Arithmetic on tikzmark points

Question

Can I do arithmetic on tikzmark points? For example if I have \tikzmark{A} and \tikzmark{B} can I find their midpoint?

Answer 1

Welcome to TeX.SE! You can use calc for that. In general, the midpoint between a and b is given by ($(a)!0.5!(b)$). One slight subtlety is that you have to prepend pic cs: to the \tikzmark coordinates, but not for \tikzmarknode nodes. The following MWE illustrates this.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{tikzmark,calc}
\begin{document}
Hello\tikzmark{A} something else\tikzmark{B}.\begin{tikzpicture}[overlay,remember picture]
\draw[red,latex-] ($(pic cs:A)!0.5!(pic cs:B)$) -- ++(0,-1);
\end{tikzpicture} \tikzmarknode{H}{Hello} world and \tikzmarknode{M}{marmots}!\begin{tikzpicture}[overlay,remember picture]
\draw[red,latex-] ($(H)!0.5!(M)$) -- ++(0,-1);
\end{tikzpicture}
\end{document}