get height and width of tikz node

Question

Consider this application:

\documentclass[border=5pt]{standalone}
\usepackage{graphicx}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \node(a) at (0.0, 0.0)[anchor=south west]{\includegraphics[width=0.3\paperwidth]{example-image-a}};
  \draw[blue, fill=red] (0.15\paperwidth, 0.0\paperwidth) rectangle (0.3\paperwidth, 0.15\paperwidth);
\end{tikzpicture}
\end{document}

My goal is to draw a rectangle over the right half of the example image. I am guessing the rectangle coordinates knowing that the width is width=0.3\paperwidth and approximately the aspect ratio.

I would like to be very precise. How can I draw the rectangle over exactly the half of example image a? Such as knowing the height and width of node a.

Answer 1

Rectangle more exactly cover image with the correcting outer sep Of image node:

\documentclass[border=5pt]{standalone}

\usepackage{graphicx}
\usepackage{tikz}

\begin{document}
    \begin{tikzpicture}
\node (a) [inner sep=0pt, 
           outer sep=-0.5\pgflinewidth] % <---
{\includegraphics[width=0.3\paperwidth]{example-image-a}};
        \draw[blue, fill=red, semitransparent] (a.south) rectangle (a.north east);
    \end{tikzpicture}
\end{document}

or by use of fit library:

\documentclass[border=5pt]{standalone}
\usepackage{graphicx}
\usepackage{tikz}
\usetikzlibrary{fit}

\begin{document}
    \begin{tikzpicture}
\node (a) [inner sep=0pt, 
           outer sep=-0.5\pgflinewidth] % <---
{\includegraphics[width=0.3\paperwidth]{example-image-a}};
\node [draw=blue, fill=red, semitransparent, inner sep=0pt,
       fit=(a.south)(a.north east)] {};
    \end{tikzpicture}
\end{document}

The result s the same as before.

Answer 2

You can use the nodes positions to position the red square precisely above the picture.

\documentclass[border=5pt]{standalone}

\usepackage{graphicx}
\usepackage{tikz}

\usetikzlibrary{calc}

\begin{document}
    \begin{tikzpicture}
        \node(a) at (0,0)[inner sep=0pt, outer sep=0pt,anchor=south west] {\includegraphics[width=0.3\paperwidth,keepaspectratio]{example-image-a}};
        \draw[blue, fill=red, opacity=0.5] (a.south) rectangle (a.north east);
    \end{tikzpicture}
\end{document}