TikZ externalize and nicematrix package

Question

How to use TikZ external library in combination with the nicematrix package?

I tried to use the external library in combination with the nicematrix package (version August 12, 2018) to increase the compilation speed. But it does not work as expected.

Hier a minimal example:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{external}
%\tikzexternalize % activate!
\usepackage{nicematrix}
\begin{document}

% Test 1
\begin{flalign}
    \begin{bNiceMatrix}
    0 & \Cdots & 0 \\\Vdots & & \Vdots \\0 & \Cdots & 0
    \end{bNiceMatrix}
\end{flalign}

% Test 2
\begin{flalign}
\begin{tikzpicture}[baseline,inner sep = 0pt]
\node {$
    \begin{bNiceMatrix}
    0 & \Cdots & 0 \\\Vdots & & \Vdots \\0 & \Cdots & 0
    \end{bNiceMatrix}
$};
\end{tikzpicture}
\end{flalign}

% Test 3
\begin{flalign}
\begin{tikzpicture}[baseline,inner sep = 0pt]
\node {$
    \begin{pNiceArrayRC}{CC}%
    C_1 & C_2 \\1 & 2 & L_1\\4 & 5 & L_2\\
    \end{pNiceArrayRC}
$};
\end{tikzpicture}
\end{flalign}

\end{document}

Here the results of the tests:

• Not external, test 1: No problem.
• Not external, test 2: No problem. Same result as for test 1.
• Not external, test 3: No problem.
• External, test 1: Error.
  • Line 1: Extra }, or forgotten $. ...nt }]\bgroup \box_use:N \l_tmpa_box \egroup
• External, test 2: No error, but output not as expacted.
• External, test 3: Error.
  • Package tikz Error: Sorry, the system call 'pdflatex -shell-escape -halt-on-e

Embedding the nicematrix in a tikz picture does not really solve the problem. Which might be doe to nested tikz pictures (see pgfmanual 50.2).

Does anyone have an idea how to overcome this issue?

Answer 1

Since the extension nicematrix uses Tikz with the options overlay and remember picture to draw the dotted lines, it's not possible to use the Tikz externalization with these elements. In fact, in versions until 2.1, we even have TeX errors when trying to use externalization with nicematrix.

In the version 2.1.1 of 2018/09/18 (available in CTAN), the processus of externalization is now deactivated in the environments of nicematrix. Of course, the only benefit is in a document with other Tikz pictures interesting to externalize.

Answer 2

I found a solution using the standalone package, that solves the issue and improves the speed. Following a short description of the solution.

Create a seperate standalone file for each matrix.

% matrix_N.tex
\documentclass{standalone}
\usepackage{nicematrix}
\begin{document}%
\ensuremath{<code of matrix>}%
\end{document}

And then include the standalone files in the main document:

% main.tex
\documentclass{article}
\usepackage[mode=buildnew]{standalone}
\usepackage{nicematrix}
\begin{document}

% Test 1
\begin{flalign}
[S]&=\tikz[baseline]{\node[anchor=center]{\includestandalone{Test1}};}
\end{flalign}

% Test 3
\begin{flalign}
[S]&=\tikz[baseline]{\node[anchor=center,yshift=1em]{\includestandalone{Test3}};}
\end{flalign}

\end{document}

Problems/Restrictions:

• Alignment in equation not perfect (manual adjustment)
• Labels and references are not supported
• Manually control the consistency of the main document and figures (font, etc.)
• Attention, if the standalone figure needs several compilations to display correctly. Compile with mode=build until the figure is correct and then change mode=buildnew or save the standalone file with a new modification date and compile until the figure is correct.

Of cause, this solution is not restricted to the nicematrix package and can be used in general. For example, if externalization with tikz is not possible.

Answer 3

From the source code:

\group_begin:
\tikzset{every~picture/.style = {overlay,
 remember~picture,
 name~prefix = nm-\int_use:N \g_@@_env_int-}}

As you can see the package draws each matrix with overlay and remember picture. So the short answer to your question how to externalize them is: you can't.

Pictures with overlay and remember picture usually require two compilations. However, the externalization mechanism is rather simple and rather than trying to produce something undefined the developers decided not to support this.

You still have the option to externalize other pictures, but if you use many nicematrix environments I would recommend to locally enable externalization for the other pictures.