Undesired equal sign in beamer using \dimexpr

Question

I'm using the following script in beamer to locate our logo at the right-most corner of all pages in the presentation,

\begin{textblock}{5}(10.0, \dimexpr 0.5\headheight - 7.5pt )
   \includegraphics[width=40pt]{logo.png}
\end{textblock}

The placement of the textblock in the page is fine, however there is an undesired equal sign (see added image) in the page. It's the first time I use \dimexpr, do I make a mistake?

I think this undesired equal sign comes from \dimexpr 0.5\headheight - 7.5pt this part of script. Because when I replace it with, for example, 1, the equal sign goes.

I just tried to do something \dimexpr 15pt - 7.5pt it still gives an undesired equal sign.

Answer 1

Another option is to use the starred version of the textblock environment. This way, you can indicate the unit and, when the calc package is loaded, you can do your usual calc computations.

\documentclass[10pt]{beamer}
\usetheme{Warsaw}
\usepackage{calc}
\usepackage{textpos}
\title{Topic name}
\author{My name}
\date{\today}
\begin{document}
\begin{frame}
\maketitle
\begin{textblock*}{5cm}(10pt,\dimexpr0.5\headheight - 7.5pt)
   \includegraphics[width=40pt]{example-image}
\end{textblock*}
\end{frame}
\end{document}

Answer 2

I suppose that testblock doesn't accept values with a unit. You have to strip it:

\PassOptionsToPackage{demo}{graphicx}%% only for demo
\documentclass[12pt]{beamer}
\usepackage{textpos}
\begin{document}

\makeatletter
\def\foo{\strip@pt\dimexpr0.5\headheight - 5pt\relax}
\makeatother
\begin{textblock}{5}(10.0,\foo)
   \includegraphics[width=40pt]{logo.png}
\end{textblock}

\end{document}

However, instead of using textblock you should redefine your header which is possible with the default macros which beamer provides.