Place image in background layer defined in frame

Question

Problem

I am working on a template for my university. I have been able to create most of it, but I am stuck trying to create a frame called titlepage. Specifically in this frame I want the user to be able to replace part of the background of an image of their choice. Like the following

Ideally I want the following syntax

\begin{frame}{titlepage, img=example-image-a, pos=0.6}
   content here
\end{frame}

or

\begin{titlepage}{img=example-image-a, pos=0.6}
   content here
\end{titlepage}

With the help of Overlay image with parts of another image and Creating a simple university Beamer template from scratch I was able to come up with the MWE at the bottom. However, I have a few problems

• The frontpage image removes all the text on the slide
• I only want the user to be able to insert an image if the frontpage i selected
• I have not been able to create the key syntax described above

uit-background-titlepage.png

Code

\documentclass[aspectratio=169]{beamer}
% \usetheme{uit}
\usepackage[english]{babel} % Main language to the right ->
\usepackage{tikz}
\usetikzlibrary{calc}

\newcommand{\UiTtitlepageBackground}{%
  \expandafter\includegraphics\expandafter[%
    width=\paperwidth,
    height=\paperheight]{uit-background-titlepage.png}%
}

\setbeamertemplate{background canvas}{
}

\usepackage{etoolbox}
\BeforeBeginEnvironment{frame}{%
    \setbeamertemplate{background}{}%
}

\makeatletter
\define@key{beamerframe}{titlepage}[true]{%
    \setbeamertemplate{background}{%
        \vbox to \paperheight{\vfil\hbox to \paperwidth{\hfil%
        \UiTtitlepageBackground\hfil}\vfil}%
    }%
}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\title{There Is No Largest Prime Number and way to to to much info}
\subtitle{subtitle na na na na na na na na na na na na na na na batman}
\date[ISPN ’80]{27th International Symposium of Prime Numbers}
\author[Euclid]{Euclid of Alexandria \texttt{[email protected]}}

\makeatletter
\AtBeginDocument{
  \newlength\beamerleftmargin
  \setlength\beamerleftmargin{\Gm@lmargin}
}
\makeatother

\newcommand{\titleimage}[2][0]{%
  \pgfmathsetmacro\xpos{max(min(#1,1),0)*0.5855}
  \begin{tikzpicture}[nodes={inner sep=0pt,anchor=south west}]
    \node (X) {%
        \hspace*{-\beamerleftmargin}%
        \vbox to \paperheight{\vfil\hbox to \paperwidth{\hfil%
        \UiTtitlepageBackground\hfil}\vfil}%
      };
    \begin{scope}[x={(X.south east)},y={(X.north west)}]
      \begin{scope}
        \clip (0.5855,0) -- (0.833,1) -| (1,0) -- cycle;
      \path let \p1=(X.north east) in (\xpos,0)
        node{\includegraphics[width=\x1,height=\y1]{#2}};
    \end{scope}
  \end{scope}
\end{tikzpicture}
}

\begin{document}

\begin{frame}[titlepage]
  \titleimage[0.55]{example-image-a}
  \titlepage
\end{frame}

\begin{frame}
  \titleimage[0.6]{example-image-c}
  \begin{theorem}
    Hmmm
  \end{theorem}
  \begin{example}
    \begin{enumerate}
      \item hello
      \item hi
      \item woah
    \end{enumerate}
  \end{example}
\end{frame}

\end{document}

Answer 1

I think I do not fully understand all problems you have but one of them seems to be to pass a set of options to the titleimage template. Since you are using pgf, this is easy. The following contains an example, which can be polished once I understand all aspects of the question. The template is now parsing the pgf keys and using them. For instance, if you are saying

\begin{frame}[t,titleimage={file=example-image-a}]
\frametitle{Something}
  \begin{theorem}
    Hmmm
  \end{theorem}
  \begin{example}
    \begin{enumerate}
      \item hello
      \item hi
      \item woah
    \end{enumerate}
  \end{example}
\end{frame}

you get

and

\begin{frame}[titleimage={file=example-image-duck,pos=0.6}]
\frametitle{Something else}
\begin{itemize}
 \item blub
 \item quack
 \item pft
 \item meow
\end{itemize}
\end{frame}

The important thing is that we added an optional key pos here. If you do not specify it, it assumes some default value. Crucially, you can add as many keys of that sort as you want, and you can always upgrade your template to allow for additional keys without losing backwards compatibility.

Here is the full code.

\documentclass[aspectratio=169]{beamer}
% \usetheme{uit}
\usepackage[english]{babel} % Main language to the right ->
\usepackage{tikz}
\usetikzlibrary{calc}

\newcommand{\UiTtitlepageBackground}{%
  \expandafter\includegraphics\expandafter[%
    width=\paperwidth,
    height=\paperheight]{uit-background-titlepage.png}%
}

\setbeamertemplate{background canvas}{
}

\usepackage{etoolbox}
\BeforeBeginEnvironment{frame}{%
    \setbeamertemplate{background}{}%
}

\makeatletter
\define@key{beamerframe}{titlepage}[true]{%
    \setbeamertemplate{background}{%
        \vbox to \paperheight{\vfil\hbox to \paperwidth{\hfil%
        \UiTtitlepageBackground\hfil}\vfil}%
    }%
}


\define@key{beamerframe}{titleimage}[]{%
    \Titleimage{#1}%
}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\title{There Is No Largest Prime Number and way to to to much info}
\subtitle{subtitle na na na na na na na na na na na na na na na batman}
\date[ISPN '80]{27th International Symposium of Prime Numbers}
\author[Euclid]{Euclid of Alexandria \texttt{[email protected]}}

\makeatletter
\AtBeginDocument{
  \newlength\beamerleftmargin
  \setlength\beamerleftmargin{\Gm@lmargin}
}
\makeatother
\pgfkeys{/title image/.cd,pos/.initial=0,file/.initial=example-image-duck}
\newcommand{\Titleimage}[1]{%
\setbeamertemplate{background}{%
  \pgfkeys{title image/.cd,#1}% 
  \pgfmathsetmacro\xpos{max(min(\pgfkeysvalueof{/title image/pos},1),0)*0.5855}%
  \begin{tikzpicture}[nodes={inner sep=0pt,anchor=south west}]
    \node (X) {%
        \vbox to \paperheight{\vfil\hbox to \paperwidth{\hfil%
        \UiTtitlepageBackground\hfil}\vfil}%
      };
    \begin{scope}[x={(X.south east)},y={(X.north west)}]
      \begin{scope}
        \clip (0.5855,0) -- (0.833,1) -| (1,0) -- cycle;
      \path let \p1=(X.north east) in (\xpos,0)
        node{\includegraphics[width=\x1,height=\y1]{\pgfkeysvalueof{/title
        image/file}}};
    \end{scope}
  \end{scope}
\end{tikzpicture}}
}
\newcommand{\titleimage}[2][0]{%
  \pgfmathsetmacro\xpos{max(min(#1,1),0)*0.5855}
  \begin{tikzpicture}[nodes={inner sep=0pt,anchor=south west}]
    \node (X) {%
        \hspace*{-\beamerleftmargin}%
        \vbox to \paperheight{\vfil\hbox to \paperwidth{\hfil%
        \UiTtitlepageBackground\hfil}\vfil}%
      };
    \begin{scope}[x={(X.south east)},y={(X.north west)}]
      \begin{scope}
        \clip (0.5855,0) -- (0.833,1) -| (1,0) -- cycle;
      \path let \p1=(X.north east) in (\xpos,0)
        node{\includegraphics[width=\x1,height=\y1]{#2}};
    \end{scope}
  \end{scope}
\end{tikzpicture}
}

\begin{document}

\begin{frame}[t,titleimage={file=example-image-a}]
\frametitle{Something}
  \begin{theorem}
    Hmmm
  \end{theorem}
  \begin{example}
    \begin{enumerate}
      \item hello
      \item hi
      \item woah
    \end{enumerate}
  \end{example}
\end{frame}

\begin{frame}[titleimage={file=example-image-duck,pos=0.6}]
\frametitle{Something else}
\begin{itemize}
 \item blub
 \item quack
 \item pft
 \item meow
\end{itemize}
\end{frame}

\end{document}

As I said, it is not streamlined yet because I have the strong feeling that I did not address all the points you raised (but I could not understand them, also because I fail to understand e.g. "I only want the user to be able to insert an image if the frontpage i selected" grammatically and logically.)