
}

\begin{document}
\[\begin{bmatrix}
     1 \\ \tikzmarkin[expr, mark at=0.135]{a}2\rho \pi \\ 3\tikzmarkend{a}
\end{bmatrix}
\]
% store the point in a coordinate: let it be a bit shifted for clarity
\tikz[remember picture,overlay]\coordinate[use marker id,xshift=0.1cm] (A) at (0,0);

\[\begin{bmatrix}
     1 \\ 2\rho \pi \\ \tikzmarkin[number, mark at=0.135]{b}3\tikzmarkend{b}
\end{bmatrix}
\]
% store the point in a coordinate: let it be a bit shifted for clarity
\tikz[remember picture]\coordinate[use marker id,xshift=0.25cm] (B) at (0,0);

% draw the curve
\tikz[remember picture,overlay]\draw[blue] 
 (A) edge[bend left=60,stealth-stealth](B);
\end{document}

The result:

The markings option is a feature of version 0.3 of the package, highly based on the nice answer Jake gave in How to draw tangent line of an arbitrary point on a path in TikZ. It exploits the TikZ markings library, thus it is possible to deploy markers as a number from 0 (start of the path) to 1 (end of the path). To show where the markers are placed, use the show markers key; for example:

\[\begin{bmatrix}
     1 \\ \tikzmarkin[expr, show markers, mark at=0.135]{a}2\rho \pi \\ 3\tikzmarkend{a}
\end{bmatrix}
\]

...

\[\begin{bmatrix}
     1 \\ 2\rho \pi \\ \tikzmarkin[number,show markers, mark at=0.135]{b}3\tikzmarkend{b}
\end{bmatrix}
\]

gives:

Moreover, the package allows to define aspects style in terms of shading: they can be adopted separately each time one highlights a formula. One way to use them, is the following:

• define some general aspects style, say aspect x;
• define styles containing offset definition plus an argument: this will be in charge to select the proper aspect style; an example: expr=aspect x.

This is an example covering some of the possibilities:

\documentclass[11pt,a4paper]{article}
\usepackage{amsmath, amssymb}
\usepackage[customcolors,shade,markings]{hf-tikz}
\usetikzlibrary{calc}

\tikzset{aspect 1/.style={
    set border color=green!70!black,
    top color=white,
    bottom color=green!50!lime!60,
    disable rounded corners=true
  },
  aspect 2/.style={
    set border color=cyan!50!blue,
    bottom color=white,
    top color=cyan!50!blue!60,
    disable rounded corners=true
  },
  aspect 3/.style={
    set border color=white,
    inner color=white,
    outer color=red!75!black!30,
    disable rounded corners=true
  },  
  vert offsets/.style={
    above offset=0.32,
    below offset=-0.08
  },
  expr/.style={
    left offset=-0.05,
    right offset=0.25,
    vert offsets,
    #1,
  },
  number/.style={
    left offset=-0.1,
    right offset=0.1,
    vert offsets,
    #1
  }
}

\begin{document}
\[
\begin{bmatrix}
     1 \\ \tikzmarkin[expr=aspect 1, mark at=0.135]{a}2\rho \pi \\ 3\tikzmarkend{a}
\end{bmatrix}
% store the point in a coordinate: let it be a bit shifted
\tikz[remember picture,overlay]\coordinate[use marker id,xshift=0.1cm] (A) at (0,0);
\hspace{1cm}
\begin{bmatrix}
     1 \\ \tikzmarkin[expr=aspect 2, mark at=0.135]{a2}2\rho \pi \\ 3\tikzmarkend{a2}
\end{bmatrix}
% store the point in a coordinate: let it be a bit shifted
\tikz[remember picture,overlay]\coordinate[use marker id,xshift=0.1cm] (A2) at (0,0);
\hspace{1cm}
\begin{bmatrix}
     1 \\ \tikzmarkin[expr=aspect 3, mark at=0.135]{a3}2\rho \pi \\ 3\tikzmarkend{a3}
\end{bmatrix}
% store the point in a coordinate: let it be a bit shifted
\tikz[remember picture,overlay]\coordinate[use marker id,xshift=0.1cm] (A3) at (0,0);
\]


\[
\begin{bmatrix}
     1 \\ 2\rho \pi \\ \tikzmarkin[number=aspect 3, mark at=0.135]{b}3\tikzmarkend{b}
\end{bmatrix}
% store the point in a coordinate: let it be a bit shifted
\tikz[remember picture]\coordinate[use marker id,xshift=0.25cm] (B) at (0,0);
\hspace{1cm}
\begin{bmatrix}
     1 \\ 2\rho \pi \\ \tikzmarkin[number=aspect 1, mark at=0.135]{b2}3\tikzmarkend{b2}
\end{bmatrix}
% store the point in a coordinate: let it be a bit shifted
\tikz[remember picture]\coordinate[use marker id,xshift=0.25cm] (B2) at (0,0);
\hspace{1cm}
\begin{bmatrix}
     1 \\ 2\rho \pi \\ \tikzmarkin[number=aspect 2, mark at=0.135]{b3}3\tikzmarkend{b3}
\end{bmatrix}
% store the point in a coordinate: let it be a bit shifted
\tikz[remember picture]\coordinate[use marker id,xshift=0.25cm] (B3) at (0,0);
\]


% draw the curve
\tikz[remember picture,overlay]\draw[cyan!50!blue] 
 (A) edge[bend left=60,stealth-stealth](B);
\tikz[remember picture,overlay]\draw[red!75!black] 
 (A2) edge[bend left=60,stealth-stealth](B2);
\tikz[remember picture,overlay]\draw[green!70!black] 
 (A3) edge[bend left=60,stealth-stealth](B3);
\end{document}

The result: