Problems with TikZ overlay in table with varios subfigures

Question

I am trying to overlay a TikZpicture over my table and would like it to look like this:

My code is the following

\documentclass{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[pdftex]{graphicx}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{shapes}
\usetikzlibrary{tikzmark}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{booktabs}
\usepackage {tabularx}
\usepackage{multirow}

\begin{document}

\newcommand{\newfig}[3]{\begin{subfigure}[t]{\linewidth}\centering\includegraphics[width=#1]{#2}\vspace{-4pt}\caption*{\footnotesize\centering #3}\end{subfigure}}
\newcommand{\newfigg}[3]{\begin{subfigure}[t]{\linewidth}\centering \includegraphics[height=#1]{#2}\vspace{-4pt}\caption*{\footnotesize\centering #3}\end{subfigure}}
\newcommand{\myCoord}[1]{
  \tikzmark{#1}
    \tikz\draw[fill=red,color=red] (pic cs:#1) circle[radius=3pt];
}

\begin{table}%
\centering
\begin{tabularx}{\textwidth}{m{0.2\linewidth}|X}
  \toprule
  Teilaufgabe&Lösungsvarianten\\
    \midrule
  Heizelement&{\begin{tabularx}{\linewidth}{X|X|X|X}
        \myCoord{a}\newfig{2.2cm}{example-image.png}{elektrischer Widerstand}&
    \newfig{2.2cm}{example-image.png}{Heizkartusche}&
    \newfig{2.2cm}{example-image.png}{Nickelchrom-draht}&
    \newfig{2.2cm}{example-image.png}{Verbrennung}
    \end{tabularx}}\\
  \midrule
  Temperatur- messung&{\begin{tabularx}{\linewidth}{*{2}{X|}X}
        \newfigg{1.3cm}{example-image.png}{Widerstands-thermometer}&
        \myCoord{b}\newfigg{1.3cm}{example-image.png}{Thermistor}&
        \newfigg{1.3cm}{example-image.png}{Thermoelement}
    \end{tabularx}}\\
    \midrule
    Mischverfahren&{\begin{tabularx}{\linewidth}{*{4}{X|}X}
        \centering aktiv&\multicolumn{4}{c}{passiv}\\ \midrule
        \newfig{1.8cm}{example-image.png}{Mischstab}&
        \newfig{1.8cm}{example-image.png}{gestufte Labyrinthmischdüse}&
        \myCoord{c}\newfig{1.8cm}{example-image.png}{Linear Edge Melt Filter}&
        \newfig{1.8cm}{example-image.png}{Statische Mischdüse}&
        \newfig{1.8cm}{example-image.png}{Statische Mischdüse}
    \end{tabularx}}\\
\bottomrule
\end{tabularx}

\begin{tikzpicture}[remember picture]
  \draw[overlay,color=red] (pic cs:a) -- (pic cs:b) -- (pic cs:c);
\end{tikzpicture}

\end{table}

\end{document}

And I'm getting the following result:

I'm thankful for any help!

Answer 1

Drawing on top of pictures generally requires some manual adjustments. But don't mix between overlaying and direct drawing. Because direct drawing can destroy your original figure alignment. I removed the \myCoord command and did all drawings in an overlay style.

\documentclass{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[pdftex]{graphicx}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{shapes}
\usetikzlibrary{tikzmark}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{booktabs}
\usepackage {tabularx}
\usepackage{multirow}

\begin{document}

\newcommand{\newfig}[3]{\begin{subfigure}[t]{\linewidth}\centering\includegraphics[width=#1]{#2}\vspace{-4pt}\caption*{\footnotesize\centering #3}\end{subfigure}}
\newcommand{\newfigg}[3]{\begin{subfigure}[t]{\linewidth}\centering \includegraphics[height=#1]{#2}\vspace{-4pt}\caption*{\footnotesize\centering #3}\end{subfigure}}

\begin{table}%
\centering
\begin{tabularx}{\textwidth}{m{0.2\linewidth}|X}
  \toprule
  Teilaufgabe&Lösungsvarianten\\
    \midrule
  Heizelement&{\begin{tabularx}{\linewidth}{X|X|X|X}
        \newfig{2.2cm}{example-image.png}{elektrischer Widerstand}\tikzmark{a}&
    \newfig{2.2cm}{example-image.png}{Heizkartusche}&
    \newfig{2.2cm}{example-image.png}{Nickelchrom-draht}&
    \newfig{2.2cm}{example-image.png}{Verbrennung}
    \end{tabularx}}\\
  \midrule
  Temperatur- messung&{\begin{tabularx}{\linewidth}{*{2}{X|}X}
        \newfigg{1.3cm}{example-image.png}{Widerstands-thermometer}&
        \newfigg{1.3cm}{example-image.png}{Thermistor}\tikzmark{b}&
        \newfigg{1.3cm}{example-image.png}{Thermoelement}
    \end{tabularx}}\\
    \midrule
    Mischverfahren&{\begin{tabularx}{\linewidth}{*{4}{X|}X}
        \centering aktiv&\multicolumn{4}{c}{passiv}\\ \midrule
        \newfig{1.8cm}{example-image.png}{Mischstab}&
        \newfig{1.8cm}{example-image.png}{gestufte Labyrinthmischdüse}&
        \newfig{1.8cm}{example-image.png}{Linear Edge Melt Filter}\tikzmark{c}&
        \newfig{1.8cm}{example-image.png}{Statische Mischdüse}&
        \newfig{1.8cm}{example-image.png}{Statische Mischdüse}
    \end{tabularx}}\\
\bottomrule
\end{tabularx}

\begin{tikzpicture}[remember picture]
  \draw[overlay,color=red] ([shift={(-1.1cm,.9cm)}]pic cs:a)node[circle,fill=red,color=red]{} -- ([shift={(-1.6cm,.6cm)}]pic cs:b)node[circle,fill=red,color=red]{} -- ([shift={(-.9cm,.7cm)}]pic cs:c)node[circle,fill=red,color=red]{};
\end{tikzpicture}

\end{table}

\end{document}

Answer 2

Although your original mycoord command introduced misalignments, it can be modified to do what you wanted. In following code, mycoord command is similar to newfig or newfigg but it declares a TiKZ node surrounding includegraphics. This nodes can be later used to draw links between them.

\documentclass{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[pdftex]{graphicx}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{shapes}
\usetikzlibrary{tikzmark}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{booktabs}
\usepackage {tabularx}
\usepackage{multirow}

\begin{document}

\newcommand{\newfig}[3]{\begin{subfigure}[t]{\linewidth}\centering\includegraphics[width=#1]{#2}\vspace{-4pt}\caption*{\footnotesize\centering #3}\end{subfigure}}
\newcommand{\newfigg}[3]{\begin{subfigure}[t]{\linewidth}\centering \includegraphics[height=#1]{#2}\vspace{-4pt}\caption*{\footnotesize\centering #3}\end{subfigure}}

\newcommand{\myCoord}[4]{%
\begin{subfigure}[t]{\linewidth}%
\centering%
  \begin{tikzpicture}[remember picture] 
   \node[inner sep=0pt, outer sep=0pt] (#1) {\includegraphics[#2]{#3}};
   \fill[red] (#1) circle (3pt); 
  \end{tikzpicture}%
\vspace{-4pt}\caption*{\footnotesize\centering #4}\end{subfigure}}

\begin{table}%
\centering
\begin{tabularx}{\textwidth}{m{0.2\linewidth}|X}
  \toprule
  Teilaufgabe&Lösungsvarianten\\
    \midrule
  Heizelement&{\begin{tabularx}{\linewidth}{X|X|X|X}
        \myCoord{a}{width=2.2cm}{example-image.png}{elektrischer Widerstand}&
    \newfig{2.2cm}{example-image.png}{Heizkartusche}&
    \newfig{2.2cm}{example-image.png}{Nickelchrom-draht}&
    \newfig{2.2cm}{example-image.png}{Verbrennung}
    \end{tabularx}}\\
  \midrule
  Temperatur- messung&{\begin{tabularx}{\linewidth}{*{2}{X|}X}
        \newfigg{1.3cm}{example-image.png}{Widerstands-thermometer}&
        \myCoord{b}{height=1.3cm}{example-image.png}{Thermistor}&
        \newfigg{1.3cm}{example-image.png}{Thermoelement}
    \end{tabularx}}\\
    \midrule
    Mischverfahren&{\begin{tabularx}{\linewidth}{*{4}{X|}X}
        \centering aktiv&\multicolumn{4}{c}{passiv}\\ \midrule
        \newfig{1.8cm}{example-image.png}{Mischstab}&
        \newfig{1.8cm}{example-image.png}{gestufte Labyrinthmischdüse}&
        \myCoord{c}{width=1.8cm}{example-image.png}{Linear Edge Melt Filter}&
        \newfig{1.8cm}{example-image.png}{Statische Mischdüse}&
        \newfig{1.8cm}{example-image.png}{Statische Mischdüse}
    \end{tabularx}}\\
\bottomrule
\end{tabularx}

\begin{tikzpicture}[remember picture]
  \draw[overlay,color=red] (a.center) -- (b.center) -- (c.center);
\end{tikzpicture}

\end{table}

\end{document}