beamer: Add Transparency to Elements of \titlepage

Question

Intro

• I want to spice my title page up by using a fancy background picture.
• Due to a bad contrast (text/picture) the readability is poor.
• I added a background color (bg =) to the elements of the \titlepage (see MWE).

Questions

1. Is it possible to define the background color (bg =) with a transparency level?
2. I figured out that \setbeamertemplate{title page}[default][rounded=true] changes the text boxes but I couldn't add (--> error) transparency (e. g. opacity=0.3).
3. I do not understand why I need to add [default] to the \setbeamertemplate{title page} in order to apply the rounded=true (I don't need that when setting a color like in \setbeamercolor).
4. Is it possible to change the width of the boxes (other then increasing the margin locally)?

\documentclass[aspectratio=169]{beamer}
\usepackage{graphicx}

\begin{document}

% Define Title Page
\title{Title of Presentation}
\subtitle{Subtitle}
\author{Dr. Manuel Kuehner}
\institute{Where I Work}
\date{\today}

\setbeamercolor{title}{fg=white,bg=black}
\setbeamercolor{subtitle}{fg=white,bg=black}
\setbeamercolor{author}{fg=white,bg=black}
\setbeamercolor{institute}{fg=white,bg=black}
\setbeamercolor{date}{fg=white,bg=black}

\setbeamertemplate{title page}[default][rounded=true]

% Print Title Page
{ % Begin Group
\usebackgroundtemplate{\includegraphics[width=\paperwidth,height=\paperheight]{example-image.png}}%
\begin{frame}[plain]
\titlepage
\end{frame}
} % End Group

\end{document}

Own Partial Solution

By adding \pgfsetfillopacity{0.65} just before the \titlepage command is kind of a manual solution (bus with some problems, see output).

\begin{frame}[plain]
\pgfsetfillopacity{0.65}
\titlepage
\end{frame}

Answer 1

tl;dr: If you don't like the below answers, define your own titlepage with semi-transpartent boxes of your own choice, for example from the tcolorbox package.

---

So if you would have asked "How to define a title page with semi-transparent blocks of variable size" you would have gotten a more useful answer, but here are the answers to the questions you asked:

1. Is it possible to define the background color (bg =) with a transparency level?

Never say never in latex, but it might involve rewriting beamer ...

2. I figured out that \setbeamertemplate{title page}[default][rounded=true] changes the text boxes but I couldn't add (--> error) transparency (e. g. opacity=0.3).

The arguments used in the titlepage template (see beamerinnerthemedefault.sty for the definition) are passed on to beamercolorboxes, which have no parameter for transparency. So no big surprise that random arguments, which are not defined, do not work.

3. I do not understand why I need to add [default] to the \setbeamertemplate{title page} in order to apply the rounded=true (I don't need that when setting a color like in \setbeamercolor).

You need the default, because you are not setting your own content, you are just loading a predefined template called default (defined in beamerinnerthemedefault.sty) If you would set your own, you could use the same syntax you are used with colours:

\setbeamertemplate{title page}{%
    \begin{frame}
       bla
    \end{frame}
 }

4. Is it possible to change the width of the boxes (other then increasing the margin locally)?

Yes, there are several. See for example Changing default width of blocks in beamer

Your black line problem

The "problem" is that the top area and the body of the boxes overlap - this ensures, that there will be no ugly viewer artefacts at the boarder. But with transparent areas you can see the overlap. A fix to this can be found at Untitled Block transparency (Beamer)